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 lg ml )1 for azoxystrobin to 5-25 lg ml )1 for kresoximmethyl, 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 EC 50 values >500 lg 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, Co´rdoba 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 Almerı´a 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.
When fungicide efficacy declines due to the development of resistance in the pathogen population, growers have to either change to an alternative mode of action or adjust their treatment programme. Adjustments may include either decreasing (or stopping) use of the mode of action, or increasing the total dose applied (by increasing number of applications and/or dose per application, where permitted) to try to maintain effective disease control. This study explores the circumstances under which increasing/decreasing total applied fungicide is financially optimal. A model based on field data is used to optimize the dose of fungicide applied when fungicide resistance develops in a pathogen population. The model is used to explore contrasting pathosystems and fungicide classes. When qualitative fungicide resistance develops, the shape of the disease-yield loss relationship determines whether the optimal total dose increases or decreases with increasing frequency of resistance in the pathogen population. When quantitative fungicide resistance develops, such that effective control can still be obtained with doses close to the maximum permitted dose, the optimal dose increases with increasing frequency of resistance in the pathogen population.
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