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 controls. Inhibition of conidial germination increased with incubation time of slurries over 5 to 7 d and persisted for at least 14 d, at which time experiments were terminated. Compost slurries became anaerobic within 1 h when incubated without aeration. Aeration of slurries decreased efficacy of the resulting extracts compared to non-aerated controls. When aerated slurries were allowed to incubate without further aeration, extracts regained efficacy, becoming not significantly different from non-aerated controls. There was no difference in efficacy between filtered (0.1μm) and untreated extracts. Passage of filtrate through microconcentrators with molecular cut-off limits ranging from 100 to 3 kDa did not diminish activity of the extracts. Autoclaved extracts were less effective than mutreated extracts but retained most of their efficacy. Extracts produced from sterile SMS were virtually ineffective compared with those from non-sterile SMS. When small volumes of slurry 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 compost.