Factors Affecting Control of Blossom Blight in a Peach Orchard with Low Level Benomyl-Resistant Monilinia fructicola. R. M. Sonoda, Professor, University of Florida, IFAS, Agricultural Research Center, Fort Pierce 33450. J. M. Ogawa, Professor, and B. T. Manji, Staff Research Associate IV, University of California, Davis 95616; E. Shabi, Division of Plant Pathology, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel; and D. Rough, Farm Advisor, San Joaquin County, Cooperative Extension Service, Stockton, CA 95205. Plant Dis. 67:681-684. Accepted for publication 2 December 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-681.

Benomyl was applied to peach blossoms in a peach orchard where 36% of the Monilinia fructicola on sources of inoculum (mummified fruit) were resistant to benomyl at 0.5–3 μg/ml. Benomyl at 1.1 and 2.2 kg a.i./ha reduced blossom blight to 56 and 43%, respectively, of that on unsprayed trees. Benomyl was highly effective against benomyl-sensitive isolates; 98–99% of isolates from blighted blossoms in benomyl-treated plots were benomyl resistant. Forty-eight percent of blighted blossoms from unsprayed trees harbored only benomyl-sensitive isolates. The difference in blighted blossoms between benomyl-sprayed trees and unsprayed trees could be accounted for by assuming that benomyl controlled blossom blight on the 48% of blighted blossoms attacked by only benomyl-sensitive isolates. Studies showed that there was a mean of almost two distinct M. fructicola isolate-types on each blighted blossom. The small number of isolates per blighted blossom, resulting in the probability that only sensitive isolates would attack some blossoms in the peach orchard, helps explain the reduction in blossom blight in the orchard. Results also indicate that benomyl treatments had little effect on M. fructicola resistant to benomyl.