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

Summary

South Tyrol, the main Italian apple growing area, is characterised by an highly intensive soil cultivation. Previous investigations shows the existence of replant disorders although it has not been evaluated which are the main causes. A survey has been carried out in this area with two main aims I) to evaluate the role of soil borne pathogens in apple replant disease and II) to evaluate the effect of soil management toward soil borne pathogens causing replant diseases. The experimental sites were chosen in order to obtain three couples of contiguous conventional and organic apple orchards.
Soil sickness test with young apple plants gave a significant growth reduction in all soil samples if compared to a peat control. Among all root colonising fungi (Fusarium oxysporum, F. solani, Aphanomyces sp., Cy/incrocarpon sp., Rhizoctonia sp. and Pythium sp.) some Rhizoctonia solani strains and all Pythium spp. were the most pathogenic. In all cases organic management seems to reduce the soil sickness severity caused by root rot fungal pathogens.