Frequency of Benomyl-Resistant Botrytis cinerea Causing Gray Mold of Wild Blackberry in the Willamette Valley of Oregon. K. B. Johnson, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902. K. M. Theiling, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331-2902. Plant Dis. 74:331. Accepted for publication 1 February 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0331C.
Since the late 1970s, resistance of Botrytis cinerea Pers.:Fr. to benomyl has been a problem in control of gray mold of small fruit, vegetable, and floricultural crops grown in the Willamette Valley of Oregon. As It consequence, use of benomyl for this purpose declined greatly during the 19805. In the fall of 1988 and 1989, the frequency of benomyl resistance in the B. cinerea population was estimated by isolating the fungus from fruit of wild blackberry (Rubus spp.) found along edges of fields in agricultural areas throughout the valley. Sites sampled were scattered along a transect about 130 km long. At each site, either one infected berry (1989) or two or three closely grouped infected berries (1988) were touched to petri dishes containing potato-dextrose agar (PDA) amended with benomyl (10 mg a.i.f L) and streptomycin sulfate (300 mgl L). Three to seven isolations were made at each site. After 1-2 days of incubation, dishes were observed for active mycelial growth of B. cinerea. To reduce contamination by other fungi, developing colonies were transferred to benomyl- and streptomycin-amended PDA. where they sporulated after 7- 10 days. In 1988. resistant B. cinerea was isolated at 11 of 13 sites; 37% of isolations yielded resistant B. cinerea. In 1989, 30 of 40 sites yielded resistant isolates, and resistant B. cinerra Was recovered from 41% of the isolations. Both the number of sites with resistant isolates and their overall frequency indicate that resistance of B. cinerea to benomyl rcemains widespread despite declining use of the fungicide.