Resistance to DMI fungicides in Venturia inaequalis from Pennsylvania

• Published in: Phytopathology 2010-06, Vol.100 (6), p.S100-S100
• Main Authors: Pfeufer, EE, Travis, J W, Ngugi, H K
• Format: Article
• Language: English
• Subjects: Malus; Venturia inaequalis
• ISSN: 0031-949X

Apple scab, caused by Venturia inaequalis, is the most economically important disease of apple in the eastern United States. Over the past 25 years, apple growers have relied on sterol demethylation inhibiting fungicides (DMIs) for scab control, but reduced efficacy has recently been noted. The aim of this study was to evaluate the sensitivity of V. inaequalis isolates from Pennsylvania to DMI fungicides. In 2009, leaves and immature fruit with scab symptoms were collected from 14 commercial orchards. Growers provided management history of the sampled plots. A total of 296 single-spore cultures were isolated from the tissues and maintained individually. Each isolate was tested for sensitivity to DMIs on 1/4-strength PDA plates amended with a range of concentrations of myclobutanil, fenbuconazole, or difenoconazole. Relative growth (RG) values were calculated and isolates with RG >75% on plates amended with 0.5 mg/ml were scored as resistant to the particular fungicide. About 14% of the isolates were cross-resistant to all three fungicides. Age of trees, size of orchard, number of DMI sprays in 2009, and lack of dormant copper sprays were positively correlated (0.0001 < P < 0.05) with the incidence of resistant isolates. Use of dormant copper sprays reduced the odds of an isolate being resistant to myclobutanil by about half (odds ratio = 0.446; 95% confidence interval = 0.239 to 0.832; P = 0.011). Management practices that reduce the risk of resistance to DMI fungicides in V. inaequalis were identified.