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Influence of Temperature and Other Factors on Initiation of Tobacco Brown Spot. J. R. Stavely, Research Plant Pathologist, Crops Research Division, ARS, USDA, Beltsville, Maryland 20705; C. E. Main, Assistant Professor of Plant Pathology, Department of Plant Pathology, North Carolina State University, Raleigh 27607. Phytopathology 60:1591-1596. Accepted for publication 2 June 1970. DOI: 10.1094/Phyto-60-1591.

Infection of tobacco by Alternaria tenuis, incitant of brown spot, was most severe at constant 20 C, and was progressively less at 24 and 16 C. Least infection occurred at 28 C or in a cycle of alternating 12-hr periods of 20 and 28 C. Inoculation of the lower leaf surfaces resulted in 2 to 3 times more lesions than when leaves were inoculated on the upper surface. Temperature effects and relative sensitivity of tobacco cultivars to the disease were similar when plants were inoculated with conidia from dead, infected tobacco leaves from the field or from pathogenic single conidium isolates grown on V-8 juice agar. Addition of a carbon source to the inoculum increased the number of brown spot lesions on all cultivars at suboptimal temperatures and on tolerant cultivars at 20 C. Other critical factors for infection included maintenance of moisture on the leaves for at least 3 days following inoculation and use of an inoculum concentration of 30,000 conidia of a proven pathogenic isolate of the fungus/ml inoculum suspension. Relative sensitivity of tobacco cultivars to the disease was the same with controlled inoculations as with natural infection in the field. Optimum temperature for radial growth of five single conidium isolates of the fungus on agar media was 27 C.

Additional keywords: Nicotiana tabacum, growth of pathogenic and nonpathogenic isolates.