Strategies to reduce copper use in organic apple production

Various strategies for controlling apple scab (Venturia inaequalis) were studied under field conditions over two growing seasons. An experimental organic orchard, designed especially for this trial, was planted with four partially scab-resistant cultivars ('Rubinstep-Pirouette', 'Reinette des Capucins', 'Reinette de Waleffe' and 'Pinova', which was used as the control) and four Vf scab-resistant cultivars (Topaz, Zvatava, Initial and JN 20/33/58). As the virulent scab race 7 is present in Belgium, one of the objectives was to test strategies to prevent Vf resistance breakdown. In order to drastically limit copper use, seven spray treatments were tested, including: 1. wettable sulphur (WS), 2. copper and wettable sulphur combined (CS), 3. wettable sulphur and lime sulphur combined (WSLS), 4. potassium bicarbonate (PB), 5. silicon (Si), 6. untreated control (Control) and 7. conventional fungicide control (IFP). The timing of fungicide applications was determined by the RIMPRO software warning system on the presence of airborne scab ascospores in orchards, taking into account forecasted weather conditions. A prototype of the 'tunnel sprayer' machine was used to apply fungicides. Compared with the control, fruit scab severity on cv. Pinova was significantly reduced by 99, 99, 91 and 68% in 2003 and by 100, 100, 92 and 52% in 2004 on trees sprayed with IFP, CS, WSLS and PB, respectively. These results were obtained with only 10 spray applications at lower fungicide rates despite the higher scab infection pressure in 2004. Good timing seems to be the most important factor in a spray treatment. Most of the treatments (IFP, CS, WSLS, WS and PB) also significantly reduced powdery mildew (Podosphaera leucotricha) infections. In both years, treatments with sulphur, copper and lime sulphur did not cause any phytotoxicity on leaves and fruits. A quantitative provisional apple scab control programme using copper and wettable sulphur was set up, including all potential climatic situations and degrees of infection risk in Belgium. The potential use and limits of potassium bicarbonate as an active substance for controlling apple scab and powdery mildew are discussed.