Dicarboximide-Resistant Isolates of Botrytis cinerea from Table Grape in Chile: Survey and Characterization. B. A. LATORRE, Facultad de Agronomia, Pontificia Universidad Catolica de Chile, Casilla 306-22, Santiago. V. FLORES and A. M. SARA, BASF Chile, Santiago; and A. ROCO, Facultad de Agronomia, Pontificia Universidad Catolica de Chile, Casilla 306-22, Santiago. Plant Dis. 78:990-994. Accepted for publication 15 July 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/PD-78-0990.

Two to four applications annually of the dicarboximide (DC) fungicides iprodione and vinclozolin have been used widely for 10-15 yr to control gray mold, caused by Botrytis cinerea, of grapes in Chile. Control failures attributable to field resistance to DC fungicides have not been reported, although the frequency of low-level resistant (LLR) isolates increased from 2 to 74.9% between the 1987-1988 and 1993-1994 growing seasons; 0.3 and 0.6% of the isolates tested during 1992-1993 and 1993-1994, respectively, were highly resistant based on mycelial growth inhibition on PDA amended with 10 mg/L of vinclozolin. The EC50 for mycelial growth varied from 2.51 to 9.02 and from 2.00 to 18.16 mg/L of vinclozolin among isolates from commercial plantations during 1992-1993 and 1993 1994, respectively. The resistant factor (RF) for the most resistant LLR isolate was 60.13, although most LLR isolates had RF values of <30. Cross-resistance among DC fungicides and to dicloran and PCNB was demonstrated. Although resistant isolates lost some fitness attributes, e.g., higher osmolarity sensitivity, they were virulent and equally inhibited when inoculated nectarine fruit were treated with commercial rates of iprodione, procymidone, and vinclozolin. Conidial germination and mycelial growth of highly sensitive (HS) isolates (EC50 for mycelial growth = ?0.5 mg/L) collected from grapevines never exposed to DC fungicides were completely inhibited by 10 mg/L of iprodione or vinclozolin. Conidial germination of LLR isolates (EC50 for mycelial growth = 2-5 mg/L) was inhibited by only 0.7-9.7%, whereas mycelial growth was arrested by 89.0-91.6%. Inhibition of growth of LLR isolates by iprodione or vinclozolin may partially explain the relatively high degree of control of gray mold of table grape that is still possible with DC fungicides after 10-15 yr of continuous use.