BSPP ‘Plant Doctor’ Quiz #22 – January 2023

The correct answer is:

Cleistothecia

These small black dots are occasionally seen in the white powdery mass that is typical of powdery mildew.  The numbers seen here are exceptional.  The cleistothecia are the sexual stage of the fungus which hold many hundreds of ascospores within.  The cleistothecia are resistant to desiccation and can be considered the resting stage of this fungus.

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Sclerotia

Unlike the small black cleistothecia seen in the previous question, these are large 5-10mm hard black lumps.  They can be formed by many fungi but are most commonly seen in Sclerotinia infections.  The sclerotia are often surrounded by a mass of white hyphae as seen here.

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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.

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Another spore type of the same fungus:

Rust fungi produce several types of spore.  The black regions are pustules of the same fungus: they are the teliospore phase of the disease.

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Bacteria:

This is the bacterial streaming test.  In some cases of bacterial wilt, it is possible to see the millions of bacteria oozing from the cut end of a stem. This is one of the few occasions it is possible to actually see plant pathogenic bacteria.

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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.

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Nematode cysts

These are the dead bodies of the female nematodes that are packed with nematode eggs.  Unlike galls, which often develop as part of the root, the cysts are very much attached to the root rather than part of it.

Photo credit: US department of Agriculture

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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.

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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.

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They are too small to be seen except with a high power light microscope:

There are two types of mite that frequently attack plants, the larger, highly mobile, web-producing red spider type of mite and the much smaller, eriophyid mite which looks like a hermit crab in an ice cream cone. These are much smaller and very difficult to see.