Spread and Persistence of Benomyl-Resistant Monilinia fructicola in South Carolina Peach Orchards. Eldon I. Zehr, Professor, Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634. Joe E. Toler, and Lynn A. Luszcz. Assistant Professor, Department of Experimental Statistics; and Research Assistant, Department of Plant Pathology and Physiology, Clemson University, Clemson, SC 29634. Plant Dis. 75:590-593. Accepted for publication 23 November 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-0590.

The spread and persistence of benomyl-resistant Monilinia fructicola were studied in an experimental peach orchard, and results were compared with conditions in several commercial peach orchards in which resistance to benomyl had occurred. One fungus strain that was resistant to iprodione was also examined in the experimental orchard. Benomyl-resistant strains became established and spread in the experimental orchard, but the frequency of isolation of these strains relative to sensitive strains declined as the season progressed unless the trees were sprayed with benomyl. The iprodione-resistant strain spread slowly but was not detected by the end of the ripening period. Benomyl-resistant strains introduced in 1984 and 1986 were not detected in the orchard in 1987. In contrast, resistant strains that developed naturally in commercial orchards persisted for 2 yr or more when benomyl use was reduced or eliminated altogether. The elimination of benomyl-resistant strains of M. fructicola is probably not possible with management practices that are practicable in large commercial orchards in the southeastern United States.