Another surprise to corn was the emergence of the rare gray leaf spot disease in the 1970s. It was mostly undetected after the initial description in 1925. After outbreaks in the humid valleys of the eastern USA, I was among others who thought that was the primary environment for this disease. Then it was found in Ohio River valley, then humid river bottoms in Iowa and then, by early 80s in Nebraska. Combination of wide use of susceptible hybrids, including a few with extreme susceptibility and increased use of minimum tillage allowed proliferation of the fungus across the US corn belt. Currently it is found in humid, warm corn areas of all continents.
Gray leaf spot is caused by the fungi Cercospora zeae-maydis and, occasionally, Cercospora zeina. Most fungal corn leaf pathogens infect corn by spores germinating and penetrating the corn leaf within a few hours after exposure to moisture on the leaf surface. Spores of the gray leaf pathogens germinate on leaf surface but the hyphae grow on the surface of the leaf for a prolonged time that is mostly determined by the humidity. The hyphal growth continues with 90-100% humidity and temporarily stopped when the air is drier. After about 100 hours of the high humidity it establishes the flat bed (appresorium) over a stomata, from which it grows into the corn leaf. Once inside the leaf, the fungus produces a toxin called cercosporin. This toxin, activated by light, reacts with oxygen to produce a metabolite that damages cell membranes. Leaked contents from the damaged cells serves as nutrition source for the fungus. The damage is restricted by the plant to areas between leaf vascular bundles, resulting in the diagnostic narrow, elongate lesions. Corn resistance is reflected in limiting the size of the lesions.
The fungus remains in the dead leaf tissue even after harvest but apparently does not compete well with other fungi if buried in soil. Left on the soil surface, it produces spores (conidia) with warmth and moisture. spores easily distributed in the wind to begin the next cycle. This could be a source of continued infection for much of the corn season.
Unexpected arrival and spread of this disease is another example of conflation of susceptibility of a widely used genetics (i.e. B73) and change in agronomic practice and a potential pathogen. After acknowledging its potential damage, evaluations for resistance resulted in varieties with limited susceptibility and limited damage.
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About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.