All ‘Plant Doctor’ Quiz Questions

The correct answer is:

Does not grow in culture:

Although TMV is able to survive outside a host plant cell the replication of the virus is intimately associated with the metabolism of the living cell, and it will never replicate in culture.

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:

Botrytis does not sporulate in that way:

Whilst the observation of small black fruiting bodies within a lesion can confirm the identity of the pathogen as a fungus there are also many fungi do not produce them. Botrytis can produce sclerotia (see question 2) from which the small fruiting bodies subsequently grow but they are generally not seen embedded in the host tissue.

The correct answer is:

Manganese:

It isn’t really possible to tell which nutrient is lacking here (could have been several), but hopefully you were able to dismiss all the other options.

The correct answer is:

Healthy

Comments:

A quote from the Superman films seems apt, adapted for plants: Is it a Virus? Is it a phytoplasma? Or is it simply another varietal characteristic? The label at Wisley said ‘Lettuce leaved Basil’, indicating that this is a named variety. It could still, of course, be related to a virus ‘infection’, albeit of the harmless and valued sort.

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:

Fungus:

This is Cercospora leaf spot of fig caused by Cercospora fici. 

Perhaps the most classic of leaf spotting fungi, these are the symptoms of Cercospora.

Note the strong border and the contained aspect of the spots, it does not look like they are going to engulf the whole leaf.

Photo credit: Jonas Janner Hamann, Universidade Federal de Santa Maria (UFSM), Bugwood.org

Correct answer:

Around £2 million.

Asian longhorn beetle is a highly destructive insect pest which poses a serious threat to a wide range of broadleaf trees, including species grown commercially for timber and fruit production.

For more information on ALB:

https://www.forestresearch.gov.uk/tools-and-resources/fthr/pest-and-disease-resources/asian-longhorn-beetle-anoplophora-glabripennis/

Correct answer:

Phytophthora ramorum

Samples are taken whenever the presence of a quarantine pest or disease is suspected, and sent to a laboratory for an official diagnosis, as plant health inspectors are not trained diagnosticians. Even in cases where the inspector is confident of the diagnosis samples are still sent to a laboratory for confirmation.

For further information on FERA’s activities:

https://www.fera.co.uk/

Sample packaging:

https://www.fera.co.uk/crop-health/sample-shipping-information

The correct answer is:

Armillaria root rot.

Comments on the other options:

It looks like a boring insect, yet the extensive white mycelium gives the game away. Armillaria is truly impressive in the number of species it can affect. It’s not a serious disease in eastern DRC, but one to look out for when diagnosing Verticillium wilt, a more common and serious problem.

The correct answer is:

Fungi: Although the symptoms look like those of a downy mildew this is a fungal pathogen (Passalora fulva).  None of the other alternatives ever produce additional growth on a plant.

The correct answer is:

Downy mildew.

The soft velvety growth on the underside of the leaves could easily be mistaken for a downy mildew (although not usually as dark as this).  The mite (which is too small to be seen) has induced the soft growth on the underside of the leaf; the brown hairs are actually leaf material.  In the field, it is easy to tell the difference as the downy mildew sporulation is easy to wipe off and this is not the case with the downy growth seen here.

The incorrect answers were:

Powdery mildew: powdery mildews are always white and almost always on the upper leaf surface.

Nematode attack; is never associated with growth on leaves.

Glasshouse red spider mites: produce a lot of webbing but it never coats the leaves like this.

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:

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:

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:

The vegetative propagation of potatoes allows them to build up:.

Plant viruses are seldom “seed borne”. As potatoes are propagated vegetatively, in the form of tubers, they will accumulate viruses year on year.

Answer: Small black dots within the infected area. Rings within the spot; the small black dots within the spot are fungal structures and the rings are common in fungal infections. Spreading from the edge of the leaf and limited by leaf veins are more generally associated with bacterial leafspots.

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:

Nematode attack

The galls seen on these roots are very typical of those produced by galling nematodes, Meloidogyne species.

The correct answer is:

Phytoplasma infection: Tiny leaves “little leaves” are often a symptom of phytoplasma infection. Aphids can distort leaves and it is also a common symptom of viral infection but generally neither induce the extreme reduction in size seen here.

The correct answer is:

Powdery mildew:

It is unusual to see powdery mildew sporulating on the underside of a leaf, but it does happen as can be seen here.

The correct answer is:

Potassium: Wood ash is a good source of potassium salts.

The incorrect answers were:

There may be trace amounts of the other nutrients but potassium is by far the most abundant.

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:

Biotic

Comments:

These are the pustules of a rust (Puccinia porrii). Look out for it on wild garlic, now flowering profusely

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:

Older leaves

In all crops it is the older leaves that show symptoms of ozone toxicity.  The most susceptible crops are beans although potatoes are mildly susceptible. Note how clearly defined and lifeless the spots appear indicating an abiotic cause.

Photo credit: Gerald Holmes, Strawberry Centre, Cal Poly San Luis Obispo, Bugwood.org

The correct answer is:

Virus:

This is a disease called lettuce big vein. The virus is spread by zoospores of a fungus (Olpidium virulentus) released from sporangia that live in the soil and infect lettuce roots. The virus also enters resting spores, allowing it to survive for years in soil, in the absence of a growing host plant.

The correct answer is:

Powdery mildew:

It is unusual for powdery mildew to cause anything other than a white dusty growth over the upper surface of a leaf, but here there is considerable host reaction and the sporulation is on the lower surface.

The incorrect answers were:

Viral infection; viral ringspots are unusual but can look like this; if you missed the white powdery growth on the leaf at the top of the photo viral infection was a good answer.

Unidentified fungal infection; There are fungi that induce considerable reddening of the host (blackberry rust is a good example), but it is unusual AND you could have spotted the sporulation on the underside of the leaf.

Bacterial infection; infection by a bacterial pathogen does not generally cause anything other than tissue death.

The correct answer is:

The concentric rings seen on this onion leaf are typical of a viral ringspot.

The fluffy purple growth in the centre of the spot leaves no doubt as to the cause of the problem.

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:

Ignore them but they are reducing potato fruit production

They do a small amount of damage to the potato fruit which is beneficial to the grower as the fruits are not edible and are not harvested.

Answer: The virus requires a helper virus to be transmitted and this does not survive in parsnips. The aphids feed on parsnips and the virus is present but it is never spread from one parsnip plant to another.

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:

Roadside dust. These beans were planted close to a very dusty road and became coated in dust. Note that the colour of the leaves is the same as the soil.

The correct answer is:

Fungal infection

This is shot hole disease commonly seen on Prunus species. It  is caused by Stigmina fungus (Wilsonomyces carpophilus).  The infection elicits such an extreme reaction by the host that the infected area drops from the leaf leaving the characteristic (if you know) hole.

Photo credit: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

Answer: Phytophthora ilicis

Phytophthora ilicis causes holly leaf blight and is found only affects Ilex species. This disease can cause defoliation; leaf blotches and kill bark. Where holly has been planted in hedges, defoliation can appear as arches where the defoliation is at its widest nearer the ground.

© Crown copyright. Forest Research

The correct answer is:

Verticillium

The early symptoms of Verticillium dahliae infection are characterized by unilateral leaf yellowing. A wilt may develop but the leaves become necrotic as the disease progresses.

The correct answer is:

Abiotic

Comments:

An older leaf loses its vigour. Note the ‘classic’ indicator of lack of nutrients with the veins remaining green and the outer regions yellowing first.

The correct answer is:

Nectar pollen and honeydew:

The adults are not the natural enemies; it is the larvae that are voracious predators.

The incorrect answers were:

Aphids; are the main food of hoverfly larvae

Mealybugs; not eaten by hoverfly larvae

Caterpillars; not eaten by hoverfly larvae

The correct answer is:

Fungus

The spot has a strong border which indicates a fungal pathogen is responsible but, most significantly, there are small black dots littering the spot, these are the fungal fruiting bodies leaving no doubt that the cause is a fungal pathogen.

The correct answer is:

Bacterial infection:

Ralstonia solanacearum is a bacterial pathogen that frequently causes a rapid wilt.  The wilt is generally the first sign of disease, the plants will survive for several days (recovering from the wilt at night) before finally turning yellow and necrotic.

The correct answer is:

All of the above

Conspicuous and often numerous sunken, bright orange-brown cankers can occur on the bark of young branches of infected trees. The cankers caused by chestnut blight can eventually girdle branches and stems, with all live tissue above the canker dying. On grafted trees, infections are most frequently found in areas close to the graft where callusing occurs, whereas in orchard and coppiced trees infections usually occur at the base of the stem (collars or insertion points). Masses of pin-head sized, yellow-orange to reddish-brown pustules may also be evident on infected bark. These fruiting bodies erupt through lenticels (gas exchange structures) in the bark and exude long, yellow-orange tendrils/threads of spores in humid and wet weather. Another obvious symptom of this disease is wilted and brown dying and dead leaves which are retained by the tree above a girdling canker. Below the canker, epicormic growth (prolific sprouting) may be visible.

IMAGE: Crown Copyright. Forest Research

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:
Moves into the plant will relocate within the plant.
Systemic chemicals enter the plant and will be moved around to new areas.  They generally move up and seldom get into the roots.

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:

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:

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:

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:

In the host plant and the insect vector:

Phytoplasmas are vectored in a similar way to plant viruses but generally have a much more intimate relationship with the vector and usually replicate in both hosts.  Unlike mycoplasmas phytoplasmas cannot be cultured.

The correct answer is:

Orobanche, parasitic plant. Note the distinct purple shoots coming from the potato tuber.

Comments:

Orobanche, or broomrape, is a genus with many species infecting a wide range of plants. It’s a widespread cause of significant losses in potatoes.

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:

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:
If the product being used contains only one active ingredient.
If the commercial product purchased from the ag chem store contains only one active ingredient it is important to mix the fungicide with another product.  Repeated use of a single active ingredient will lead to resistance.

The correct answer is:

There is no pathogen involved

There is no pathogen involvement in these symptoms.  This is “bitter pit” symptom in apple caused by a calcium shortage.  Rather like the more familiar blossom end rot in tomato.

The correct answer is Confused.

No idea, but unlikely to be biotic given the uniform curling in all the leaves.

Diseases (and pests) are progressive, so you see different stages of symptom development and in different parts of the plant.

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:

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:

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:

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:

Aphids don’t lay eggs:

Aphids give birth to live young, they do not lay eggs. That is one reason why the populations can rise so rapidly.

The incorrect answers were:

See above.

The correct answer is:

Three pathogens.

Rust, powdery mildew and Septoria are all infecting this leaf.

The correct answer is:

Phytophthoras:

Late blight on potato and tomato is an extremely common disease, but there are relatively few other common examples of Phytophthora attacking above ground, they are mostly pathogens of roots and lower stems.  Here we see Phytophthora palmivora attacking the cocoa pods.

The correct answer is:

Ice crystals:

Frost has come down and coated these leaves with a wonderfully delicate array of crystals.

The correct answer is:

Viral infection:

The patchy green and pale green pattern is typical of a viral infection. Often described as a mosaic.

The incorrect answers were:

Nutrient deficiency: yellow and green regions on the same leaf are typical of some kinds of nutrient deficiency, but the sharp lines between the zones indicate that is not the problem here.

Nematode attack: the symptoms of nematode attack are a general yellowing of the plant; some portions will remain green and slowly fade to yellow.  There will never be the sharp lines dividing the colours as seen here.

Water mould (downy mildew); it is common for pale patches to appear on leaves due to downy mildew infection, but here, the whole leaf has become pale with darker green regions.  The sharp lines between the colours indicate this is not due to downy mildew BUT there is an important and common exception to this rule; cucumber downy mildew.

The correct answer is:

Insect feeding damage; this is speckle due to micro-browser feeding, in this case thrips.

The correct answer is:

Biotic

Comments:

I’m not sure if this is late blight or not. But it’s definitely biotic. The necrotic lesions have a defined border, unlike the more diffuse and spreading areas I associate with Phytophthora infestans. A colleague who worked with potatoes and helped to run a diagnostic lab in Scotland said that they often put leaves into a sealed bag overnight to see if the suspect late blight symptoms developed a furry surface.

The correct answer is:

Pot bound:

This plant had outgrown its pot, the lateral roots appear healthy, but the main root growing down doubled back when it could not penetrate the base of the pot.

The correct answer is:

Pileus:

There are lots of mycological terms which often do not help in a field diagnosis, but if you want to impress your friends you could refer to the cap of the toadstool as the pileus.

The correct answer is:

Small discrete spots with no yellowing around the necrosis:

In this case there is a clear line between the healthy and the diseased tissue, the spots do not appear to be spreading rapidly and there is almost no yellowing. All these symptoms (along with the concentric circles within the spots which cannot be seen here) are indicative of early blight.

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

The correct answer is:

Nitrogen fixing nodules

These growths are pink and displaced to the side of the root, these two characteristics make a diagnosis simple.

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:

Remove viruses from the planting material

The tiny bundle of cells at the very tip of the plant is the only area that remains free of virus. The removal, sterilisation and then growth (on nutrient media) of the tips is the only way to eradicate viruses from potatoes.  The small plants are multiplied in the field many times before being sold as ware potatoes.

The correct answer is:

Aphids

It is unusual for aphids to attack underground but, as you can see, it does happen.

The correct answer is:

Ophiostoma novo-ulmi

Dutch elm disease is a highly destructive disease of several species of elm (trees in the Ulmus genus). It is caused by two related fungi, Ophiostoma novo-ulmi and Ophiostoma ulmi, although almost all cases are now caused by O. novo-ulmi. The fungus is spread from tree to tree by elm bark beetles.

It has killed tens of millions of elm trees in the United Kingdom since the second wave caused by O. novo-ulmi was accidentally introduced, probably during the 1960s.

IMAGE: Crown Copyright. Forestry Commission

The correct answer is:

Basidiomycete pathogen

The only fungi that produce large fruiting structures such as toadstools, mushrooms, or (as in this case) brackets, are the Basidiomycetes.

Photo credit : Andrej Kunca, National Forest Centre – Slovakia, Bugwood.org

The correct answer is:

Downy mildew infection.

This is the only case I know of where a downy mildew attack below ground.  The dark staining (it is not a rot) is extremely characteristic; once you know.

Photo credit Eric Boa

Answer: Neonectria neomacrospora. A fungal disease which can cause severe dieback and eventual death of true firs. Originally recorded in North America, there were old records of its presence in Europe (outside Denmark and Norway), although not causing severe symptoms. Although known to be present in the UK, reports have increased since 2015. Tree deaths have often occurred on fir species in landscape plantings, Christmas tree plantations, and forest stands.

© Crown copyright. Forest Research

Correct answer:

A nursery that imports cuttings from non-EU countries for growing on and sells these plants to garden centres.

Low risks sites trade in UK origin plants and sell to the end user (e.g. garden centre customers). High risk sites buy in from third country outside of the EU, propagate plants (as opposed to growing them on), and sell plants on to other distributers.

Surveys are not undertaken randomly, they are done on a risk-related basis taking account of business activity, hosts traded, origin of plants, volume and trade and level of distribution.

The correct answer is:

Virus: The symptoms could undoubtedly be described as a mosaic, a symptom often synonymous with viral infection.  Phytoplasmas do not generally create a mosaic, they turn leaves yellow. Nematodes cause a general yellowing of the leaves and not a mosaic. Nutrient deficiencies will often cause a mottle in which the yellow and green areas are not so distinct from each other as seen here.

The correct answer is:

Physiological condition; no pathogen involved

This is “black heart” or “hollow heart” of potato. There are no external symptoms. It can develop in the field or in storage. This physiological condition is associated with low oxygen levels.

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:

Fungal disease: Without prior knowledge and based on the photo alone it would be reasonable to suggest that the problem was a fungal (or bacterial) pathogen. Nematodes could cause the general dieback but look at the healthy regions on the lower leaves, this does not look like a general decline.  Herbicides can cause similar symptoms but nearby plants are unaffected. A nutrient deficiency could not have caused this as the plant is too large to suddenly suffer from such severe symptoms.

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:

Aphids:

A very common pest characterised by: the pear-shaped soft body, obvious legs and, normally, a lack of wings on most of the population.

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:

Thrips:

Thrips can get into the apex of a plant and the feeding may cause the developing leaves to appear distorted once they have expanded.

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:

It is not possible to grow any downy mildews in culture.

They have to be maintained as resting spores or actively growing on a susceptible host.

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:

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:

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:

An aphid type insect called Phylloxera:

Very unusual symptom this pest has devastated the grape industry in the late 19^th century.

Nematodes very rarely attack leaves and when they do the do not form galls. Bacterial galls are rarely found on leaves and they turn brown as they develop. Downy mildews do not form galls.

The correct answer is:

Marginal necrosis (edges of the leaves going brown) in the lower leaves:

Can be confused with wind damage or bacterial infection but the symptoms are always seen in the lower leaves first.

The incorrect answers were:

Reddening and stunting: you were probably getting confused with phosphate deficiency which has these symptoms.

Yellowing: you were probably getting confused with nitrogen deficiency which has these symptoms.

Marginal necrosis (edges of the leaves going brown) in the upper leaves; a deficiency of all the major nutrients (NPK) are seen in the lower leaves first.

The correct answer is:

Fungal rot (Athelia rolfsii)

Comments:

Previously known as Sclerotium rolfsii, this is a widespread pathogen affecting many hosts. Note the distinctive sclerotia – the small white spheres – at the bottom of the photo. The abundant mycelium is also distinctive for this pathogen.

The correct answer is:

Herbicide damage:

Potatoes are difficult to kill with herbicides but these are the symptoms of low levels of Glyphosate on a potato volunteer.

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:

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:

Adventitious roots

The roots seen here were found growing inside the slightly swollen stem of a tomato plant.  The swelling appeared to be associated with Fusarium infection (comments welcome).

The correct answer is:

Xylella fastidiosa

Xylella is a plant disease which can affect several species of broadleaved trees widely grown in the UK, such as oak, elm and plane, as well as a wide range of other commercially grown plants. Among them are grape, citrus, coffee, olive, almond and blueberry species, and many herbaceous plant species.

In nature, the bacteria are exclusively transmitted by insects from the Cicadellidae and Ceropidae families, which feed on plants’ xylem fluid. There are several species of insects in the UK which could act as vectors (agents of spread) of X. fastidiosa bacteria, including the common froghopper (Philaenus spumarius). Although such insects usually only fly short distances of up to 100 metres, they can be carried much longer distances by the wind.

IMAGE: David Cappaert, Bugwood.org

Correct answer:

All of the above.

We use a wide range of scientifically relevant sources, trade intelligence, feedback from our clients. Early identification is important to help combat quarantine pests and diseases. If you find a quarantine organism or suspect one may be present, you should contact your local PHSI immediately to alert them. In England and Wales: To report suspicion of a quarantine plant pest or disease, or for other plant health queries not specified above, telephone 0300 1000 313 or email planthealth.info@apha.gov.uk

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:

Bacteria: Without prior knowledge of this complaint it would be a reasonable to suggest that the cause was a bacterial (or fungal) pathogen. Viruses and insects do not generally cause tissue death as seen here and nematodes will only rarely attack above ground.

The correct answer is:

Aphid feeding

Feeding by the asparagus aphid Brachycorynella asparagi induces this proliferation of leaves

Photo credit :  William M. Brown Jr., Bugwood.org

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:

Late blight (Phytophthora infestans)

The classic necrosis surrounded by the white bloom of sporulation is clear in this photo. Powdery mildew is extremely rare on potatoes and is not typical of powdery mildews when it does occur and there is no downy mildew of potato (although Phytophthora is a close relative).

The correct answer is:

The skin of the potato “sets” i.e. matures.

All the time the tops are green, the tubers will be expanding, and the skin of the potato will be stretching. Once the tops are dead the potato skin will mature and seal the potato tuber.

Photo credit: Farmers Guardian

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:

Biotic

Comments:

This is a rust, common on several onion relatives – the chives in my garden get it. Look out for rusty leeks bought from shops (though less common than it used to be).

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:

Mites:

As stated in the question, this leaf distortion is due to a fungal pathogen (Taphrina deformans). These symptoms are more typical of the distortion sometimes caused by mites.

The incorrect answers were:

Scale insect; never induces leaf distortion.

Downy mildew; can occasionally produce very limited leaf distortion

Viral infection; can distort leaves but the whole leaf would generally be affected

The correct answer is:

Botrytis cinerea:

This opportunist pathogen is extremely common and attacks weakened tissue flowers and fruits.  It causes Grey mould and the grey sporulation can be seen on top of the rotting bud.

The correct answer is Biotic.

This is Bean mosaic virus.

Note the distinctive markings on the leaf and loss of colour

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:

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:

Honey dew. There is no sooty mould growing on the honey dew which is why it remains clear (and sticky). Insects in the upper branches have deposited it on the leaves, where it will remain, until rain washes it off.

The correct answer is:

Zinc.
It is very unusual to see a deficiency in a nutrient causing in leaf distortion, but it does happen occasionally.  Zinc deficiency can be manifest in many different ways depending on the crop.  In this case the leaves of the potato are folded and narrow due to a shortage of zinc however in other crops the symptoms of zinc deficiency are very different

The correct answer is:

Brown are parasitized; green are healthy:

The brown aphids are empty shells, the parasitoid has eaten the interior and has emerged. The green aphids are currently healthy.

The incorrect answers were:

Brown are males; green are females; brown male aphids do exist, but almost all aphids are female.

Two different species of aphid; no, they are all the same species.

The brown insects are not aphids and are predators of the aphids; no, they are all (or were all) aphids.

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:

Biotic

Comments:

Looks like another virus to me. Not sure which one. Causing little or no obvious damage. By the way, I suspect I’m in a minority in looking for plant health problems as I walk round Wisley (which is close to where I live).

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:

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:

The mites are so tiny they cannot be seen.

Most pests can be seen with the naked eye or at least with a hand lens.  Inspection of plants infested with rust mites will reveal nothing, often leading to a misdiagnosis.

The correct answer is:

Mineral deficiency:

This is “tip burn” and is the lettuce equivalent of blossom end rot. In the same way as the tissue at the end of a tomato will break down, because the plant cannot mobilise calcium to the area fast enough, something similar is happening here. The edge of the leaf is growing so quickly it becomes short of calcium and the tissue dies.

The correct answer is:

The one with black coloured antennae.

This is the famous Colorado beetle Leptinotarsa decemlineata an innocuous insect which only became a serious pest of potato when the potato was introduced to its native range.  The beetle with the tan antennae is the sunflower beetle Zygogramma exclamationis

Photo credit:  Marco Verch Black antennae beetle; Keith Roragen, Tan antennae beetle both Flickr

The correct answer is:

Herbicide.

Potatoes are difficult to kill with herbicide as they often have considerable underground reserves. Glyphosate will kill them, but it requires a considerable dose.  This yellowing at the base of the leaves is the symptom of a glyphosate treated potato plant.

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:

Phytoplasma. I was first alerted to this ‘infection’ by my colleague Yaima Arocha. She spotted the strappy, thin leaves and contrasted them with the broader leaves (look bottom left) in Uganda. The plants tested positive for phytoplasma. Such symptoms are widespread in the tropics, though whether you consider this plant diseased is another matter.

Comments on the other options:

Yes, it could be a virus, but I’d expect to see less vigorous growth and perhaps death of leaves. You could argue that this is a varietal characteristic: this is what people are used to seeing and the plant is growing vigorously. Poinsettias are ‘infected’ by a phytoplasma and sold on their compact shape. Definitely not insect feeding. Ari was smiling because I saw an infected plant and he saw normal growth.

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:

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:

Fungal infection:

These lesions are due to Cercospora leaf spot caused by Cercospora asparagi. Symptoms are small, oval, grey to tan lesions with reddish brown borders on the needles and small branches. Spores of the fungus are produced on the lesions and are dispersed by wind and rain.

Photo credit : Brian Olson, Oklahoma State University, Bugwood.org

The correct answer is:

European mountain ash ringspot associated virus

The European mountain ash ringspot associated virus (EMARaV) is a pathogen that infects mountain ash trees (Sorbus aucuparia). The pathogen attacks the leaves of its hosts where it causes mottling and discoloration. In extreme cases the productivity of the tree can be affected, which can result in its gradual decline.

IMAGE: Adrian Fox, Fera Science Ltd.

The correct answer is:

Abiotic

Comments:

Fire damage, probably because the farmer has burnt the understorey crop in preparation for a new sowing.

The correct answer is:

All of the answers are correct.

It prevents spores of late blight dripping onto the tubers and prevents light from turning the potatoes green, it extends the stems from which the potatoes develop, and it also protects from frost and allows the soil to warm up quickly.

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:

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:

Phytoplasma. Lethal yellowing of coconut palms, known locally as St Paul’s wilt, is a devastating disease, with variants found around the world. The sudden wilting of the canopy is linked to the ultimate collapse and failure of conducting vessels (phloem).

Comments on the other options:

Rhinoceros beetle is also damaging but crowns tend to become ragged and lack this sudden collapse of fronds. Waterlogging would affect more palm trees at the same time. Fungal root rot is a ‘good’ guess, and Phytophthora is widespread. Difficult to distinguish without testing of samples.

The correct answer is:

These downy mildew symptoms could be mistaken for viral infection:

The yellowing of the leaves with the green streaks do look very much like viral symptoms.  The yellowing is also typical of nutrient deficiency but note none of the nearby plants have the same symptom.

The correct answer is:

There is no pathogen involved:

The symptoms on this butternut squash are due to accidental glyphosate drift.

The correct answer is:

Verticillium wilt

Verticillium albo-atrum causes a disease often referred to as “Verticillium wilt” however a wilt is often not the first symptom and may be absent all together following infection.

Photo credit; Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, Bugwood.org

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:

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:

It has a very damaging impact on the health of the plant but localised to the place of infection.

Despite the ability of mistletoe to photosynthesise, it does draw a lot of photosynthate and minerals from the host tree. Epiphytes are plants that use larger plants for support, but this is a parasitic plant, draining resources from the tree.

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:

Banana weevil (Cosmopolites sordidus)

Comments:

There are many reasons why bananas decline, and the only way to identify the most likely cause is by looking inside the pseudostem and low down for tell-tale signs.- in this case the black pockets associated with weevil damage. Look closely and you can see the larva top left. Also known as the banana borer.

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 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:

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:
Some fungicides will control oomycetes:
Oomycides or watermouldicides don’t often enter into our language. People talk of fungicides controlling the oomycetes, however although many fungicides have activity against oomycetes many do not (Metalaxyl as a good example).

The correct answer is:
The active ingredient of the product:
The mode of action (how the chemical kills the fungus) is due to the active ingredient(s).  The remainder of the canister is important but are unrelated to mode of action.

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:

Phytoplasma. Look at the clumped growth, reduced internode length and reduced leaflet size. The difference between unhealthy and normal growth is stark and dramatic – a good starting point for thinking ‘phytoplasma’

Comments on the other options:

Yes, it’s similar to herbicide damage – but drift is unlikely to reach the top parts of the crown. Virus infection is also a possibility; distinguishing little leaves from reduced growth is not easy and requires close examination (even then I’m easily confused).

The correct answer is Healthy.

It’s not a rust, simply pollen scattered over an adjacent leaf.

Sometimes the most obvious answer is the correct one.

Don’t over-complicate diagnoses. Flower production is big, big business in Colombia and growers are always on the look out for imperfections. This isn’t one.

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:

It is possible to culture Xylella on special media.

There is special medium that will support the growth of Xylella, it  grows slowly and produces small colonies..

Photo credit:   John Hartman,  University of Kentucky Bugwood.org

The correct answer is:

They cannot be cultured.

Nematodes are found in a great number of environments, but the plant parasitic nematodes do not grow without the presence of a living plant

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:

There is no pathogen involved

Incredible as it may seem, there is no pathogen infection on these leaves.  The necrotic areas are generally described as “stress symptoms” but they can develop even in the absence of any apparent stress on the plant.

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:

Crystals of the virus:

Although it is often said (said by me) that viruses are far too small to be seen, even with a high power light microscope, this is a crystal of TMV. Tobacco mosaic virus is the only virus that reaches such high levels in the host that it actually crystalizes and, with careful microscopy, these crystals can sometimes be seen in the epidermal cells of infected plants.

Photo credit  Entsar A. Nassar, Kh. A. El-Dougdoug, M.E. Osman, Rehab A. Dawoud and Aliaa H. Kinawy, 2012. Characterization and Elimination of a TMV Isolate Infecting Chrysanthemum Plants in Egypt. International Journal of Virology, 8: 14-26.

The correct answer is:

Viral infection

These are the symptoms of Potato spindle tuber viroid (PSTVd).  A viroid is effectively a virus that does not have a protein coat and survives as naked nucleic acid. It is spread via mechanical transmission on tractor wheels and trousers. It can be spread by aphids but only when plants are also infected with Potato leaf roll virus (PLRV).

Photo credit: European and Mediterranean Plant Protection Organization , Bugwood.org

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.