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Acquisition of Resistance to Sterol Demethylation Inhibitors by Populations of Venturia inaequalis

Acquisition of resistance to sterol demethylation inhibitors (DMIs) by populations of Venturia inaequalis was investigated using a microscopical method developed by C. Siebels and K. Mendgen. Microscopical analysis of conidiophore formation enabled the earlier detection of resistance and a clearer distinction between DMI-resistant and DMI-sensitive populations than other in vivo methods commonly used to analyze inhibitory effects of fungicides. In addition, because observations were made on the level of individuals, quantitative measures of the composition of conidial populations were obtained. The development of DMI sensitivity was followed over a period of 3 years in control apple orchards that had never been treated with fungicides and in orchards with DMI history. The 50% effective dose values determined by microscopical evaluation of conidio-phore development for untreated populations revealed the baseline sensitivities of 0.3, 0.96, 0.09, 1.22, and 1.92 mg/liter for flusilazole, fenarimol, difenoconazole, tebuconazole, and pyrifenox, respectively. As compared with the baseline sensitivity, all populations with DMI history showed significant resistance to flusilazole. A strong nonlinear correlation (R = 0.96) was found between the resistance factors and the sum of all DMI treatments of the 3 years before taking the sample. According to this correlation, resistance can be expected in all apple orchards of the fruit-growing area along Lake Constance, Germany, in which more than two DMI treatments per season have been applied. Due to cross-resistance, the recently introduced DMI fungicides difenoconazole, tebuconazole, and pyrifenox did not allow the control of V. inaequalis populations resistant to flusilazole.