Selection for and evaluation of an avocado orchard soil microbially suppressive to Phytophthora cinnamomi

Abstract

A survey was undertaken in southern California in 1999–2000 to search for an avocado soil that exhibits natural suppression to avocado root rot caused by Phytophthora cinnamomi. The Somis-1 soil, which had consistent massive degradation of P. cinnamomi hyphal mats, low P. cinnamomi populations and good tree health, was shown to be a soil with a high level of microbial suppression to P. cinnamomi in greenhouse studies. Other soils chosen for study, which seemed to demonstrate some type of suppression in the field as evidenced by high P. cinnamomi soil populations yet with good tree health, did not demonstrate microbial suppression in greenhouse studies. These soils may represent a type of suppression which is ephemeral or highly dependent on specific environmental factors. The suppression of the Somis-1 soil was transferable to a conducive soil with as little as 1% natural soil mixed with 99% fumigated soil. The suppression was gradually eliminated in soil pre-treated at various temperatures from 25° to 90°C. The suppression in the Somis-1 soil did not correspond with cellulase or laminarinase activity or soil microbial activity. The suppression appeared to correspond with moderately well-drained soils as found in the Somis-1 soil, which drained at a rate that might be conducive to the growth and activity of microorganisms antagonistic to P. cinnamomi.