VIEW ARTICLE

Relation of Postinoculation Leaf Wetness to Initiation of Tobacco Brown Spot. J. R. Stavely, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705; L. J. Slana, Research Plant Pathologist and Research Assistant, Tobacco Laboratory, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705. Phytopathology 65:897-901. Accepted for publication 18 March 1975. DOI: 10.1094/Phyto-65-897.

Leaf wetness was essential for initiation of brown spot on inoculated greenhouse tobacco plants by Alternaria alternata. The number of leaf lesions per plant increased significantly as wet periods were lengthened from 8 to 24, 48, 72, and 96 hours. A few lesions developed after as little as 4 hours of wetness. Resistance of cultivars could be differentiated with as little as 48 hours of wetness, but 72 hours was better for this purpose. Total hours of wetness was critical, but the results were the same, whether the exposure was continuous or broken by dry intervals. Plants were always allowed to dry after spray inoculation with conidial suspensions, before they were exposed to wetness. Lengthening this dry period from the minimum time to 20 days decreased disease severity, but lengthening it only to 10 or fewer days had little effect. The results suggest that dew may supply enough moisture for disease initiation, and that rain has an additional favorable effect.