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External Growth, Penetration, and Development of Cercospora zeae-maydis in Corn Leaves. Peter M. Beckman, Graduate research assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27650; Gary A. Payne, assistant professor, Department of Plant Pathology, North Carolina State University, Raleigh 27650. Phytopathology 72:810-815. Accepted for publication 30 October 1981. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-810.

The influences of plant age, genotype, leaf age, and moisture on symptom expression of gray leaf spot caused on corn (Zea mays) by Cercospora zeae-maydis were studied. External growth, penetration, internal colonization, and sporulation were characterized histologically. Spores germinated after 24 hr at 2230 C when plants were exposed to 12 hr of mist. Where no free water accumulated, germ tubes showed positive stomatal tropism. The presence of free water on the upper leaf surfaces reduced tropistic responses toward stomata, appressorium formation, and subsequent penetration. On the lower leaf surfaces, abundant appressoria formed over stomata 45 days after inoculation and penetration occurred after 67 days. In mature plants, chlorotic dots formed at 9 days, elongated discolored streaks at 12 days, and necrotic and sporulating lesions at 1621 days. Young corn plants were also susceptible and developed sporulating lesions 34 days earlier than mature plants. Leaf age and plant genotype had little effect on symptom expression under greenhouse conditions. Internal colonization was intercellular and confined to the mesophyll. Delimitation of hyphal growth lateral to the vascular system by sclerenchyma tissue surrounding the major veins resulted in typical long, narrow, parallel bordered lesions. Fungal stroma developed in the substomatal cavities coincident with necrosis of the lesion and gave rise to numerous conidiophores and conidia. These results suggest that sustained periods of high relative humidity are more important for the development of C. zeae-maydis than plant or leaf age. The typical late-season appearance of this disease in the field is likely due to extended periods of high humidity provided by the canopy of mature corn plants.

Additional keywords: histopathology.