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Microbial Antagonism to the Imperfect Stage of the Apple Scab Pathogen, Venturia inaequalis. John H. Andrews, Department of Plant Pathology, University of Wisconsin, Madison 53706; Flora M. Berbee(2), and Erik V. Nordheim(3). (2)Department of Plant Pathology, University of Wisconsin, Madison 53706; (3)Departments of Forestry and Statistics, University of Wisconsin, Madison 53706. Phytopathology 73:228-234. Accepted for publication 16 July 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-228.

Fifty microorganisms from McIntosh apple leaves were screened for antagonism to two strains of the apple scab pathogen, Venturia inaequalis. The eight most antagonistic microorganisms (Flavobacterium sp., Cryptococcus sp., Aureobasidium pullulans, Trichoderma viride, Chaetomium globosum, Microsphaeropsis olivacea, and two unidentified actinomycetes) were ranked as to their efficacy. The ranking resulted from integration of data on inhibition of V. inaequalis from three in vitro and three in vivo assays: growth on nutrient agar, germination and germ tube lengths on agarose-coated slides, lesion size, overall symptom development, and conidial production on infected leaves. The best and most consistent antagonist was C. globosum. Average rank order of the antagonists was not influenced appreciably by the strain of V. inaequalis but, except for C. globosum, ranks differed markedly among assays. Impact of antagonists on germination and germ tube lengths of conidia of V. inaequalis on leaf surfaces was predicted by the agarose slide method 72 and 83% of the time, respectively. Ability of antagonists to reduce disease development was positively correlated with their suppression of conidial production by V. inaequalis (p = 0.651). The presumptive modes of antagonism determined in vitro are nutrient competition and antibiosis.

Additional keywords: biological control of aerial or foliar plant pathogens, Spilocaea pomi, screening for microbial antagonists.