Evaluation of apple replant problems based on different soil disinfection treatments-links to soil microbial community structure?

Replant problems or soil sickness are known phenomena but still unsolved. The aims of this study were (i) to set up a test system for detecting replant problems using in vitro propagated apple rootstocks (M26) based on different soil disinfection treatments and (ii) to explore the treatment effects on root morphology and soil microbial community structure. The bio-test involved soil with apple replant problems (apple sick) and healthy soil from an adjacent plot, both either untreated, or submitted to treatments of 50 and 100 A degrees C, or the chemical soil disinfectant Basamid. Histological analyses of roots and denaturing gradient gel electrophoresis (DGGE) fingerprints in rhizosphere soil collected at the final evaluation were performed. After 10 weeks, shoot dry mass on apple sick soil was 79, 108 and 124 % higher for soil treated at 50 A degrees C, 100 A degrees C and with Basamid, respectively, compared to the untreated soil. Roots in untreated apple sick soil showed destroyed epidermal and cortical layers. DGGE fingerprints revealed treatment dependent differences in community composition and relative abundance of total bacteria, Bacillus, Pseudomonas and total fungi. The clear differences detected in soil microbial communities are the first steps towards a better understanding of the causes for apple replant problems.