Primary scab control using a "during-infection" spray timing and the effect on fruit quality and yield in organic apple production

Organic apple production in Europe depends to a great extent on the use of copper fungicides for scab control (Venturia inaequalis). The objective of this 6-year study (2003-2008) conducted in Belgium was to determine measures for reducing the use of copper fungicides in organic apple production. The effectiveness of a "during-infection" spray strategy using wettable sulphur (with or without copper), lime sulphur, potassium bicarbonate, silicon and five natural plant extracts (orange peel, soapbark, tea seed, quinoa seed and grapefruit seed) for controlling primary scab was investigated in a split-plot field experiment. Four apple cultivars that express a gradient of partial scab resistance were included: a high scab-susceptible cultivar (cv. 'Pinova'), a medium scab-susceptible cultivar (cv. 'Pirouette') and two old cultivars expressing low to very low scab susceptibility (cvs. 'Reinette Hernaut' and 'Reinette des Capucins'). Apart from these cultivars, four monogenic Vf scab-resistant cultivars (cvs. Initial', 'Topaz', 'Zvatava' and 'JN20/33/58') were also included in the experimental orchard. In order to reduce the amount of fungicide required, two strategies were used: a specific spray timing involving spraying during the infection processes, before fungal penetration, determined by the RIMpro software warning system, and a tunnel sprayer machine for optimal treatment applications. Depending on the year, a total of 8-12 applications were made annually. Under field conditions that were highly conducive disease, low rates of elemental sulphur (<= 40 kg. ha(-1) per year) combined with low rates of copper (<= 2.1 kg. ha(-1) per year) provided the best scab control and reduced scab severity on the leaves and fruits by 85-100%, depending on the year and cultivar, compared with the untreated control. In most cases, the lime sulphur spray treatment, which used more elemental sulphur but did not use copper, provided a similar level of scab control to the combined wettable sulphur and copper spray treatment. Sulphur, potassium bicarbonate and all plant extracts significantly reduced scab severity on leaves and fruits. In general, the treatments increased the yield of the high and medium scab-susceptible cultivars as well as that of the low and very low scab-susceptible cultivars. Under these experimental conditions, none of the treatments caused phytotoxicity, increased fruit russet or led to undesirable soil and fruit residues at harvest. The potential and limitations of "during-infection" spraying as a protection strategy against apple scab in organic farming are discussed.