Infection and Development of Target Spot of Flue-Cured Tobacco Caused by Thanatephorus cucumeris. H. D. Shew, Associate Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. C. E. Main, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 74:1009-1013. Accepted for publication 12 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-1009.

The effects of misting frequency and temperature regime on basidiospore infection and lesion development of target spot of tobacco were determined. Three isolates of Thanatephorus cucumeris (Rhizoctonia solani AG-2-2) produced hymenia and basidiospores equally when exposed to moderate temperatures and extended periods of high relative humidity. Optimum temperatures for hymenium production, infection, and lesion development were 16–30, 20–26, and 20–30 C, respectively. Hymenium production was very minimal at temperatures above 30 C or at any temperature in the absence of misting. Lesion development was limited at high temperatures. Basidiospores germinated by the production of a single germ tube that terminated in an appressorium. Following direct penetration, a stroma was formed in the infected epidermal cell before further colonization of host tissue. Initial colonization resulted in the formation of a primary lesion 1–2 mm in diameter that remained distinct even if lesion expansion occurred. Lesions expanded by hyphal growth through the leaf tissue and by hyphae that grew out of stomata and across the leaf surface and then penetrated other stomata. The relationship between environmental conditions required for development of target spot and disease forecasting is discussed.