| In California, avocado root rot caused by Phytophthora cinnamomi is one of the limiting factors to avocado production. This study describes the search for and characterization of an avocado orchard soil that exhibits natural microbial suppression to avocado root rot.; A survey undertaken in southern California in 1999-2000 to search for an avocado soil that exhibits natural suppression to avocado root rot resulted in four soils chosen for study over a two year period. Out of the four soils studied, one soil demonstrated microbial suppression in greenhouse trials. The microbial suppression was transferable to a conducive soil and was gradually eliminated in soil pre-treated at various temperatures from 25° to 90°C. The microbial suppression did not correlate with cellulase or laminarinase activity or soil microbial activity and appeared to correlate with moderately well-drained soils.; In vitro studies demonstrated significant Phytophthora cinnamomi hyphal mat degradation and chlamydospore damage at soil matric potentials between -15 mbar to -100 mbar. There was no significant chlamydospore damage at -10 mbar, -5 mbar or at 0 mbar (saturation); however, there was significant hyphal damage at all matric potentials except 0 mbar.; The hyphal and chlamydospore formation life stages were degraded by microbial antagonists in the suppressive soil. Sporangial development, zoospore release, zoospore movement and zoospore attraction to avocado roots did not appear to be affected by microbial antagonists.; Bacterial rRNA genes associated with the naturally suppressive avocado soil were examined using Oligonucleotide Fingerprinting of rRNA Genes (OFRG). OFRG identified three bacterial groups that positively correlated with P. cinnamomi hyphal degradation and which had high sequence identity to rRNA genes from Cellvibrio sp., a pseudomonad, and Cellulomonas variformis, an actinomycete. As these bacteria secrete cellulase, they may be involved in the degradation of P. cinnamomi cell walls.; The combination of Trichoderma aureoviride and fosetyl-Al resulted in significantly healthier and more abundant roots than with Trichoderma alone or fosetyl-Al alone in avocado seedlings infected with Phytophthora cinnamomi. However, a decline in the P. cinnamomi soil population did not occur with any of the treatments. |
