Biological control of Phytophthora root rot of avocado with microorganismsgrown in organic mulches

Organic mulches colonized with microbial biocontrol agents, termed bioenhan ced mulches, were tested for their ability to reduce Phytophthora root rot of avocado (Persea americana Mill.). Benomyl-resistant mutants of Gliocladi um virens (KA 230-1) and Trichoderma harzianum (KA 159.2) isolated from sup pressive soils and selected as efficient antagonists of P. cinnamomi were e valuated for their ability to colonize different mulches under controlled l aboratory conditions. Sudangrass and a coarse yardwaste were found to be be tter substrates than a fine yardwaste, woodwaste or rice hulls for biocontr ol agents propagules production. The most suitable conditions for colonizat ion were an optimum temperature of 24 degreesC, a moisture content of 20% f or sudangrass and 30% for the coarse yardwaste, and a continuous light expo sure during a 15-day incubation period. In the greenhouse, fresh sudangrass and a coarse yardwaste colonized with G. virens and used as a surface mulc h proved to be the best combination for reducing the population of P. cinna momi in 4-liter pots containing artificially-infested soil. Healthy avocado roots made up 31-37% of the roots in the G. virens-mulch combinations comp ared to 0% healthy in infested controls after two months.