Occurrence of Phytophthora cinnamomi on Roots of Azalea Treated with Preinoculation and Postinoculation Applications of Metalaxyl. D. M. Benson, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 71:818-820. Accepted for publication 30 April 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0818.

Phytophthora cinnamomi was isolated from roots of azaleas treated with metalaxyl at 37 or 75 mg a. i./L before, on the day of, or after inoculation with colonized oat grains of the pathogen. Recovery of P. cinnamomi from roots of treated plants ranged from 0 to 100% depending on timing of the metalaxyl application and year of the experiment. Recovery of P. cinnamomi was lowest from roots of plants treated on the day of inoculation. In general, percent recovery of P. cinnamomi was lower when plants were treated with 75 mg/L of metalaxyl rather than 37 mg/L. Fresh top weights and root rot ratings for plants treated with metalaxyl did not differ significantly from those of untreated, uninoculated plants for azaleas treated up to 14 days after inoculation even though recovery of P. cinnamomi ranged from 0 to 83% depending on time of treatment after inoculation and year of the experiment. Preinoculation drenches of metalaxyl were ineffective in preventing subsequent root infection even though top weights and root rot ratings did not differ from those of untreated, uninoculated controls. Mobility of the fungicide in soil may account for the frequent isolation of P. cinnamomi from plants given preinoculation drenches of metalaxyl.