Strategies to Reduce Copper Use in Organic Apple Production

Jamar, L., Lateur, M.
Centre de Recherches Agronomiques, Département Lutte Biologique et Ressources Phytogénétique, Gembloux, Belgium.

Different strategies to control apple scab, Venturia inaequalis, were studied under field conditions during two growing seasons. An experimental organic orchard designed for this trial was planted with four partial scab-resistant cultivars (‘Rubinstep-Pirouette’, ‘Reinette des Capucins’, ‘Reinette de Waleffe’, Reinette des Capucins and ‘Pinova’ used as 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 is to test different strategies with the aim to avoid Vf resistance breakdown.

In order to drastically limit copper use, seven alternative spray schedules were tested. These included: 1. Wettable sulphur (WS), 2. Copper and wettable sulphur combination (CS), 3. Wettable sulphur and lime sulphur combination (WSLS), 4. Potassium bicarbonate (PB), 5. Silicon (Si), 6. Untreated control (Control) and 7. Conventional fungicides control (IFP). Fungicide application timing was defined by the ‘RIMPRO’ software warning system closely connected with the presence of airborne scab ascospores in the orchards taking weather forecast into account. A prototype of “tunnel” sprayer machine was used for fungicide applications.

Compared to the control, fruit scab severity on ‘Pinova’ was significantly reduced in 2003 by 99%, 99%, 91% and 68%, and in 2004 by 100%, 100%, 92% and 52% on trees sprayed respectively with IFP, CS, WSLS, and PB. These results were obtained with only ten spray applications and at lower fungicide rates despite a higher 2004 scab infection pressure. A better spray timing strategy seems to be the most important factor.

Most of the schedules (IFP, CS, WSLS, WS and PB) reduced powdery mildew (Podosphaera leucotricha) infections significantly. In both years, treatments with sulphur, copper and lime sulphur did not cause any phytotoxicity on leaves or fruits.

A quantitative provisional apple scab control programme using copper and wettable sulphur was set up to include all potential climatic situations and infection risk degrees in Belgium. Potential use and limitations of potassium bicarbonate used as an active substance to control apple scab and powdery mildew are discussed.