Occurrence and distribution of resistance to QoI fungicides in populations of Podosphaera fusca in south central Spain

Cucurbit powdery mildew caused by Podosphaera fusca limits crop production in Spain. Since its management is strongly dependent on chemicals, the rational design of control programmes requires a good understanding of the fungicide resistance phenomenon in field populations. Fifty single-spore isolates of P. fusca were tested for sensitivity to three quinone-outside inhibiting (QoI) fungicides: azoxystrobin, kresoxim-methyl and trifloxystrobin. Minimum inhibitory concentration (MIC) values for QoI-sensitive isolates were found to range from 0.25 to 10 μg ml⁻¹ for azoxystrobin to 5–25 μg ml⁻¹ for kresoxim-methyl, using a leaf disc-based bioassay. High levels of cross-resistance to QoI fungicides were found. Eleven isolates showed resistance to the three QoI fungicides tested with MIC and EC₅₀ values >500 μg ml⁻¹ resulting in RF values as high as >715 and >1000 for trifloxystrobin and azoxystrobin, respectively. A survey of P. fusca QoI resistance was carried out in different provinces located in the south central area of Spain during the cucurbit growing seasons in 2002, 2003 and 2004. Examination of a collection of 250 isolates for QoI resistance revealed that 32% were resistant to the three fungicides tested; the provinces of Ciudad Real, Córdoba and Murcia being the locations with the highest frequencies of resistance (44–74%). By contrast, no resistance was found in Badajoz, and relatively low frequencies were observed in Almería and Valencia (10–13%). Nearly 50% of resistant isolates were collected from melon plants. Based on these data, recommendations about the use of QoI fungicides for cucurbit powdery mildew management in the sampled areas are made.