BSPP ‘Plant Doctor’ Quiz #4 – July 2021

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.