PUTATIVE MECHANISM AND DYNAMICS OF INHIBITION OF THE APPLE SCAB PATHOGEN VENTURIA-INAEQUALIS BY COMPOST EXTRACTS

Clarified water extracts of slurries of spent mushroom substrate (SMS) inhibited in vitro germination of conidia of the apple scab pathogen Venturia inaequalis by up to 98% relative to germination in Water cont rols. Inhibition of conidial germination increased with incubation tim e of slurries over 5 to 7 d and persisted for at least 14 d, at which time experiments were terminated. Compost slurries became anaerobic wi thin 1 h when incubated without aeration. Aeration of slurries decreas ed efficacy of the resulting extracts compared to non-aerated controls . When aerated slurries were allowed to incubate without further aerat ion, extracts regained efficacy, becoming not significantly different from non-aerated controls. There was no difference in efficacy between filtered (0.1 mu m) and untreated extracts. Passage of filtrate throu gh microconcentrators with molecular cut-off limits ranging from 100 t o 3 kDa did not diminish activity of the extracts. Autoclaved extracts were less effective than untreated extracts but retained most of thei r efficacy. Extracts produced from sterile SMS were virtually ineffect ive compared with those from non-sterile SMS. When small volumes of sl urry from raw SMS were added to slurries of sterile SMS and incubated for an additional period, efficacy of the inoculated SMS was enhanced compared to uninoculated controls. We conclude that a major inhibitory principle of the SMS extract is a low molecular weight, heat-stable, non-protein metabolite produced by anaerobic microorganisms in the com post. Copyright (C) 1996 Elsevier Science Ltd