BSPP ‘Plant Doctor’ Quiz #1 – April 2021

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.