Abstract
Apple and pear production is localized in specific regions within Europe. As different as their climatic conditions, as special are their production requirements. The severe restrictions on the use of plant protection products in organic farming imply that not every fruit growing area is suitable for organic pome fruit production. Considering the susceptibility of different crops and varieties to fungal diseases is therefore a basic principle for effective organic production systems in most growing areas. European regulation constitutes the legal framework for the registration of active substances of plant protection products within the EU, but each substance and commercial product must then be authorized on a national level. Copper-and sulphur-based products as well as lime sulphur, used in targeted spray programs, are most commonly applied for the control of fungal diseases such as apple scab and powdery mildew. In some countries, mainly in the northern part of Europe, copper and lime sulfur are not registered. In other countries lists of so-called plant strengtheners, not regulated by the EU and containing active substances with fungicidal (side) action (for example acid clays, carbonates and phosphonates), exist. At the moment the continuity of these lists is uncertain. The main fungal diseases, scab (Venturia inaequalis) and powdery mildew (Podosphaera leucotricha), certainly are a real challenge for organic pome fruit production, and their biology and control strategies have been studied extensively and almost exclusively over the last 15 years (MacHardy, 1996; Holb, 2007; Chapman et al., 2011). However, also the so-called secondary fungal diseases such as alternaria blotch (Alternaria mali), sprinkler rot (Phytophtora spp.), bull's eye rot (Pezicula malicorticus), sooty blotch (complex of different fungi), white haze (Tilletiopsis spp.), etc. (Weber, 2009; Baric et al., 2010; Mayer et al., 2010), are increasingly becoming of concern. These diseases, with their origin and development mostly unexplored, used to occur occasionally, but are now appearing with increasing frequency and intensity. Therefore, an urgent need for the development of new control strategies against these " new " fungi, compatible with organic farming, exists.