Petals of a Black-eyed Susan

by Richard Haynes, Missouri U.S.A.

My September article on Black eyed Susan, Rudbeckia missouriensis, had a question for Micscape readers as to the identity of the two small "critters" or whatever, seen at 400X on petals in the final two photos, figures 34 and 35, (shown again here).

Almost immediately after publication, I had e-mail answers from several readers, notably William Dembowski, FRAS, Bob Goldsack, Robert Gorkin and Dan Holloway. Each of these gentlemen informed me that I had photographed Alternaria, a common fungus spore that afflicts plants. Bill Dembowski even sent one of his photomicrographs that showed the Alternaria spore. It was identical to the two above.

After gratefully receiving such timely and useful help, I located the website of George Barron, University of Guelph, Canada, which has an excellent picture of Alternaria spores along with much information on the fungus and from which I'll quote a bit. It may prove useful to other Micscape readers.

"...Alternaria...produces spores. The nuclei result from mitosis and the spores are genetically identical (mitospores). The spores in Alternaria are multi-celled and pigmented and they are produced in chains or branching chains. The spores have a distinctive appearance that makes them easy to recognize. They are broadest near the base and taper gradually to an elongate beak. Alternaria species are cellulolytic (breakdown cellulose to glucose for energy) and commonly grow on dead plant materials, particularly cereals and grasses. Some species are also parasitic on living plants and cause early blight of tomato and potato... When Alternaria attacks the host leaf, it produces a series of concentric rings around the initial site of attack. This gives a "target spot" effect that is associated with early blight..." - George Barron

Alternaria also drifts with the wind and can cause allergies in persons who are susceptible. Wind drift is undoubtably how my two spores came to be on the petals of the Black eyed Susan I was studying.

I thank each of these four sharp-eyed readers, Bill, Bob, Robert and Dan, to whom I am very much indebted. Without their input, I'm sure I would still be trying to name my "critters".

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Fig. 34 400X	Fig. 35 400X

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