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

Abstract

Kunz, S., Deising, H., and Mendgen, K. 1997. Acquisition of resistance to sterol demethylation inhibitors by populations of Venturia inaequalis. Phytopathology 87:1272-1278.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 conidiophore 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 fruitgrowing 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.The success of modern, integrated disease control strategies strongly depends on the availability of efficient curative fungicides (2). Until 1995, sterol demethylation inhibitors (DMIs) were the only curative fungicides registered for use in apple orchards in Germany. Since DMI fungicides represent single-site inhibitors interfering with C-14 demethylation of 24-methylendihydrolanosterol in ergosterol biosynthesis (6,18), a high risk of development of resistance exists (18). In fact, resistance problems have been observed in several pathogens including powdery mildews (10,11,25), Penicillium digitatum (7), Pyrenophora teres (26), Sclerotinia homoeocarpa (9), and Venturia inaequalis (13). Isolates of V. inaequalis with reduced sensitivities to several DMI fungicides have been reported to occur worldwide (4,13,23,24,31,32). Resistance to DMI fungicides is under multigenic control, increasing gradually by additive action of the different resistance genes (6,15). Depending on the stringency of the selection pressure, sensitivity of V. inaequalis populations may, therefore, shift gradually from baseline sensitivity to a distinct le...

Sensitivity of Venturia inaequalis Chilean Isolates to Difenoconazole, Fenarimol, Mancozeb, and Pyrimethanil

“…The wild isolates showed a high sensitivity to difenoconazole and reflected a great intrinsic activity of this fungicide when compared to fenarimol's lower activity. Similar results were obtained by Kunz et al (1997) when assessing the sensitivity of in vivo DMIs where difenoconazole was ranked as the most active fungicide against V. inaequalis when compared to flusilazol, fenarimol, tebuconazole, and pyrifenox.…”

“…Discriminatory doses to separate sensitive from resistant isolates were proposed based on wild sensitivities according to Köller et al (1991). Resistance factors (RF) were calculated for each fungicide as the ratio between mean sensitivity of the orchard population divided by the mean sensitivity of the wild population (Kunz et al, 1997).…”

Researches on Methodologies to Verify Reduced Sensitivities of <i>Venturia inaequalis</i> in Field to Difenoconazole and First Indications of a Survey in Italy

“…It resulted also able to exceed resistance to previous IBS's as fenarimol, myclobutanyl and fenbuconazole [11]. By the late Author, the cause could be the major activity of difenoconazole, instead of a moderate cross-resistance with the other IBS fungicides.…”

Inhibition of Efflux Transporter-Mediated Fungicide Resistance in Pyrenophora tritici-repentis by a Derivative of 4′-Hydroxyflavone and Enhancement of Fungicide Activity

“…1). That the fungicide application regimen affects the sensitivity of a population has been shown for the powdery mildews of barley and cucurbits, the wheat eyespot fungus Pseudocercosporella herpotrichoides, and the apple scab fungus Venturia inaequalis (2,7,19,22).…”