Venturia inaequalis demethylation inhibitor fungicide sensitivity after orchard sanitation

Abstract: Venturia inaequalis, commonly known as apple scab, causes large crop losses world-wide and in South Africa if not controlled by fungicide applications. Integrated disease management approaches such as sanitation practices for inoculum reduction are thought to help maintain effectiveness of chemical control. Applied research in an orchard environment can give an indication of the efficiency of a combination or single application of different management strategies. In this study monitoring of fungicide sensitivity of V. inaequalis populations was applied in field trials where leaf shredding, or a standard fungicide spray program during the growing season combined with the leaf shredding treatment had been applied. It was compared to an untreated orchard section, or a standard spray program alone. The aim of this study was to evaluate the impact of sanitation treatments in the orchard on levels of fungicide sensitivity of one of the most commonly used systemic fungicide classes in South Africa, namely demethylation inhibitors. Two populations from sanitation trials were tested for their fungicide sensitivity to flusilazole using a mycelial growth test of single spored isolates. In the orchard sections, where leaves had been shredded after leaf fall in combination with fungicide application, a significant reduction in resistance levels was observed (EC50 = 0.11 μg/ml) when compared to the negative control (EC50 = 0.27 μg/ml).