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 microscop ical method developed by C. Siebels and K. Mendgen. Microscopical anal ysis of conidiophore formation enabled the earlier detection of resist ance and a clearer distinction between DMI-resistant and DMI-sensitive populations than other in vivo methods commonly used to analyze inhib itory effects of fungicides. In addition, because observations were ma de on the level of individuals, quantitative measures of the compositi on of conidial populations were obtained. The development of DMI sensi tivity 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 eva luation of conidiophore development for untreated populations revealed the baseline sensitivities of 0.3, 0.96, 0.09, 1.22, and 1.92 mg/lite r for flusilazole, fenarimol, difenoconazole. tebuconazole, and pyrife nox, respectively. As compared with the baseline sensitivity, all popu lations with DMI history showed significant resistance to flusilazole, A strong nonlinear correlation (R = 0.96) was found between the resis tance factors and the sum of all DMI treatments of the 3 years before taking the sample. According to this correlation, resistance can be ex pected in all apple orchards of the fruit-growing area along Lake Cons tance, Germany, in which more than two DMI treatments per season have been applied. Due to cross-resistance, the recently introduced DMI fun gicides difenoconazole, tebuconazole, and pyrifenox did not allow the control of V. inaequalis populations resistant to flusilazole.