DICARBOXIMIDE-RESISTANT ISOLATES OF BOTRYTIS-CINEREA FROM TABLE GRAPEIN CHILE - SURVEY AND CHARACTERIZATION

Two to four applications annually of the dicarboximide (DC) fungicides iprodione and vinclozolin have been used widely for 10-15 yr to contr ol gray mold, caused by Botrytis cinerea, of grapes in Chile. Control failures attributable to field resistance to DC fungicides have not be en reported, although the frequency of low-level resistant (LLR) isola tes increased from 2 to 74.9% between the 1987-1988 and 1993-1994 grow ing seasons; 0.3 and 0.6% of the isolates tested during 1992-1993 and 1993-1994, respectively, were highly resistant based on mycelial growt h 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 mo st resistant LLR isolate was 60.13, although most LLR isolates had RF values of <30. Cross-resistance among DC fungicides and to dicloran an d PCNB was demonstrated. Although resistant isolates lost some fitness attributes, e.g., higher osmolarity sensitivity, they were virulent a nd 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 (EC 50 for mycelial growth = less-than-or-equal-to 0.5 mg/L) collected fro m grapevines never exposed to DC fungicides were completely inhibited by 10 mg/L of iprodione or vinclozolin. Conidial germination of LLR is olates (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 ex plain the relatively high degree of control of gray mold of table grap e that is still possible with DC fungicides after 10-15 yr of continuo us use.