First Report of Practical Resistance to QoI Fungicides in Venturia inaequalis (Apple Scab) in Chile

Plant Disease
B V SallatoG Aylwin

Abstract

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...Continue Reading

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