Scab control in organic apple production: conclusions of an eight year study in temperate weather conditions

Summary

The objective of this study, conducted over eight growing seasons (2002−2009), is to identify an innovative way for reducing the use of fungicides and in particular, copper fungicides, for the control of apple scab (V. inaequalis) in organic apple production. Special emphasis is put on
cultivar traits, sanitation practices and primary scab infection control during spring season. An original
approach is proposed for defining a specific spray timing involving spraying during the infection processes, especially before fungal penetration, determined by the RIMpro software warning system.
This ‘during-infection’ spray strategy allows reducing from 30 to 50% the amount of fungicide usually used for effective apple scab control, on high scab-susceptible cultivars. Potassium bicarbonate, lime sulphur, and three plant extracts such as peel orange extract, among 60 alternative products tested, have the potential to reduce copper use. However, copper use, even with low doses, seems to be still
necessary in presence of scab-susceptible cultivars. The results obtained in these experiments could not be attributed to the specific technical performances of the tunnel sprayer used, which however, offer valuable environmental benefits. This work shows that (i) planting cultivars with polygenic scabresistance traits, (ii) increasing accent on sanitation practices aimed at reducing initial inoculum in autumn, and (iii) applying an accurate “during-infection” spray strategy in spring, are the three most promising approaches for substantial further reductions in protection products fully compliant with international organic crop production standards.