All ‘Plant Doctor’ Quiz Questions

The correct answer is:

Bacteria: It is common for bacterial infection sites to be water soaked. If a spot is water soaked there is strong probability bacteria are the cause, however, bear in mind that bacterial infections are not always water soaked.

The incorrect answers were:

Fungi: leafspots caused by fungi are very rarely water soaked.

Water moulds: despite the name, the downy mildews and Phytophthora do not cause water soaked lesions.

Viruses: mostly cause leaf distortion and mosaic patterns. They do not cause water soaking.

The correct answer is:

Bacterial pathogen: This is bacterial blight of cassava. It is a well-known disease found in all areas where cassava is grown. It is caused by the bacterium Xanthomonas axonopodis pv. manihotis. The symptoms initially appear as water-soaked areas before the leaves turn brown and dry out.

The incorrect answers were:

Fungal pathogens: can cause something similar, but do not start specifically at the leaf edge.

Viral pathogens: almost never cause tissue death such as this; they generally produce leaf distortion and mosaics.

Nematodes: can cause wilt and death of plants, but generally the whole plant will suffer and not just an isolated leaf or an area of a leaf.

The correct answer is:

Fungal pathogen: This is brown leaf spot of cassava caused by a fungus. The small black dots seen in the leaf spot are the actual fungus. If the dots are present then fungus is definitely the cause. However, some fungi do not produce black dots, so if the dots are absent then a fungus may still be the cause.

The incorrect answers were:

Bacterial pathogens: commonly cause leaf spots such as this but they never produce structures that can be seen (even with a hand lens).

Water moulds: include the downy mildews and Phytophthora; neither produce structures such as these. They both generally produce a downy growth on the underside of the leaf (in the case of Phytophthora with a lot of necrosis (browning)).

Viruses: never produce structures that can be seen and they very rarely produce leaf spots. Leaf spots caused by viruses are usually in the form of rings.

The correct answer is:

Nitrogen: A deficiency of the major nutrients (NPK) is always seen in the lower leaves first. This tip dieback of the leaf is characteristic of nitrogen deficiency in maize but not in other crops. The general yellowing of the lower leaves is a more general symptom.

The incorrect answers were

Potassium: in the case of potassium (K) shortage it is the outsides of the leaves that are the first to show symptoms.

Sulphur: as sulphur is not a major nutrient, the symptoms are seen in the upper leaves first. The symptoms of sulphur deficiency is a general yellowing of the whole leaf.

Phosphate: shortage of phosphate (P) induces a reddening of the lower regions of the plant.

The correct answer is:

Mites: Mites are a very common pest on many crops especially those grown under cover, however, despite the many and severe symptoms they can induce, they never cause plants to wilt. It is not clear why mites are more of a problem in a protected cropping environment but it may be due to not being rained upon.

The incorrect answers were:

Nematodes: cause considerable damage to the roots of plants attack and commonly cause wilts

Bacteria: it is common for stem infecting bacteria to cause wilts.

Insects: both stem borers and root dwelling insects commonly cause wilts by attacking the stem or roots.

The correct answer is:

Herbicide damage: the dramatic bleaching of the upper leaves is a symptom associated with herbicide treatment. The plants on the right appear not to have been treated, note the very sharp line between the healthy plants and those with symptoms.

The incorrect answers were:

Nematodes: attacking the roots can cause the leaves to turn yellow but usually the whole plant will have this symptom and not just the upper leaves.

Nutrient deficiency: is a common cause for plants to turn yellow but compare the plants at the edge of the field which appear to have no symptoms. It is unlikely that there would be this much variation in the soil over such small distances.

Viral infection: the symptoms of viral infection are often a mosaic rather than the bleaching seen here and the symptoms would not be restricted to only on a section of the crop (see above).

The correct answer is:

Phytoplasmas: the symptoms of phytoplasma infection can look very similar to the regrowth after Glyphosate treatment. These symptoms are extremely typical of phytoplasma infection.

The incorrect answers were:

Bacteria: never induce these symptoms.

Fungi: fungal infection can induce a witches’ broom /little leaf symptom but it is an unusual symptom of fungal infection.

Nematodes: can stunt plants and the developing leaves may appear smaller but they never induce this dramatic “little leaf” formation.

The correct answer is:

Insect galls: This is the larval stage of the mango gall midge. Larvae penetrate plant tissues and live in flattened, blister-like galls. Final-instar larvae emerge from damaged tissues and fall to the soil where they pupate. When conditions are favourable a new generation of adults emerges within 4-6 days, but when conditions are unfavourable the full-grown larvae construct silk cocoons in which they rest for one or more years before emerging.

The incorrect answers were:

Frass: this would be very unusual frass. The frass from caterpillars is large and black like this but is never stuck to (or embedded into) leaf tissue. Thrips frass can be stuck to the leaves like this but is never this large.

Bacterial galls: it is unusual to see bacterial galls on a leaf but the three things which indicate that these are not bacterial galls is the fact that there is so many of them, they are all approximately the same size and that they are smooth.

Anthracnose: this is a fungal disease that causes extensive leaf spots and death of the leaves and not the small smooth disc-shaped galls we see here.

The correct answer is:

Pollen: In grasses a lot of pollen is shed and may be seen as a fine dusting on leaves.  The pollen is not a problem in itself, but it will increase the nutrients on the leaf surface which can increase infection by pathogens. This is not so much a problem in broadleaved crops, but the nutrition on the leaf surface can be affected by falling petals. The falling petals may create an environment in which the pathogen can thrive.

In this picture, the additional nutrients from the fallen flower have allowed the pathogen to attack the leaf and start an infection.

The incorrect answers were:

Frass: can appear in many different forms but is never dry and dusty like this.
Sooty mould: is commonly seen on the leaf surface growing on honeydew but is always black.
Powdery mildew: is commonly seen growing on leaf surfaces but it cannot be shaken (or blown) from the leaf and is always white.  However, it is not possible to tell that the pollen could be blown from the leaf in this photo.

The correct answer is:

Bacteria: often enter plants through small amounts of damage, will be carried on cultivation equipment and thrive under wet conditions.

The incorrect answers were:

Viruses: very few viruses are transmitted mechanically (although some of them are very important) and there is little evidence that they are readily transmitted through wounds created during cultivation. Some viruses can be transmitted by contact with contaminated equipment.

Fungi: most fungi do not generally enter plants through wounds and minor cultivation damage would not spread fungal disease greatly.

Nematodes: do not generally infect plants through damage.

The correct answer is:

Abiotic scorch of some sort.

Many things may cause the tips of leaves to die back like this, but the tan colour of the dead region and extreme transition between the dead and healthy tissue indicates scorch.  In this case, the scorch is due to ammonium nitrate solution having been applied when the sun was too strong.

The incorrect answers were:

Nutrient deficiency: much yellowing occurs. The sharp transition from healthy to dead tissue is not typical of nutrient deficiencies.

Insect larvae within the stem: Insect larvae can cause problems in wheat but the symptoms are a general withering of the plant and not the sharp transition between healthy and dead tissue.

Bacterial infection: Bacterial infection generally appears wet and, whereas the dead areas here appear very dry, you would rarely see such a marked transition between healthy and dead tissue.

The correct answer is:

Ozone

Ozone is an under-appreciated threat to agriculture, especially in polluted areas. The symptoms are generally seen on older leaves and beans are particularly susceptible. (Photo; UKCEH)

The incorrect answers were:

Water logging; roots immersed in water for prolonged periods will damage many crop plants. The symptoms are usually yellowing and wilting.

Hydrogen sulphide; can be a problem in rice crops but not usually elsewhere.

Excessive humidity; this is one of the causes that can lead to oedema and not the reddening and flecking seen here.

The correct answer is:

Mosaic

The incorrect answers were:

Mottle:   the sharp lines between the yellow are green areas on the leaf make this a mosaic. If they were blurred and the contrast in colour was not so great, it may be considered a mottle.

Yellowed : this is a term used to describe a leaf where the whole leaf has lost its green colour.

Speckled :  this is a symptom that can look similar to a mosaic but the effect is due to the empty surface cells reflecting light (empty due to feeding of a mite or thrips) and the leaf has not changed colour beneath.

The correct answer is:

Salts rising up from below and crystalizing on the soil surface.

If an irrigation system provides all the water for a crop and the water has the slightest mineral content, salts will build up over time on the surface of the soil. When installing an irrigation system, it is often necessary to install drainage too so that the soil could be flushed through occasionally.

The incorrect answers were:

Fungal mycelium growing due to the excessive moisture; this only occasionally happens in extremely humid conditions, not where an irrigation system is necessary.

Bleaching of the humus from the normal brown to white; even in the strongest sun, light does not bleach soil.

Fine root hairs searching for water on the very surface of the soil; root hairs extend only one millimetre from the root at most and will never grow to the surface.

The correct answer is:

Cut the potatoes in half and plant both halves thereby doubling your numbers of seed potatoes.

Provided it is dried off before planting, the large wound created when cutting a seed potato does not let in pathogens and is a well-known technique with potato growers (for horticultural reasons, cut them in half lengthways so both halves get the rose and stem ends of the potato).

The incorrect answers were:

Plant them as they are; do not cut the potatoes, the wound will let in opportunistic pathogens from the soil: they could be planted as they are but cutting them is an option (see above).

Slash the skin of the potato to shock it into growing: this is not a recognised technique and does not work.

Do not plant large seed potatoes as they will not sprout; go and buy some smaller ones: you may want to consider eating the larger potatoes but they will grow.

The correct answer is:

Uneven fertiliser distribution

The straight line between the stunted yellow crop and the lush green crop indicate that the difference is due to a mechanical effect. The general yellowing of the crop on the left is typical of wheat hungry for nitrogen, whereas the dark green crop on the right appears to have had too much nitrogen and that is exactly what happened.

The incorrect answers were:

Nematodes attacking the roots; nematodes can cause stunting and yellowing of plants but attack is never uniform as seen here, nor in a straight line.

Variation in soil type; the soil can have a significant effect on crop growth and variation between soils can be dramatic, but they are never in a completely straight line.

Viral infection; Viruses can cause yellowing and stunting as seen here but they do not form lines like this.

The correct answer is:

Most viruses have a specific relationship with their vectors: the relationship between the vector and the virus is complex and incompletely understood. This relationship is often very specific.

The incorrect answers were:

All plant viruses can be transmitted by insects; not true, although many viruses are transmitted by insects, other vectors include fungi, nematodes, seed or mechanical means.

All plant feeding insects can transmit viruses; not true, some groups are very commonly seen as virus vectors (such as aphids) and many cannot transmit viruses at all.

Laboratory work with micro manipulators can mimic the transmission of all insect transmitted viruses; there is a great deal more to the transmission of viruses than simple mechanical contamination of the mouthparts and micro manipulators cannot replicate this.

The correct answer is:

Genetically the two groups are completely unrelated.

Despite the many similarities between these two groups (long filamentous hyphae and spore formation), it is now known that they are completely different organisms, with different biochemical structure and evolutionary lineages.

IMAGE: Cucumber leaves attacked by a powdery mildew (fungus) left and a downy mildew (oomycete) right

The incorrect answers were:

The oomycetes are considered as the “lower fungi; this would have been the correct answer 40 years ago but it has become clear that oomycetes are not fungi and are not related to fungi.

The two groups occupy different habitats and there is very little similarity between these groups; the oomycetes and the fungi occupy very similar habitats and look very similar.

Superficially and genetically, they are closely related but oomycetes are not technically fungi; they do appear superficially similar (and the oomycetes are often referred to as “fungus-like organism”) but they are not genetically related.

The correct answer is:

The length of time the aphid remains able to transmit the virus depends on the virus/vector combination. The ability of the aphid to transmit the virus, which is technically the length of time it remains viliferous, depends on the virus and species of aphid. The aphid may remain viliferous for a few minutes or it may be for weeks.

The incorrect answers were:

The aphid will always carry the virus (and will inoculate other plants) for the remainder of its life; this is unusual in aphid/virus combinations but it is true in some cases.

The aphid will lose the virus and will not spread it after a few hours; this is true in many aphid virus combinations.

The aphid will lose the virus and will not spread it after a few days; this is true in many aphid virus combinations

The correct answer is:

They provide good control of bacterial pathogens:

Streptomycin, Tetracycline and other medically important antibiotics are effective in controlling bacterial plant diseases but their use is prohibited in many countries due to fears that the resistance developing in the environment could spread to human and animal pathogens.

The incorrect answers were:

They have no effect on bacterial pathogens that affect plants: medically important antibiotics are very effective in controlling bacterial plant pathogens but should not be used.

They will not control bacterial plant pathogens but do control viruses: this is not true -there are no known chemicals that will control viruses once within a plant.

They will remove the natural biological control on leaves and will actually make the situation worse: the antibiotic application will reduce the natural bacterial populations but if the disease is present, it will have greater effect on the pathogen population.

The correct answer is:

Generalised stress.

Reddening (just as with yellowing) is often a sign of generalised stress. This may be due to a variety of factors, for example, abiotic pathogens and insect feeding.  Phosphate deficiency will often cause reddening of the lower leaves and can be diagnostic but that is not the only problem that induces this effect.

The incorrect answers were:

Definitely indicates a deficiency of phosphate

This is a common symptom of P deficiency but is not the only cause of reddening.

Insect larvae within the stem

The stress caused by insect feeding will often cause reddening of the upper leaves but many other problems will do this too.

Root problem of some sort

Root problems may cause stress, leading to reddening, but that is not always the case.

The correct answer is:

Bacteria

Bacteria can cause leaves to collapse due to rotting, but the tissue in this image does not appear to be rotten or water soaked. It appears to be healthy apart from the areas that have split open.

The incorrect answers were:

Physiological problem

This is what I think is causing the problem (although it is very strange as the plants either side are unaffected). Plants, usually fruits, can split open if they take up water too quickly. It may superficially look like insect damage or pathogen infection, but the wound is generally clean.

Phytoplasma infection

Phytoplasma infection can cause gross morphological changes to the development of a plant. There is no evidence that this symptom is due to a phytoplasma but this is the kind of symptom they can provoke.

Animal damage

It looks like a large animal has laid on the plant and snapped the leaf blade at the point of attachment to the petiole.  There is no sign of any infection at the site of breakage.

The correct answer is:

Healthy

Despite the obvious purple colour, these potatoes were totally healthy and tasted like the more familiar white potato.

The incorrect answers were:

Fungal infection

There are several fungal pathogens that will infect potato tubers but generally they will cause the tissue to soften and lose structure.  It you look past the colour you can see that the tuber is perfectly fine.

Nutrient deficiency

Phosphate deficiency will often cause the lower leaves to become reddened (as per question 1) but this does not extend to the tubers.

Water mould infection

The only water mould that will infect the tubers of potatoes is Phytophthora.  This is definitely not Phytophthora infection; the tissue would be wet and brown, not hard and purple.

The correct answer is:

They are on the side of the root and not in the middle

When considering if a lump is a gall or a nodule, one simple rule is: if you can detach it without breaking the root it is a nodule.  Nodules are always displaced to the side of the root, whereas galls usually appear to be part of the root.

The incorrect answers were:

They are not intensely coloured

Both galls and nodules are generally whitish. Galls may be creamy coloured and nodules may have a slight pink colour.

They are less than 4mm across

Nodules are always less than 4mm across, but galls can vary in size from a few millimetres to up to 15 mm and larger.

They are round in cross section.

Both galls and nodules are normally round in cross section.

The correct answer is:

The level of transmission into seed can be significant in some cases, but the proportion of infected seed varies by crop and virus.

A few viruses will be carried over into the seed of the plant in a few cases. The percentage of infected seed varies depending on the crop and virus.  (Pea seed-borne mosaic virus is a good example).

The incorrect answers were:

All viruses are passed on at a low rate in the seed, but the industry lives with this.

No. Some viruses may be passed on into seed, but this is relatively unusual.

Some viruses are always passed on through seed as it is the only means of spread

This is not the case and all viruses have some other means of infecting plants

The process of seed production will remove all viruses and what appears to be seed infection is actually contamination from seed coat.

Contamination of the seed coat does occur in some cases (e.g. TMV on tomatoes) but not many viruses can survive outside the host.  If the seed is contaminated, the virus particles can be literally washed off with bleach.

The correct answer is:

It is extremely important as it provides the sugars to fill the grain

In wheat, the flag leaf is the most important one to keep free of diseases. If only one fungicide spray is applied to a wheat crop, it is best to do it when the flag leaf has just emerged. This is not the case in other cereals where the flag leaf may play a relatively minor role (e.g. barley).

The incorrect answers were:

The flag leaf is unimportant as the ear has already formed by the time it emerges.

It is true that the ear has formed but there is no grain in the ear.

The flag leaf will shade the ear and actually reduces yield.

The ear itself does contribute to grain fill to an extent and there will be some shading, but that is insignificant and you want the flag leaf to be in full sun.

It is just as important as any other leaf.

With regard to grain fill, the importance of the leaves falls off rapidly as you descend the plant.

The correct answer is:

Only wheat

Urediniospores will always infect the host plant on which they were produced. It is the teliospores, which will give rise to the basidiospores, that will ultimately infect the barberry. From a crop protection point of view, a farmer needs to prevent the urediniospores re-infecting the wheat.

The incorrect answers were:

Wheat and barberry

See above

Only barberry

See above

Technically neither wheat nor barberry

See above

The correct answer is:

Zymoseptoria

Even if you are not familiar with wheat diseases, you should have been able to get to the correct answer by eliminating the others.  This is Zymoseptoria (previously Septoria). It is an extremely common pathogen on wheat, carried by rain splash. If it reaches the flag leaf, the yield will be considerably reduced.

The incorrect answers were:

Phytophthora

Phytophthoras are mostly pathogens of trees; potato, tomato, capsicum pepper and soybean are the only annual crops where it is usually a problem.

Yellow rust

Yellow rusts form long stripes of yellow pustules and cause little browning and cell death. (It is still extremely damaging.)

Natural senescence of the older leaves.

The lower leaves of wheat do senesce as the crop matures but do not do so in patches as seen here.

The correct answer is:

Well-developed head capsule and no prolegs (short stubby legs on the abdomen)

The larvae of beetles are grubs. They have a large head, no prolegs but often have well developed thoracic legs (true legs). However, in borers and stem dwellers, these may be extremely small.

The incorrect answers were:

Well-developed head capsule and multiple prolegs (short stubby legs on the abdomen)

This morphology is typical of a moth larva; a caterpillar.

No head capsule and multiple prolegs (short stubby legs on the abdomen)

This morphology is typical of a fly larva; a maggot (the prolegs may be very small)

No head capsule and no prolegs (short stubby legs on the abdomen)

This morphology is typical of a fly larva; a maggot.

The correct answer is:

Two chemicals with the same mode of action will not help to prevent resistance in the pathogen population.

It is always a good idea to blend fungicides to prevent resistance developing in the pathogen population, but if the two chemicals are from the same resistance group, resistance can still develop.

The incorrect answers were:

The chemicals are both systemic and it is preferable to have contact and systemic mix. 

It is common practice to blend a systemic chemical with a contact one. This is usually done to get the benefit of the systemic action with the economy of the contact.

The chemicals are considered to be old and newer ones are available.

The triazole group of fungicides have been available for several decades but they are still an important aspect of disease control.

The chemicals will counteract each other and effectively cancel each other out.

There is no suggestion that this is correct; both chemicals will stay active (until resistance develops).

The correct answer is:

Well-defined leaf spots

The leaf spots typical of Cercospora infection often have all the characteristics of a typical fungal leaf spot: limited size with a clear, distinct border, evenly distributed across the leaf surface.

The incorrect answers were:

White powdery growth

You may be thinking of the powdery mildews. These are the only fungi that produce white powder in substantial quantities.

Wet and slimy rot

Cercosporas will kill the leaf tissue, but it generally remains hard and dry within the leaf spot.

Mosaic or mottle, often more pronounced on the older leaves

These symptoms are not associated with Cercospora infection.

The correct answer is:

Herbicide damage

The clue here is to look at the weeds near the edge of the vegetable patch.  The gardener was clearly spraying off the weeds at the edge of the bed and has just caught the potato plants with the spray. This low dose of glyphosate has produced symptoms at the top of the plant. The young leaves are turning yellow, spreading up from the base of the leaf, but notice that the distal ends of these leaves remain green; these symptoms are typical of sub-lethal glyphosate exposure.

The incorrect answers were:

Iron deficiency

The yellowing of younger leaves is typical of iron deficiency, but, characteristically, if iron shortage is the cause, the leaf veins remain green. Iron deficiency does not induce a wilt.

Phytoplasma infection

Yellowing is a common symptom of phytoplasma infection, but they very rarely cause wilts.

Fungal infection

In the absence of yellowing grass and weeds nearby, this would be a sensible suggestion, although other factors could have caused these symptoms.

The correct answer is:

Bacterial infection

There is not a great deal of information here, but the one leaf lesion we can see is: spreading from the edge of the leaf, does not have a clearly defined border and there is no structure to the lesion. All of these factors suggest that the cause is bacterial.

The incorrect answers were:

Fungal infection

It is possible that this could be a fungal infection, but appears more like a bacterial one.

Nutrient deficiency

Necrotic leaf spots such as this are not typical of nutrient deficiencies.

Water mould infection

There are very few water moulds that commonly attack the leaves of these plants, so this would be unlikely.

The correct answer is:

All of the above

All the features of these leaf spots indicate they are caused by a fungus: a clearly defined border of the spot; not spreading along veins; no visible limitation of spread by veins; they are not associated with the edge of the leaf or with damage; but most telling of all, is that you can see the fungus as rings of small raised black lumps within the spots.

The incorrect answers were:

Strong clearly defined border see above.

Circular and they are not spreading from the edge of the leaf see above.

They are not spreading from the edge of the leaf see above.

The correct answer is:

Hyperparasitism of the powdery mildew pathogen

Ampelomyces quisqualis is a fungal pathogen of fungi. It is attacking the powdery mildew growing on the surface of the leaf.

The incorrect answers were:

Powdery mildew infection

Yes, it is powdery mildew, but that is not what you can see. The powdery mildew has been over grown by the pathogen

Algal growth

This is the most convincing wrong answer and it could be algal growth, but it is not.

Sooty mould

Always black and generally not in discrete patches as seen here.

Photo credit: Michael James MoA Barbados

The correct answer is:

The downy mildew on the sow thistle could spread to the crop

The downy mildews are often very specific in the plants they can attack, but as the sow thistle and lettuce are related, both are susceptible to the same pathogen.  Bremia lactucae causes downy mildew on both plants. This is a good example of diseases spreading from weeds.

The incorrect answers were:

Neither pathogen will spread to the crop see above

Both pathogens could spread to the crop see above

The downy mildew on the sow thistle could spread to the crop see above

The downy mildew on the fat hen could spread to the crop see above

The correct answer is:

The lumpy distortion is not typical of downy mildew infection.

This pest and pathogen are often confused, but downy mildews will rarely cause leaf distortion and this lumpy distortion (rugosity) is typical of mite infestation.

The incorrect answers were:

The pale growth on the underside of the leaf is never white in downy mildew infection.

The sporulation on the underside of leaves infected with downy mildew can be white, grey, pink or purple.

There is no downy mildew that attacks grapevine.

This is not true and Grapevine downy mildew (Plasmopara viticola) is a major problem for the wine industry.

The patchy distribution is not typical of downy mildew infection.

The distribution on these leaves is very typical of downy mildew infection.

The correct answer is:

Virus infection

This is the symptom of the, aptly named, parsnip yellow fleck virus. It is an aphid transmitted virus but cannot spread between parsnip plants. The aphid can only transmit the virus if there is an additional helper virus present, but the helper virus cannot survive in the parsnip. All infections are due to aphids migrating in from wild relatives (in which, both the helper and yellow fleck virus can survive).

The incorrect answers were:

Insect feeding

Sap-feeding insects (aphids, mealybug, scale etc.,) don’t usually make much of a mark on leaves. There are exceptions, but usually they produce necrosis and not yellowing.

Bacterial infection

Bacteria can cause yellowing, but not in a mosaic pattern such as this.

Nutrient deficiency

Yellow and green regions on the same leaf can be typical of some nutrient deficiencies, but the transition between the green and yellow is usually blurred and they are not intermingled as seen here.

The correct answer is:

Nutrient deficiency

This is one symptom that you cannot get from using first principals of plant pathology. It is often seen on rapidly growing fruits, especially tomatoes and cucurbits. When growing rapidly, the plant cannot mobilise calcium to the expanding tissue fast enough and it begins to break down. This is only ever seen at the blossom end of the fruit, hence the name “blossom end rot”.

The incorrect answers were:

Sun scald

This can affect fruits and can look similar, but would be on the top of the fruit and not on the end.

Bacterial infection

This would be a very reasonable suggestion if the rot was not at the blossom end.

Fungal infection

This would be a very reasonable suggestion if the rot was not at the blossom end.

The correct answer is:

Petals add nutrients to the leaf surface and can assist infection by opportunistic pathogens.

The leaf surface can be a hostile environment for fungal pathogens, but the additional nutrients provided by the petals can provide a stimulus to allow infection.

The incorrect answers were:

Once the petals have fallen, the developing fruit is extremely exposed to pathogen attack.

This may occur to some extent, but the presence of the petals does not really protect the ovaries.

Petal fall is a good mid-point in the growth of a plant to put on a spray.

This may be the case, but crop protection should not be about calendar spraying.

The fall of petals has no bearing on fungicide application.

This is not the case, see above.

The correct answer is:

Rust fungus

The brown rusty coloured pustules are clearly visible in the more advanced stages. The small pale dots are also infection sites but the pathogen has not yet broken through the leaf epidermis (the tough layer of cells that creates the leaf surface).

The incorrect answers were:

Sucking insect probing

Many sucking pests, aphids, mealybugs, scale etc., do not create any damage at the site of penetration. Some pests do leave marks, but often they are necrotic and not pale like this.

Viral infection; a local lesion host

A local lesion host is a concept familiar to many who have worked with viruses but it is not often seen in the field.  The rust pustules give away the real cause.

Herbicide spray damage

Symptoms can appear spattered across the leaf like this, but normally those that create an extremely localised response (e.g., paraquat) produce necrotic spots, not pale ones such as these.

The correct answer is:

Physiological problem

The cause of oedema is still a mystery.  There is no pathogen or pest involved and it is entirely physiological.  The uptake of water seems to be the issue.

The incorrect answers were:

Insect feeding

Sap feeding insects do not often cause feeding marks, but when they do, they are often necrotic.

Fungal infection

Rust fungi can sometimes cause a slight blister before they burst the surface, but they would never get this large, nor so uniform.

Mineral deficiency

There are occasional unusual symptoms caused by a lack of nutrients (blossom end rot is a good example) but this is not one of them.

The correct answer is:

They are beneficial themselves as they prey upon pests.

Spiders are very rarely pests and are generally seen as useful members of the crop fauna as they will predate pests.

The incorrect answers were:

They reduce the beneficial insect numbers and should be controlled.

They can predate beneficial insects as seen in the photo, but this is unusual, and will generally consume pests.

They are generally pests themselves and should be controlled.

There are no examples of spiders as pests of crops (mites are related to spiders but are not spiders).

They have almost no effect on the pest status of a crop.

This is not true and they can reduce pest numbers considerably.

The correct answer is:

A

The necrotic region with very little yellowing is typical of late blight on tomato. The white bloom of sporulation is seen around the edge of the necrotic area. (Spreading of necrosis into the leaf, with a band of sporulation bordering it, is extremely characteristic of late blight infection).

The incorrect answers were:

B

This mottling is more typical of a mineral deficiency; possibly manganese deficiency in this case.

C

The tips of the leaves have become necrotic, but there is no sign of any sporulation.

D

The yellowing around the necrotic regions is not typical of late blight.

E

The tan necrotic regions are not typical of late blight under normal conditions.

The correct answer is:

They take up a lot of room and will crowd out the crop.

The main problem with wild oats among the crop is that; they are extremely large plants that grow very quickly, will shade the crop and remove water and nutrients from the soil. They also produce thousands of seeds which can be dormant, making them a very difficult weed to control.

The incorrect answers were:

The seeds contaminate the harvest, reducing its quality.

Unless the harvest is being saved for seed, this is not a problem.

They carry diseases over season to season, making crop rotation redundant.

This may be true to a limited extent, but is not foremost in farmers minds when they are battling wild oats.

The seeds are shed before the combine harvests the crop and do not contribute to the yield.

This is usually true, but it is not the missing yield that is the problem.

The correct answer is:

High temperatures and dry weather after the initial infection.

Foliar-attacking Phytophthora require warm and wet weather to thrive. In this case, the weather became very hot and very dry, and the Phytophthora has died. The lesions have been taken over by a saprobe and is sporulating in some cases.

The incorrect answers were:

This is a resistant variety; this is a resistant reaction. 

No; this was a very susceptible reaction and, if the weather had not changed, the whole leaf would have died.

Biological control of the pathogen is taking place.

Saprobes have moved in on the dead tissue but that is not what killed the leaf tissue (or killed the Phytophthora)

These are early symptoms and will develop into more characteristic symptoms with time.

No; early symptoms of Phytophthora infection are pale, sunken patches on the leaf which rapidly turn black.

The correct answer is:

The upper leaves are red, but the lower leaves remained green.

The symptoms of phosphate deficiency, like the other major nutrients (nitrogen and potassium), are always seen in the lower leaves first. This reddening is due to stress; caused by a stem borer living in the stem at the point where the plant turns red.  Note that the other plants around are not showing any reddening.

The incorrect answers were:

The leaves are the wrong shade of red.

The red colour develops due to stress and the diagnosis on the hue of the colour is not really possible.

The plant is not stunted.

Phosphate deficiency does usually stunt plants but not always.

Maize is the exception and does not turn red in response to phosphate deficiency.

This is not true.

The correct answer is:

Sclerotinia sclerotiorum

A very common fungal disease, which often attacks the base of plants; it is one of the few plant pathogens that will develop an extensive network of hyphae outside the plant.

The incorrect answers were:

Bacterial wilt

Bacterial wilt can cause wilting and necrosis above the site of infection (it usually spreads to all of the plant quickly) but it never produces these black structures seen within the stem.

Phytophthora infestans (Late blight)

The upper leaves have died but the lower leaves show symptoms which would be very unusual for this pathogen. Phytophthora resting spores are too small see with the naked eye and P. infestans does not produce them in many countries.

Tomato spotted wilt virus

The symptoms of TSWV are mostly ringspots and rarely a wilt. Viruses never produce structures visible to the naked eye.

The correct answer is:

About 3 – 10 minutes should be enough.

When the water is extremely still and viewed against a black background, fine wisps of milky fluid can be seen emerging from the cut end of the stem.  It is sometimes easier to take a photo and view the photo than to see it with your eye.

The incorrect answers were:

Viewed very quickly; a few seconds and you will have missed it.

This is too short. You need to allow time for the bacteria to begin to flow.

Overnight is best as this allows the bacteria to really flood out.

The bacteria will have exited the stem by this time and you will not see them in the water.

3 – 4 hours is about right.

This is too long. The bacteria will have exited the stem by this time and you will not see them in the water.

The correct answer is:

The dilution can be varied within certain parameters.

The correct amount of active ingredient should be applied to the correct location on the crop and, within certain limits, the amount of water – as a carrier fluid – can be varied.

The incorrect answers were:

The dilution of the chemical is unimportant provided the correct amount of active ingredient is applied.

This is true to a point, but once the spray becomes too dilute, the spray will run off leaves and onto the soil.

The dilution of the chemical has to be very exact to ensure best results.

The dilution of chemical does not always have to be exact and, usually, there is some sort of flexibility in the range of dilutions possible.

The more concentrated the solution the better and dilution is done only to make the concentrate go further.

A more concentrated fungicide will generally have greater capacity to kill the fungus, but it would be un-economic, illegal and would scorch the crop.

The correct answer is:

Fungus:

These are leaf spots caused by Septoria on a floating water weed.  Septoria is a Coelomycete fungus which means it makes these little black fruiting bodies within the leafspot. Zymoseptoria is a common pathogen on wheat and these pycnidia can usually be seen within leafspots at the base of wheat plants.

The incorrect answers were:

Insect feeding: rarely causes necrosis most sucking insects leave no mark on the leaves.

Water mould:  they sporulate as, either: 1) microscopic hard oospores that cannot be seen, as they are too tiny and are deep within the tissue, or 2) as fluffy growth, usually on the underside of leaves.

Rust fungus: this specialist fungal group does not create leaf spots and the host tissue will only become necrotic (dead) once the host can no longer support the demands of the pathogen.

The correct answer is:

Viral infection:

Mosaic (not seen here) and leaf distortion are both classic symptoms of infection by a virus. This is an example of leaf distortion NOT due to viral infection.

The incorrect answers were:

Bacteria: distortion of leaves is not associated with bacterial infection.

Nutrient deficiencies: there are few examples of nutrient deficiencies causing distortion in leaves but this is extremely unusual.

Water moulds: very occasionally, downy mildews can systemically infect the host, which then produces small, thick (distorted) leaves, but this is rare.

The correct answer is:

Similar to the adult, but shorter and without wings:

Pod bugs belong to the Hemiptera, which do not undergo complete metamorphosis during development. The young of Hemiptera often look broadly similar to the adults, but are smaller and often lack wings.

The incorrect answers were:

Caterpillar: All caterpillars turn into butterflies or moths.

Grub: All grubs turn into beetles.

Maggot: All maggots turn into flies.

The correct answer is:

Ringspots:

The spot on the leaf has a green centre, thus the spot is a ring.  This symptom is almost always due to viral infection and, more often than not, vectored by a nematode.

The incorrect answers were:

Frass:  frass is a term for excrement (usually applied to that of insects but not exclusively).

Mosaic: a mosaic as a symptom literally looks like a mosaic, with hard, angular edges to the blocks of colour.  Mosaics are almost always yellow and green.

Spots: the definition of a spot on a leaf is a tricky one and there are lots of definitions.  This clearly is a spot BUT ringspot is a far better answer.

The correct answer is:

They are toxic and, if present in even modest quantities, will render the grain poisonous:

Ergot are very toxic and must not be eaten. The limits on ergot contamination for animal feed is one per litre of grain, whereas for human consumption, the tolerance is nil.

The incorrect answers were:

They are a missed opportunity for grain production; each ergot produced is a loss in yield: this is true, but the numbers of ergot never substantially reduce the yield.

They have a bitter taste and will make the grain unpalatable: you don’t really want to eat one to find out.

They will discolour the flour when milled and reduce its value: it is not visible in milled flour. Historically, there are examples of unscrupulous millers selling contaminated flour and, when baked into bread, the whole village dying from the ergot poisoning.

The correct answer is:

Mites:

Galls on leaves such as those seen here are commonly produced by eriophyid mites.

Incorrect answers were:

Whitefly: small white flies producing plenty of white wax are the symptoms of whitefly, not galls.

Leaf miners: there is no leaf-mining group of insects as such; several different species of insect have adopted this habitat. Long, silvery, winding trails through the leaf are the symptoms and typically not galls.

Mealy bugs: “white slow-moving waxy insects hiding in crevices,” is how they are often described, and they are not associated with gall production.

The correct answer is:

Iron:

Interveinal chlorosis in the upper leaves is a common symptom of iron deficiency.

The incorrect answers were:

Nitrogen: a yellowing (including the leaf veins) of the lower leaves can be the result of nitrogen deficiency.

Phosphate: stunted plants, which are dark green with a slight red tinge, are symptoms associated with phosphate deficiency.

Boron: the tip of the plant suffers necrosis and tiny leaves are produced when there is a boron deficiency.

The correct answer is:

Floral parts:

As for Ergot (Question 5), smuts usually infect the flowers of a plant and grow down into the ovaries.

The incorrect answers were:

Roots: Stems: Leaves: Smuts often have complex lifecycles, and there are exceptions to the rule that all smuts infect the floral parts of the plant (see photo above), but there are not many.

The correct answer is:

An additional species of plant which is required for the pathogen to complete its full lifecycle:

Some rust fungi have complicated life cycles. Whilst farmers are generally familiar with the asexual spore production on crop plants, they are less familiar with the sexual phase of the same fungus which often takes place on an unrelated plant. It is this “other plant” that is termed the alternate host.

The incorrect answers were:

Any wild plant that is susceptible to a pathogen: this is not considered an alternate host; it is just part of the host range of the fungus.

A crop plant that is slightly susceptible to a pathogen, but not as badly as another crop: this is not considered an alternate host; it is just part of the host range of the fungus.

A plant that the pathogen will survive on when the crop is not in production: it is often the case that the alternate host allows the fungus to over-winter when the crop is not in production, but that is not what makes it an alternate host.

The correct answer is:

Genetic disturbance.

The yellow regions are distributed as zones. This strongly indicates a genetic issue within the leaf.

The incorrect answers were:

Lack of nutrient: symptoms of nutrient deficiency can affect regions of the leaf (veins, edges etc) but would not show up in zones of the leaf such as this.

Viral infection: the blocks of yellow are not typical of viral infection, it is often more of a pattern of yellow and green across the entire leaf; a mosaic.

Herbicide spray damage: yellowing is a common symptom of herbicide damage but the clear lines of yellow and green could not be due to the effects of herbicide.

The correct answer is:

Spraying around seedlings when applying herbicide:

The shroud prevents drift which theoretically makes it possible to spray round seedlings, however it is extremely easy to kill the seedlings by doing this (as I know to my cost).  As it takes several days for the seedlings and the weeds to die, you don’t know you have killed the crop until several days later.

The incorrect answers were:

Spraying insecticides: it may be appropriate to use a shroud but it would be unusual to do so.

Spraying especially toxic chemicals: the shroud is not considered part of personal protective equipment.  You should protect yourself from all agrochemicals when spraying using appropriate PPE.

Spraying fungicides: it may be appropriate to use a shroud but it would be unusual to do so.

The correct answer is:

Remove all weeds as they will impact on yield.

It is extremely unlikely (but not unknown) that a pathogen would reduce the impact of a weed population in an agricultural setting. You are better off to remove all weeds. NB. Pathogens are used very successfully in classical biological control.

The incorrect answers were:

Leave the infected weeds so that the disease could spread:  the disease may spread from the infected weeds to uninfected weeds but the competition from the original weeds would be too great to justify this plan.

The removal of the weed depends on the potential for the disease to spread to the crop: some diseases can spread from weeds to crops but see the answer above.

Leave all the weeds so that the disease can spread to healthy weeds: it would be foolhardy to leave all the weeds thinking that the pathogen would take care of them.

The correct answer is:

Mycotoxin production by the fungus causing the wheat to be unsafe to eat:

Infection of the ear will lead to mycotoxin production in the grain. These mycotoxins are usually present at extremely low levels but are extremely toxic.  They cause cancer at low concentrations and rapid death at higher concentrations. Wheat rarely has sufficient concentrations to cause immediate death but that is not the case with poorly stored maize.

The incorrect answers were:

Yield reduction due to the infection of the ear: this will have an effect but is relatively small compared to the effect of toxin accumulation.

Allows other more damaging pathogens to enter the ear: there is no evidence that this is the case.

Will return to the soil where it will cause root disease next year: Fusaria are fungi that survive well in the soil but that is not the primary problem with ear infection of wheat.

The correct answer is:

Viral infection:

The distorted growth is typical of a viral infection.

The incorrect answers were:

Bacterial infection: distorted growth is not associated with bacterial infection.

Downy mildew infection: systemic infection by a downy mildew is rare but when it occurs it can distort the leaves, this is not systemic infection by a downy mildew.

Nematode infection: wilt and yellowing are the symptoms commonly associated with nematode infection not leaf distortion.

The correct answer is:

All four:

The symptoms of sun scorch are seen in this photo.  There is always a severe transition between the healthy and dead tissue.  Other abiotic factors can do this such as fertilizer scorch and damage from some herbicides.

The correct answer is:

I don’t know and I don’t want to guess:

I appreciate that this is a bit of a cop out from me, but if anyone has any suggestion as to the cause of this extremely unusual symptom I would be glad to hear it.

The other answers were:

High temperature and dry weather: not symptoms associated with these conditions.

Mineral deficiency: there is no mineral deficiency that gives these symptoms.

Viral infection: possible as viruses can create very unusual symptoms but it is far from typical in every regard.

Herbicide damage: possible but the other leaves on the plant were unaffected.

The correct answer is:

Unrelated; two different rust fungi:

The photo on the left is brown rust and that on the right yellow rust.

The incorrect answers were:

They are the same fungus and the environment has caused the difference in colour: the environment does not cause a great deal of colour change in rust spores.

They are the same fungus but producing different spore types: rust fungi do produced different kind of spore within the same pustule but these are almost always black.

The same fungus but darker spores are older: rust spores don’t darken with age. 

The correct answer is:

Phosphate:

Stunted growth and reddening of the leaves (especially the lower leaves) is a classic sign of phosphate deficiency.

The incorrect answers were:

Nitrogen: yellowing of the lower leaves is typical of nitrogen deficiency.

Potassium: leaf edge scorch of the lower leaves is typical of potassium (K) deficiency.

Molybdenum: deficiency in molybdenum is not often seen except in brassicas. The symptoms are reduced laminar and cupping of the laminar in the upper leaves.

The correct answer is:

High humidity:

The conditions for the sporulation (production of downy growth) are high humidity and darkness. The darkness is pretty well guaranteed at night unless the leaves are in experimental conditions. High humidity often occurs when there has been rainfall and then the temperature drops. To test for a downy mildew, put a symptomatic leaf in a plastic bag overnight in the dark and look for sporulation in the morning.

The incorrect answers were:

High temperature, high light: the downy mildews do not thrive in such conditions and the infected areas often turn necrotic and die.

Low temperature; low light: under these conditions it is likely that sporulation will occur due to the concurrent increase in relative humidity but it is the humidity that is the important factor.

When free water has splashed up onto the underside of the leaf: the downy mildews will not sporulate into free water, the leaf must be dry.

The correct answer is:

Insecticides are generally more toxic than fungicides:

Fungicides are toxic but their toxicity is generally much less than that of the insecticides.

The incorrect answers were:

Fungicides are generally more toxic than insecticides: the reverse is true.

The toxicity of both fungicides and insecticides is about the same: this is not the case.

It is the formulation of the active ingredient that affects its toxicity and not the product itself: the formulation of product does not affect the toxicity of the active ingredient but it may affect your exposure to the chemical.

The correct answer is:

Bacterial leaf spotting:

What appear to be infection points spreading from the leaf edges could easily be mistaken for bacterial infection.

The incorrect answers were:

Viral infection; viruses rarely cause localised symptoms such as this and the symptoms are usually leaf distortion and mosaic.

Nematode infection; the most common symptom of nematode infection is usually a wilt, only later do the leaves begin to show other symptoms generally caused by the stress of the wilting.

Mite damage; the speckling that is usually the first sign of mite infestation may start in localised patches but will spread before those patches become very severe.

Photo courtesy of  Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

It means that the pathogen kills the host tissue before colonising it.

Necrotroph is the opposite of a biotroph; a necrotrophic pathogen will use an arsenal of toxins and enzymes to overpower the hosts defences and kill the tissue ahead of the advancing fungal mycelium.

The incorrect answers were:

It describes the fungal group that the pathogen is in; the fact that the fungus is a necrotroph has no bearing on the classification (for example genus/species) of the pathogen.

It means that the pathogen will only grow on material that has died previously; fungi that grow on dead material are generally described as saprobes or saprophytes.  They are generally not pathogens but are opportunistic / imposter pathogens.

It means that the pathogen will rot the host plant; almost all necrotrophs do rot the host tissue, as it is often killed with host cell wall degrading enzymes, but that is not what the term means.

Photo courtesy of : Howard F. Schwartz, Colorado State University, Bugwood.org

The correct answer is:

To prevent the sexual cycle of the pathogen proceeding:

This photo is of oospores; they are sexual spores of the oomycetes (water moulds). In some pathogens (such as P.infestans which causes late blight of potato or tomato) oospores will only form if there are and – strains present. Many countries only have one of the two mating types, thus preventing sexual spores being produced.

The incorrect answers were:

It is good to keep the amount of pathogen low in whatever way you can; whilst this is true, the amount of additional pathogen arriving on imports would be miniscule relative to the established population.

It will allow imports to be excluded so that local farmers can be supported; under WTO rules it is not easy to exclude produce except for phytosanitary reasons, but this is not the reason from a plant pathology viewpoint.

 The identification of the pathogen may not have been accurate and another pathogen may gain access to the country; this is unlikely as phytosanitary inspectors are well trained and are supported by expert scientists who test potential new pathogens found on imported material.

Photo courtesy of : Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

The correct answer is:

The means by which it is spread:

The means by which the pathogen moves from one plant to another can be determined (to some extent) by the development of the disease as it spreads through the crop (detailed diagnosis would still be required to determine what the pathogen is).

The incorrect answers were:

How long it can remain dormant in the field; it would be very difficult to determine this based on the pathogen spread as, once the infection is established in the field, the dormancy is no longer an issue.

Whether it is biotroph or a necrotroph; these are descriptions of how the pathogen interacts with the host and not the way in which it spreads.

Which type of pathogen group it is; there are many different pathogen groups and many of them spread in many different ways, it is not possible to infer the type of pathogen from its spread.

Photo courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

Using molecular experimental protocols, extract infected plant tissue and amplify the nucleic acids for analysis.

To amplify up the nucleic acid (usually RNA but may be DNA) and analyse this is the way in which most laboratories now identify plant viruses.

The incorrect answers were:

Squash a small amount of tissue, mount this with a cover slip and observe under a high-power microscope; this would be true for fungal pathogens, but viruses are too small to be seen with a light microscope.

Attempt to grow the virus on specialist agar from a small amount of leaf which has been briefly dipped in dilute bleach; this would be true for many fungal and bacterial pathogens, but viruses don’t grow on agar.

Isolate the virus particles from the vector; it may be possible to isolate the virus from the vector, but the virus particles will still be too small to be seen.

Photo courtesy of Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

The populations will decline simply because the food source will be reduced:

The BT chemical will kill the larvae which will reduce (or eliminate) their food source. The natural enemies will either leave or die.

The incorrect answers were:

It will have no effect as there are no natural enemies that are in the moth family; whilst there are no natural enemies in the moth family, the population will reduce as the food supply will vanish.

The spray will kill the predators as they may be eating the already poisoned larvae; the BT has a rapid knock-down action and the predators will not eat dead prey.

The predator population will increase because the toxic chemical will make the prey easier to catch; there is generally no problem with predators catching their prey and the BT will not slow the prey down (they are caterpillars after all). It will kill them rapidly and reduce the amount of food for natural enemies.

The correct answer is:

Smut fungus:

This is leaf infection of maize by Ustilago maydis (Mycosarcoma maydis). Infection causing the black sooty spores on the cobs are a much more familiar sight.

The incorrect answers were:

Crown gall caused by Agrobacterium tumefaciens; crown gall can produce galls of this size but the galls are never smooth they are usually brown and Agrobacterium does not infect grasses.

Gall-forming insect; this is reasonable suggestion but (difficult to tell from the photo) the leaf is distorted and there are no galls there.

Viral infection; the distortion seen here would not be so localised if it was caused by a virus.

Photo courtesy of Daren Mueller, Iowa State University, Bugwood.org

The correct answer is:

There are occasions when nets can be used to keep pathogens out of a crop:

This may be a bit of a trick question, but there are occasions when insect vectored viruses are kept out of a crop (such as seed potato production) with netting. It is the virus that is being excluded which happens to be inside an insect.

The incorrect answers were:

The netting has to be fine but it can be used to stop wind-blown fungal spores; no, the mesh would have to be so fine that light would not pass through.

Nets are never used to keep out pathogens; there are occasions, see above.

The netting has to be fine but it can be used to stop bacterial pathogens; no, the mesh would have to be so fine that light would not pass through.

Photo courtesy of Howard F. Schwartz, Colorado State University, Bugwood.org

The correct answer is:

Ultra Low Volume:

Application of less than 5L per Hectare (2 L per Acre) on field grown crops is generally considered to be ULV.  There are advantages and disadvantages to using this application technique; ULV sprayers are especially prone to wind drift due to extremely small droplet size.

The correct answer is:

The heat generated by the composting process will kill the pathogen spores:

If done correctly, the heat from the pile will kill the spores of pathogens. Larger piles are better as they have less surface to volume ratio.  Also, shallow compost areas are ineffective; compost should be kept heaped-up to better retain core heat.

The incorrect answers were:

Yes, the pathogens will survive and will infect any susceptible crop when compost is applied to the soil; this is not true if the composting process is done correctly.

Biocontrol will take place within the composting process and the fungal pathogens will rot along with the plant material; this is not true and the resting fungal spores are highly resistant to rotting.

Some pathogens will survive but most will not due to the length of time the composting process takes; composting should only take a few months and fungal resting spores will survive much longer than that.

The correct answer is:

Yellowing:

It is sometimes called “Chlorosis”, but yellowing is generally a good description of the symptom when the plant has no other symptoms than looking paler than a healthy plant.

The incorrect answers were:

Pale: the plant could be considered as pale, but this is not a word often used to describe plant symptoms.

Necrotic: this term is used to describe tissue that has turned brown and is dead.

 Nitrogen deficient: this may be the cause of the symptom, but it is not the name of the symptom.

The correct answer is:

Wilt:

This is a common symptom caused by abiotic factors, insects and pathogens.  It always refers to a drooping of the leaves due to a lack of water in the leaves. Woody tissue cannot wilt as the structure is not maintained through water pressure, however the leaves on a woody branch may wilt.

The incorrect answers were:

Flaccid: it is true that the leaves are flaccid, but that is not the usual word to describe a plant in this state.

Collapsed: wilt is a far better word to describe the symptom.

Drooped: yes, the plant is drooped, but wilt is a much better description because it indicates why the plant is drooped (lack of water in the leaves).

The correct answer is:

Enations:

A seldom seen symptom, but it is characteristic of some diseases.

The incorrect answers were:

Galls: this term usually applies to undifferentiated growth on the surface of a plant.

Phyllody: this term may be coming up later in the quiz.

Fasciation: this term may be coming up later in the quiz.

The correct answer is:

Canker:

Canker is almost always a symptom of woody tissue, the pathogen attempts to invade the host and the host lays down material to prevent it. This on-going battle leads to a wound with raised edges.

The incorrect answers were:

Gall: this is an undifferentiated growth of host tissue and does not have this wound aspect to it.

Tumour: not a word that is frequently used in plant pathology.

Lesion:  the symptom could be described as a lesion, but as it is woody with enlarged edges, canker is a better term.

The correct answer is:

Phyllody:

Not a common symptom (except in sesame): phyllody is the production of leaves in place of petals.  Phytoplasmas, and occasionally viruses, can induce this symptom.

The incorrect answers were:

Witches’ broom: it could be considered to be a type of witches’ broom, but, as the leaves are replacing the petals and other floral parts, it is phyllody.

Proliferation: there is considerable proliferation at the top of the plant, but that is a general term for excessive growth.

Fasciation: there is no evidence of fasciation here.

The correct answer is:

Halo:

This phrase is used to describe the distressed host tissue at the edge of a leaf spot.  It is almost always yellow as seen here.

The incorrect answers were:

Corona: this is not a term used in plant pathology.

Border: the yellow halo is generally considered to be outside the border of the leaf spot.

Toxin zone: there may be toxins leaching out into the host tissue creating the halo, but that is not what it is called.

The correct answer is:

Cysts:

These are the cysts of the potato cyst nematode.  They are attached to roots just like nematode galls, but, unlike a gall, they are not made up of the plant tissue; this is the swollen body of the female nematode, packed full of eggs.

The incorrect answers were:

Galls: these are commonly associated with nematode infection (but not all nematodes cause galls). Cysts are much smaller than galls and are always of extremely similar size and appear clean, despite being in the soil.

Eggs: difficult to imagine what animal would have laid eggs in this haphazard way, most insects / animals will lay eggs bunched tightly together.

Sclerotia: they are always black and usually formed within the tissue and (it is difficult to tell) are much larger than nematode cysts.

Photo courtesy of Wikipedia

The correct answer is:

Gummosis:

When damaged by a pest, pathogen or even mechanical damage, some trees will start to produce resinous sap which will dribble down the trunk.  The technical word for this is ‘gummosis’.

The incorrect answers were:

Ooze: often called ooze, but this is not the correct term.

Bleeding: often called bleeding, but this is not the correct term.

Resination: this is a word I have just made up, no-one calls it resination.

The correct answer is:

Rugose:

The leaves are clearly distorted but not thickened into true blisters and the leaf is its normal thickness.  It is a very common kind of leaf distortion often called blistering but this is actually incorrect.

The incorrect answers were:

Blistered: the leaves have not swollen into blisters.

Crinkled : not a word that is used in plant pathology.

Shrinkled: a word that is sometimes used in some countries but is not an official word.

The correct answer is:

Mosaic:

This term is often used to describe virally infected plants, but all viral infections do not lead to mosaic symptoms and not all mosaics are due to viral infection. Note that this leaf is also rugose.

The incorrect answers were:

Yellowed: some of the leaf is undoubtedly yellow, but it is the relationship between the yellow and green areas that are important.

Variegated: the leaf is variegated, and this is not wrong, but mosaic is the usual term used when describing viral symptoms.

Blotchy: it could be described as blotchy, but this term is not often used in plant pathology.

The correct answer is:

Septoria apicola:

You are probably not familiar with the spores of Septoria, but none of the other pathogens produces spores so it has to be the fungal pathogen.

The incorrect answers were:

Cucumber mosaic virus; viruses have no resting structures such as spores and almost all of them (TMV is a major exception) have no survival outside a host or vector.

Erwinia amylovora; a very common bacterial pathogen, bacterial plant pathogens do not produce spores.

Xanthomonas campestris; see above.

The correct answer is:

Provides the operator with a false sense of security:

Unless the PPE is actually fit for purpose, it may provide the illusion of protection to the operator so they will believe they are protected and change their behaviour and ultimately become more exposed.

The incorrect answers were:

Provides limited protection to the operator; there may be very limited protection offered but this is often tiny compared to the extent the operators believe they are protected. It is my personal opinion that these masks that offer almost no protection should be banned.

Reminds the operator that the chemical is potentially hazardous; this could be the case but it also reminds the operator that they think they are protected when they are not.

Has no effect on the behaviour of the operator or their exposure; providing a mask will alter the behaviour of the operator exposing themselves to greater hazard.

The correct answer is:

Use another pesticide container of a different type:

This is not good practice, but if a farmer is to split a pack, at least get them to use a pesticide container. That way, if there is a mix up, the worst that can happen is that the wrong chemical is sprayed onto a crop.

The incorrect answers were:

Use plastic fizzy drinks bottles as they are tough and will withstand pressure; this is very common behaviour in some regions but is the worst choice. It can easily lead to accidental death when the pesticide is drunk by a third party.

Use containers that cannot be sealed in-case pressure builds up inside the container; any container containing a pesticide should be sealed.

Use metal food containers as they are the strongest; the strength of the container can be an issue but, most importantly, it should NOT be an old food container.

The correct answer is:

All three can generally cross a graft:

The graft is generally not a barrier to pathogens.  A few viruses cannot cross the graft, but they are the exception and most can.

Photo by kind permission from the real English fruit company. https://realenglishfruit.co.uk/

The correct answer is:

Chemical.

This may seem a surprise, but an extract from a plant contains chemicals that will control the pests and diseases.  The origin of the chemical (in this case a plant) does not alter the fact that chemicals are being used to control the problem.

The incorrect answers were:

Biological; no, even though the chemical is from a biological source, it is not biological control.

Cultural; it may be part of the local culture, but this is not how the word “cultural control” is used.

Organic; there are very strict rules with regard to what can be sprayed onto crop plants when in an organic regime.   Just because the chemical is from a natural source does not make it approved for organic use.

The correct answer is:

The decision as to what to do will depend on the crop and pest:

Generally, it is appropriate to destroy the crop trash after harvest (ploughing or composting or burning), however there are times when it may be appropriate to remove it and store it.  If the pest is not mobile; the natural enemies are mobile and there is no threat from other pests or diseases, then this may be a sensible strategy.

The incorrect answers were:

Burn it or compost it as far as is practical; this is a safe option and is usually the correct one.

Store the debris nearby to allow the natural enemies to emerge; this can be a good practice but should not be universally adopted unless you are aware of all the pests and diseases on the trash.

Take it to a field where the same crop is being grown and spread it onto the soil; this is never appropriate and would completely undermine most of the benefits of crop rotation.

The correct answer is:

Powdery mildew:

They are very unusual pathogens and will prosper under dry conditions.  Free water will actually inhibit the germination of powdery mildew spores.

The incorrect answers were:

Downy mildew; all downy mildews require free water and high humidity to grow and spread.

Bacterial disease; bacterial diseases are seldom a problem in dry weather.

Rusts; once inside the leaf, rust pustules will develop within a dry leaf, but spore germination requires free water.

The correct answer is:

One sprayer is required:

Provided the sprayer is well washed out between use, there is no requirement to have more than one sprayer on the farm.

The incorrect answers were:

See above.

The correct answer is:

There is nothing that can be done to prevent the spread of a virus within a plant:

There are no chemicals that will prevent the spread within a plant and there are many areas that are symptomless but still carry the virus.

The incorrect answers were:

Antiviral sprays work well and should be applied as soon as symptoms are seen; they do not work.

Cut off the regions showing symptoms to prevent the spread to healthy areas; there will be areas of the plant infected but not showing symptoms.

Insecticide sprays will prevent insects from transferring the virus to new areas of the same plant; repeated infection from outside is not necessary for a virus to spread within a plant.

The correct answer is:

The walking speed of the operator:

The speed at which the operator walks through the crop will affect the volume of diluted spray applied to an area of land.

The incorrect answers were:

The concentration of the mix in the tank; this will not affect the amount of diluted liquid exiting the nozzle.

The size of the crop; the spray operator would have to slow down for a larger crop if it was to be fully coated, but the size of the crop will not affect the amount of spray required.

The correct answer is:

Squirrel damage. They strip the bark to feed on the sweet sap below. Once the stem is ringed the leaves above die quickly. Frequently seen in some years on isolated or small groups of trees.

Comments on the other options:

The large area with dying leaves suggests that a major branch has been damaged below. Wilt pathogens result in a more uneven pattern of leaf death; and I don’t know of any such pathogens affecting sycamore, or Acer species in the UK. The tree was too far from the sea to be affected by salt damage, and the pattern of damage would have been less severe and more diffuse.

The correct answer is:

Bacterial disease. Pseudomonas savastanoi pv. fraxini.

Comments on the other options:

‘Genetic abnormality’ might appear an attractive option yet these are distinctive symptoms with a scattered appearance, more indicative of a precise cause. Insects and mites are common gall-formers on leaves and fleshy tissue – not on woody stems. Yes, there is a fungal canker on ash (Neonectria ditissima), which kills tissue rather than stimulating new growth, as seen here. P. savastanoi is an unusual and relatively unknown pathogen, yet widespread and damaging on olive trees (related to ash).

The correct answer is:

Fire damage. This symptom really confused me when I carried out a survey of neem pests and diseases in Niger and Nigeria in the 1990s. Yet looking at afresh it is much clearer that someone has lit a fire below.

Comments on the other options:

Stem borers could cause such symptoms, but not here. The leaves have died suddenly and in a defined area – not an indication of a wilt pathogen. High winds can damage leaves, with the margins drying up and browning. Symptoms would be less severe and more diffuse in a larger area of the crown.

The correct answer is:

Physiological stress (I think). The staining around the exit of this spectacular oozing might suggest something pathological, yet on balance I think this is abiotic. The crowns were healthy and the trees productive, giving further weight to my rapid field diagnosis (I was on holiday).

Comments on the other options:

Fungal gummosis is common on citrus species and is associated with Phytophthora species (I know, it’s not a fungus – but no one calls this Oomycete or Fungus-like gummosis ). The gummosis is lower down on the trunk and accompanied by heavy staining below and dieback and decline in the crown. Gummosis is a common symptom in trees and though pathogens are involved it’s often a sign of physiological disturbance.

The correct answer is:

Leaf miner. The holm oaks around where in live in SW London are plastered with infestations. Doesn’t seem to affect the growth and health of the trees, There are two species found on holm oak. This appears to be Phyllonorycter messaniella, the leaf blotch leafminer.

Comments on the other options:

These are superficial symptoms caused by caterpillars feeding beneath the leaf epidermis. A fungal leaf spot would kill the part of the leaf affected and, typically, have a more defined shape. This evergreen oak has evolved to keep its leaves over winter and though low temperatures could kill off young growth in early spring, for example, it wouldn’t produce these blotches. Nor would a virus.

The correct answer is:

Phytoplasma – maybe. Similar symptoms have been associated with witches’ brooms on Salix spp. in Central Asia, Iran. I’ve seen this in Ukraine and Poland and I expect it to be more widespread yet under-recorded.

Comments on the other options:

There’s no diagnostic evidence to link this with phytoplasma in the UK, yet there’s a clear similarity to symptoms widespread on willows elsewhere. It’s too distinctive and extensive to be a ‘genetic abnormability, an otherwise attractive option for unusual growths. But also consider what insect feeding (not here, in my opinion) and bacterial infections can induce (see ash example earlier). Waterlogging? Nuh!

The correct answer is:

Salt damage. Winters are cold and icy, hence the liberal application of salt on the roads.

Comments on the other options:

Initially confused by the uniform development of symptoms, Dr Phil suggested that salt had been translocated to the leaves, where high concentrations had resulted in this browning. Pests and diseases would produce more varied and scattered symptoms, with evidence of feeding damage, necrosis and so on. High temperatures for prolonged periods are more likely to induce leaf curling prior to death.

The correct answer is:

Fungal (Hymenoscyphus fraxineus) and abiotic dieback. Let’s not forget that ash dieback of the abiotic variety has been around for decades in th UK. Separate the wispy dieback (fungal) around the crown periphery from the larger dead branches I associate with root disturbance and other abiotic factors. Described as stag-headed because they look like antlers.

Comments on the other options:

Air pollution is one of those fall-back options: when pest and disease options have been ostensibly eliminated look for something that could have a widespread impact on tree growth. Nutrient deficiencies is another fall-back option. I’ve not included lightning damage. I said diagnosing tree health problems wasn’t going to be easy …

The correct answer is:

Virus-like particles. Closely associated with this disease, known as Mundulla Yellows yellows. But there are other theories, including poor soils and herbicide damage.

Comments on the other options:

I’m going to stick with the virus-like particles option, partly because the symptoms have that sort of feel: yellowing, unusual growth patterns. Opinion is however divided in the absence of what some say is conclusive proof. Xylella fastidiosa has a huge host range and is a remote possibility – I took this photo from some distance away so didn’t have a chance to examine the leaves. The herbicide damage theory gains little support from this site.

The correct answer is:

Taphrina betulina. Yes, fungi can cause witches’ brooms and other strange growths, such as peach leaf curl (also a Taphrina sp.) and – my favourite – mango malformation (Fusarium!).

Comments on the other options:

Too many, wrong shape and too densely packed to be birds’ nests (says a self-confessed non-birder). Yes, phytoplasmas do frequently induce witches’ brooms, but not on birch. Mistletoes are green, are more regular in shape and less dense.

The correct answer is:

Symptoms are seen in the lower leaves first:

Nitrogen, Phosphate and Potassium deficiencies have different symptoms, but share the common factor that it is the lower leaves where symptoms are first seen.

The incorrect answers were:

The leaves turn yellow; the lower leaves turning yellow is the major indication that the plant is short of nitrogen, but that is not true of the other phosphate or potassium shortage.

The very tip of the plant is the first to show symptoms; few nutrition deficiencies are initially seen in the tip of the plant. Symptoms due to calcium and boron are seen here.

Symptoms are seen in the young leaves first; a deficiency of almost all of the minor nutrient’s symptoms are seen in the young leaves first.

The correct answer is:

Moth

Many caterpillars have four pairs, but some have fewer, as seen here. It is not difficult to identify these caterpillars as “loopers”, based on the way they arch their body as they move.

The incorrect answers were:

Sawfly; sawfly larvae are often mistaken for caterpillars but, rather than having the typical 4 pairs of false legs or fewer, they have more than four pairs.

Beetle; beetle larvae (grubs) don’t have false legs like this, but they do have thoracic legs.

Fly; fly larvae (maggots) have no false legs, and no thoracic legs either.

The correct answer is:

Insects:

These symptoms are not proof that insects are responsible, but strongly suggest they are the cause. Many sucking pests leave no marks at all, but others such as leafhoppers and planthoppers cause this symptom.

The incorrect answers were:

Bacteria; bacterial leaf spots are often associated with the leaf edge or damage and are water-soaked and of variable sizes.

Fungi; fungal leaf spots are usually larger than this and are not quite so uniform in their appearance.

Viruses; this would be a very unusual symptom for a virus.

The correct answer is:

Recently infected material, where the symptoms are still developing:

This is where the pathogen will be most active and the symptoms not modified by other microbes moving in on the dead tissue.

The incorrect answers were:

Dead material, where the symptoms have been fully expressed; it is difficult to work out why a leaf or plant has died as they often look the same (black and shrivelled).

Asymptomatic tissue: after infection but before symptoms appear; if there are no symptoms (even if there is infection) it is not possible to do a field diagnosis.

Material that is almost dead but not quite; the amount of saprobes that will be colonising the tissue will make for confusing symptoms.

The correct answer is:

Micro-browsing insects.

The contents of the surface cells of the leaf have been sucked out.  Pests that commonly do this are thrips and mites, but, in this case, it was leaf hoppers that did the damage.

The incorrect answers were:

Powdery mildew; this common fungal pathogen rarely attacks potatoes. It is not the cause here.

Phytophthora; late blight is very common on potato crops, but the symptoms are not like this.  Rapidly expanding black zones, with a pale perimeter are the typical symptoms.

Virus; speckle such as this can be mistaken for a viral mosaic but, look carefully at the photo, this is not a mosaic, there are no panels of yellow and green.