A low input strategy for scab control in organic apple production

Summary

The effectiveness of compounds such as sulphur, lime sulphur, copper and potassium bicarbonate for controlling primary scab was investigated in field experiments over two growing seasons in Belgium on high and medium scabsusceptible cultivars, 'Pinova' and 'Pirouette', respectively, and two Vf scabresistant cultivars, 'Initial' and 'Topaz'. With the aim of reducing the amount of fungicide to be applied, two strategies were evaluated: (i) timing fungicide applications so that spraying occurred during the infection process, as determined by the RIMpro software warning system and (ii) using a tunnel sprayer machine for treatment applications. In both years of the study, although there was high disease pressure, low rates of elemental sulphur (<40 kg ha-1 year-1), combined with low rates of copper (<3 kg ha-1 year-1) provided the best scab control and reduced scab severity on the fruit of 'Pinova' by ≥97% compared with a water control. Lime sulphur was more effective than wettable sulphur and appeared to be effective at temperatures below 10°C. Potassium bicarbonate significantly reduced apple scab and was, in some cases, as effective as wettable sulphur, using the same dosage. All treatments increased the yield of the scab-susceptible cultivars as well as that of the scabresistant cultivars. Under these conditions, none of the treatments caused phytotoxicity, increased fruit russet, affected the summer density of the predatory mite, Typhlodromus pyri or resulted in undesirable residues on fruit at harvest.