Microbial Antagonism to the Imperfect Stage of the Apple Scab Pathogen,Venturia inaequalis

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Abstract
Fifty microorganisms from ''McIntosh'' apple leaves were screened for antagonism to 2 strains of the apple scab pathogen, Venturia inaequalis. The 8 most antagonistic microorganisms (Flavobacterium sp., Cryptococcus sp., Aureobasidium pullulans, Trichoderma viride, Chaetomium globosum, Microsphaeropsis olivacea and 2 unidentified actinomycetes) were ranked as to their efficacy. The ranking resulted from integration of data on inhibition of V. inaequalis from 3 in vitro and 3 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.