Resistance to the QoI fungicide (kresoxim-methyl) was detected in Venturia inaequalis (Cooke) Winter isolates from apple scab lesions collected from apple orchards in Chile after 5 to 6 years of commercial use of QoI fungicides. To reduce or prevent resistance development, QoI fungicides have been applied in blocks of two or three consecutive applications followed by an application of a fungicide with a different mode of action. However, the efficacy of kresoxim-methyl declined dramatically, providing almost no disease control in several orchards under high scab pressure in southern Chile, and resistance to the QoI fungicides was suspected. Conidia from at least five sporulating leaf scab lesions were collected from each of 10 and 14 orchards in 2003 and 2004, respectively. Fungicide sensitivity was determined by using a conidial germination test on potato dextrose agar acidified with 0.5 ml/liter of 95% lactic acid (APDA) containing kresoxim-methyl (Stroby 50 SC) at 0.0, 0.01, 0.1, and 1.0 μg/ml. The kresoxim-methyl concentration (0.01 μg/ml) has been used as a discriminatory dose in previous resistance studies (2). Germination was assessed for at least 100 conidia at each inhibitor concentration after 18 to 24 h of incubation at 20°C (1,2). Sensitivities were expressed as the percentage of germination relative to that on unamended APDA (1,2). These assays were repeated and all isolates with 50% effective dose (ED50) at >0.1 μg/ml were considered resistant. Isolates from eight of 10 and two of 14 orchards were resistant to kresoxim-methyl in 2003 and 2004, respectively. Relative germination varied from 42.4 to 100% for isolates from resistant orchards on 0.1 μg/ml of kresoxim-methyl, with eight isolates having ED50 at >0.1 μg/mL. Conidia from isolates obtained in 2003 also germinated on APDA amended with 1.0 μg/ml of kresoxim-methyl, with two isolates having ED50 at >1.0 μg/mL. Relative to the mean ED50 of 0.02 μg/ml of kresoxim-methyl determined for relative germination in North America baseline population of V. inaequalis (2), resistance factors for these Chilean isolates were estimated to range from 5 to 50. Conidia of resistant isolates continued to germinate in the presence of 0.1 μg/ml of kresoxim-methyl when 100 μg/ml of salicylhydroxamic acid was added to the amended APDA, suggesting that an alternative oxidase pathway was not responsible for the in vitro results obtained (2). A G143A target mutation has been detected in a V. inaequalis isolate from Chile, although no details concerning this finding were reported (Fungicide Resistance Action Committee, online publication, 2004 [http://www.frac.info]). The poor control of apple scab with QoI fungicides in Chilean orchards appears to be due to the development of resistant strains of V. inaequalis that occurred despite the limited use of these materials in blocks of two or three consecutive applications, as initially suggested for resistance management. Farmers have been advised to make only one application of QoI fungicide per year in a tank mixture with another type of fungicide or even to abandon the use of QoI fungicides for apple scab control. To our knowledge, this is the first report of a severe outbreak of apple scab due to the presence of populations of V. inaequalis resistant to QoI fungicides under commercial conditions in Chile.
References: (1) R. B. Küng Färber et al. Pest Manage. Sci. 58:261, 2002. (2) G. Olaya and W. Köller. Plant Dis. 83:274, 1999.
