Replant problems in South Tyrol: role of fungal pathogens and microbial population in conventional and organic apple orchards

The South Tyrol, the main apple-growing area in Italy, is characterised by intensive soil cultivation. Previous investigations have revealed the existence of replant disorders although the main causes of these have not been evaluated. A survey was carried out in this area with two main aims: to evaluate the role of soil-borne pathogens in apple replant disease and to compare the impact of organic and conventional management on replant diseases caused by soil-borne pathogens. The experimental sites were chosen to obtain three pairs of contiguous conventional and organic apple orchards. There was a high level of organic matter in both the organic and the conventional apple orchards with no appreciable differences in humic fractions. The soil sickness test with apple seedlings revealed a significant reduction in growth for all soil samples as compared to the peat control. The growth score on soil samples from organic orchards was significantly higher than that obtained on conventional soil. Total fungal population in soil samples was positively correlated to the apple seedlings growth score and negatively correlated to the frequency of root colonisation. Most of the root-colonising pathogens belonged to the well-known root rot complex, Rhizoctonia solani and Pythium spp. being the most pathogenic on apple seedlings.