Effects of water extracts of a composted manure-straw mixture on the plant pathogen Botrytis cinerea.

Manure-straw mixtures were composted and water extracts, made by incubating compost in water for 3 to 18 days, were assessed for antagonistic activity against Botrytis cinerea, using a range of tests. Extracts of all ages inhibited conidial germination on glass slides and reduced mycelial growth on agar. Mixing extracts of all ages with droplets of suspensions of B. cinerea conidia on detached Phaseolus bean leaves suppressed lesion development, but only 3- to 8-day-old extracts had an effect when sprayed onto leaves 2 days before inoculation. Extracts contained a large and varied microbial population of actinomycetes (0.3 to 2.4×10(5) c.f.u.ml(-1)), bacteria (1.5 to 5.6×10(10) c.f.u.ml(-1)), filamentous fungi (25.0 to 45.5 c.f.u. ml(-1)) and yeasts (26.1 to 62.6 c.f.u.ml(-1)). Eight- and 18-day-old extracts lost activity completely on filter sterilization or autoclaving. Weekly sprays of 8-day-old extracts onto lettuce in the glasshouse had no effect on the incidence of grey mould, but significantly reduced its severity and increased marketable yield. The use of compost extracts in biocontrol of plant diseases and their possible mode of action is discussed.