Residual Activity of Metalaxyl and Population Dynamics of Phytophthora cinnamomi in Landscape Beds of Azalea. D. M. Benson, Department of Plant Pathology, North Carolina State University, Raleigh 27695-7616. Plant Dis. 71:886-891. Accepted for publication 22 May 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0886.

Metalaxyl applied at 0.0014 kg a.i./5.5 m² of soil surface on a 30-day schedule during the growing season for two seasons resulted in continued suppression of inoculum of Phytophthora cinnamomi up to 18 mo after the last fungicide application. Inoculum density of P. cinnamomi in plots treated with metalaxyl on a 60-day schedule was similar to that in untreated plots after 18 mo, but plant mortality was nil and plant size was significantly greater than in the untreated plots. No suppression of inoculum of P. cinnamomi was found after 18 mo in plots treated with fosetyl-Al, even though plant mortality was low compared with that of plants in untreated plots. The accuracy of the semiautomatic elutriator assay method for P. cinnamomi was improved by including a 9% correction factor to account for propagules that passed through the smallest sieve used. The precision of the elutriator method did not increase with more than three subsamples assayed per soil sample as determined by estimating the variance of the general mean procedure with increasing numbers of subsamples. Inoculum density of P. cinnamomi was greatest in the root zones of plants with no foliar symptoms or with only initial symptoms of root rot and lowest in soil from plants with severe symptoms or in plots without plants.