BSPP ‘Plant Doctor’ Quiz #14 – May 2022

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:

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:

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:

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:

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:

Spread pattern on the leaf/plant:

Paraquat is a chemical that induces sun-scorch, even at low light intensity. The damage appears the same; the only way of telling them apart is the pattern of damage on the plant.

The incorrect answers were:

Sun-scorch will develop as yellow regions before the tissue dries: this is not true; the tissue does not yellow before turning brown.

Sun-scorch will develop slowly over time: this is not true; the tissue will die immediately.

Fruiting bodies may be seen in the sun-scorch lesion; both sun-scorch and paraquat are abiotic causes and would never have fruiting bodies within the lesion.

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:

Water mould (downy mildew):

In cool damp weather, downy mildews will sporulate profusely on the underside of the leaf and may cause some necrosis.  In hot sunny weather they will induce much greater necrosis and sporulation is much reduced.

The incorrect answers were:

Virus; never produce spreading lesions such as these.

Fungus; the symptoms could be seen as fungal, but note the sporulation on the underside of the leaf, something that fungi rarely do.

Bacterial; the sporulation clearly indicates that this is not a bacterial infection.

The correct answer is:

Algae:

Growth of algae on leaves can be common on perennial crops, especially on the lower (shaded) leaves.  There are very few algae that damage plants (as pathogens).

The incorrect answers were:

Powdery mildew; are always white.

Sooty mould; is always black.

Downy mildew; is generally pink, purple, violet or white on the underside of the leaf.

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.