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Disease Control and Pest Management

Protective Applications of Conidia of Nonpathogenic Alternaria sp. Isolates for Control of Tobacco Brown Spot Disease. Harvey W. Spurr, Jr., Research Plant Pathologist, Southern Region, Agricultural Research Service, U.S. Department of Agriculture, Oxford Tobacco Research Laboratory, Oxford, NC 27565; and Professor of Plant Pathology, North Carolina State University, Raleigh, NC 27607; Phytopathology 67:128-132. Accepted for publication 15 June 1976. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-128.

Nonpathogenic isolates of Alternaria, applied to leaves of tobacco before inoculation with the pathogen, Alternaria alternata, reduced tobacco brown spot. Brown spot severity was reduced 60% in laboratory experiments and 65% in artificially induced field infections. Level of disease reduction was related to the concentration of conidia applied as a protectant. In the laboratory, protection was maximum when the protective conidia were applied to the leaves 2-3 days before inoculation. Nonpathogenic conidia germinated and grew on the leaf surface with no evidence of antagonism toward the pathogen. Colonization by the protective fungus on the leaf surface usually altered host penetration by the pathogen to one of epiphytic growth on the leaf surface.