Plant Doctor Quiz for Plant Health Week 2025

The correct answer is:

Viruses
The mosaic seen here is extremely typical of virus infection. The stark contrast between the panels of yellow and green on all the leaves gives the plant a striking appearance.

The incorrect answers were;

Fungi: Yellowing of the whole leaf or various regions of a leaf due to fungal infection is common, but mosaic symptoms are not associated with fungal infection.

Mineral deficiency: Yellow and green leaves are very common symptoms of a deficiency of several minerals, but mineral deficiency patterns are never a stark mosaic like that seen here. A general mottled yellow pattern is much more typical of mineral deficiency.

Phytoplasmas: The yellowing due to phytoplasma infection is almost always a general yellowing of the infected leaves and never a mosaic such as this.

The correct answer is:

Powdery mildew and a rust fungus: The white powdery growth on the upper leaf surface with no browning is typical of powdery mildew infection; the orangey pustules bursting out from within the leaf are typical of rusts.

The incorrect answers were;

Two species of rust: Rusts can sometimes have two different coloured pustules on the same plant, but they are often black and orange.  The white growth is not typical of a rust, nor is it a pustule.

Powdery mildew and downy mildew: The white growth is a powdery mildew, but the downy mildew doesn’t form pustules and there are few examples of them attacking grasses.

Bacteria: Bacteria never produce structures that can be seen, and they are almost always associated with rotting or browning of the host plant.

The correct answer is:

Left Bacteria;  Right Fungus:

It is not possible to say with certainty, but the water soaking of the leafspot on the left spreading down the leaf veins is common in bacterial leaf spots, whereas the dry leaf spot on the right is more typical of fungal leaf spots.

The correct answer is:

Fasciation and Phyllody: The stem of the plant has developed multiple meristems (growing points), and these all grow together, producing a stem that is a fusion of multiple stems.  The cause is difficult to determine, it has been associated with some bacterial infections and with mite damage.  Phyllody is the production of leaves where petals should be. The cause can be due to phytoplasma infection, but there are other causes.

The incorrect answers were;

Enations: Small outgrowths perpendicular to the surface of leaves.

Little leaf: The plant produces little leaves, developing a bonsai-type appearance, which is often associated with witches broom and generally caused by phytoplasma infection.

Oedema: Warty/corky outgrowths on the surface of fruit. There is no pathogen involvement, and they are due to physiological conditions

The correct answer is:

Bacterial galls: The stem of the plant has developed multiple meristems (growing points), and these all grow together, producing a stem that is a fusion of multiple stems.  The cause is difficult to determine, it has been associated with some bacterial infections and with mite damage.  Phyllody is the production of leaves where petals should be.  The cause can be due to phytoplasma infection, but there are other causes.

The incorrect answers were;

Nitrogen fixing nodules: They are too large to be nitrogen fixing nodules, they are not on the roots but also on the stem, and they are rough, whereas nodules are smooth

Nematode galls: They are on the roots but also the stem, and they are rough and on the side of the stems. Nematode galls are generally part of the root and not growing on its side.

Insect galls: They could be insect galls, but this is unlikely as they are rough and appear to be on both the stem and roots.

The correct answer is:

A single cause: The fungal pathogen Sclerotinia has attacked the base of the plant, causing a rot. The fungus can be seen as fluffy white mycelium and the black (Sclerotia) resting structures.

The correct answer is:

The small dots within the leafspots: All of the options listed would tend to indicate a fungal cause, but the presence of the small dots within the leafspots makes it certain.  These are the fungal fruiting bodies; you can see the fungus itself.  However, there are large numbers of fungal plant pathogens that never produce such structures, and their absence does not mean the cause is not fungal.

The correct answer is:

Downy mildews: The soft, downy growth on the underside of the leaf with very little host reaction is typical of the downy mildews. There is no downy mildew on tomato, despite what Google will tell you.

The incorrect answers were;

Bacterial infection: Bacteria never produce structures that can be seen with the naked eye, and infection is almost always associated with necrosis.

Anthracnose: A fungal disease associated with leaf and fruit spots.

Powdery mildew: Always white and almost always present on the upper leaf surface.

The correct answer is:

Downy mildews: This is the systemic response to downy mildew infection.  In a few plants (Pea is one of them), the pathogen actually grows into the meristem, and all subsequent host growth is infused with the pathogen hyphae.  Under conditions of darkness and high humidity, the whole light coloured region of the plant will bear pinky sporulation.

The incorrect answers were;

Rust infection: There are some examples of rusts producing galls and other unusual growths, but this distorted growth is not one of them.

Bacterial infection: Bacterial infection is almost always associated with necrosis.

Viral infection: Distortion is a common feature of viral infection, but these waxy, thickened leaves are not typical of the distortion usually associated with viral infection.

The correct answer is:

Viral infection: Ringspots such as these are typical of infection by viral pathogens.  The “rings” sometimes appear as arcs or commas.  The presence of asymptomatic tissue within a ring strongly indicates a viral cause.

The incorrect answers were;

Bacterial infection: Bacterial infection is almost always associated with necrosis, spots are common, but not with an asymptomatic centre.

Insect feeding: Feeding marks are unusual, but they can occur. The Hemiptera are known to leave marks, but they are not generally ringspots like this.

Nematode attack: There are a few examples of foliar symptoms caused by nematodes attacking the leaves. They are rare, and they do not cause ringspots like these.