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Sorghum Downy Mildew: Biology of Systemic Infection by Conidia and of a Resistant Response in Sorghum. Ying Yeh, Former graduate research assistant, Department of Plant Sciences, Texas A&M University, College Station, 77843; R. A. Frederiksen, professor, Department of Plant Sciences, Texas A&M University, College Station, 77843. Phytopathology 70:372-376. Accepted for publication 29 October 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-372.

Sorghum plants inoculated with conidia of Peronosclerospora sorghi were held under environmental conditions known to favor disease development. Hyphae from germinating conidia penetrated indirectly most successfully in shoot tissues. The fungus penetrated and established systemic infection only during early stages of seedling development. Resistance to sorghum downy mildew was manifested within 4 days after seeds of IS12661C and an IS12661 derivative imbibed water; whereas, the susceptible cultivar, TX412, remained susceptible for 7 days. A histological study revealed a hypersensitive-type reaction that restricted fungal development in resistant cultivars and was associated with the precocious formation of primary haustoria by the fungus in epidermal cells. Primary haustoria formed in mesophyll cells of susceptible cultivars and were associated both with unrestricted fungal development and, ultimately, either systemic symptoms or large local lesions. Histological studies provided evidence that resistance is manifested by different genotypes at different stages of the host-parasite interaction.