die as spot size increases into longer, narrower leaf lesions. Although initially brownish and yellow, the characteristic grey color that follows is due to the production of grey fungal spores (conidia) on the lesion surface. These symptoms that are similar in shape, size and discoloration are also prevalent on the corn husks and leaf sheaths. Leaf sheath lesions are not surrounded by a yellow radiance, but rather a brown or dark purple radiance. This dark brown or purple discoloration on leaf sheaths is also characteristic to northern corn leaf blight (Exserohilum turcicum), southern corn leaf blight (Bipolaris maydis), or northern corn leaf spot (Bipolaris zeicola). Corn grey leaf spot mature lesions are easily diagnosed and distinguishable from these other diseases. Mature corn grey leaf spot lesions have a brown, rectangular and vein-limited shape. Secondary and tertiary leaf veins limit the width of the lesion and sometimes individual lesions can combine to blight entire leaves. Pathogenesis One reason for the pathogenic success of Cercospora zeae-maydis is the production of a plant toxin called cercosporin. All members of the genus Cercospora produce this light-activated toxin during infection. In the absence of light, cercosporin is inactive, but when light is present, the toxin is converted into its excited triplet state. Activated cercosporin reacts with oxygen molecules, generating active single oxygen radicals. Oxygen radicals react with plant cell lipids, proteins, and nucleic acids, damaging and killing affected cells, and nutrients released during the cell rupture and death feed the Cercospora fungus. A study of mutant Cercospora lacking the gene responsible for cercosporin production demonstrates that, though unnecessary for infection, cercosporin increases the virulence of Cercospora fungi. Disease cycle Cercospora zeae-maydis survives only as long as infected corn debris is present; however, it is a poor soil competitor. The debris on the soil surface is a cause for primary inoculation that infects the incoming corn crop for the next season. By late spring, conidia (asexual spores) are produced by Cercospora zeae-maydis in the debris through wind dispersal or rain. The conidia are disseminated and eventually infect new corn crop. In order for the pathogen to actually infect the host, high relative humidity and moisture (dew) on the leaves are necessary for inoculation. Primary inoculation occurs on lower regions of younger leaves, where conidia germinate across leaf surfaces and penetrate through stomata via a flattened hyphal organ, an appressorium. Cercospora zeae-maydis is atypical in that its