Detection and Quantification of Resistance of Venturia inaequalis Populations to Sterol Demethylation Inhibitors

Abstract

Monoconidial isolates of Venturia inaequalis were collected in 1990 and 1991 from orchards in New York, Michigan, and Nova Scotia that had never or only sporadically been treated with fungicides acting as sterol demethylation inhibitors (DMIs). Sensitivities of isolates to two representative DMIs (fenarimol and myclobutanil) were determined by a sensitivity test based on the relative growth (RG) of mycelial colonies at one discriminatory dose. Mean isolate sensitivities were not significantly different (P > 0.2) for the majority of the populations tested, and all sensitivity data obtained from these sites were combined to provide a baseline distribution of isolate sensitivities for both fenarimol and myclobutanil. The baseline distributions were compared with isolate sensitivities determined for an experimental orchard in Nova Scotia with a documented case of DMI resistance and for a commercial orchard in Michigan with a long history of DMI use and first evidence of practical DMI resistance. For both DMIs tested and in both treated orchards, frequencies of isolates with RG values <80 had decreased or only slightly increased compared to the baseline population. In contrast, frequencies of isolates with RG values >80 had increased more than 20-fold over baseline levels. Thus, isolates with RG values >80 were rated DMI resistant. The validity of a qualitative isolate classification was tested in controlled infection studies. At doses of fenarimol and myclobutanil recommended for commercial control of apple scab, reproduction of a typical sensitive isolate on treated apple leaves was suppressed completely. Lesions caused by a resistant isolate continued to expand and produced abundant conidia. Statistical analysis of orchard sensitivities revealed that the analysis of isolate counts grouped into the categories DMI sensitive or resistant was most indicative in comparisons of orchard sensitivities aimed at detection of practical DMI resistance. A high degree of cross-resistance between fenarimol and myclobutanil indicated that sensitivity tests with one of the DMIs employed as the diagnostic tool in this study can serve as a test for other DMIs.

Detection and management of myclobutanil-resistant grapevine powdery mildew (Uncinula necator) in Ontario

“…Not only were most of the sensitive isolates eliminated, but more isolates emerged with significantly higher EC 50 values than were detected in the June collection. This directional shift in response to fungicide applications satisfied one of the criteria for judging the development of resistance to a fungicide (Köller 1991;Köller et al 1997). This change occurred in a vineyard that was considered to have had practical resistance to myclobutanil at 80 g/ha at the start of the year.…”

Screening of Botrytis cinerea isolates from vineyards in Israel for resistance to fungicides

“…The RG values of mycelial colonies at the DD were used as a simplified sensitivity test. EC 50 values for these isolates were established based on the relationships between the EC 50 values of the isolates, whose growth was evaluated on media containing several concentrations of the fungicides, and the RG scores of these isolates on CDA amended with the DDs of those same fungicides (EC 50 (D) ) (Jo et al 2006;Köller et al 1997). …”

Overexpression of the 14α-Demethylase Target Gene ( CYP51 ) Mediates Fungicide Resistance in Blumeriella jaapii

“…DMIs were introduced for plant disease management in the 1970s. Since the 1980s, clearly defined field resistance to DMIs has been reported for powdery mildews of barley, cucumber, and grape (4,12,27) and reduced sensitivity to DMIs has been reported for several other fungal pathogens, including the apple scab pathogen Venturia inaequalis (21,24).…”