Effect of leaf litter management on scab development in an organic apple orchard

Ascospores of Venturia inaequalis produced on scabbed leaves in the leaf litter are the main source of primary inoculum, causing infections in apple orchards. The purpose of this 2-year experiment, carried out in a commercial organic orchard, was to assess the effect of combining leaf sweeping from the alleys with leaf ploughing in within the row on scab inoculum and development. In 2003, scab severity was monitored at different distances from the unremoved leaf areas to estimate ascospore spreading. Scab incidence and severity were low in 2003 and high in 2004. At fruit harvest, both years, the leaf litter removal method reduced fruit scab incidence by 82.5% and 54.6% respectively, and fruit scab severity by 74.0% and 67.7%, respectively. Measures of scab lesion gradient indicated that ascospore spreading was not important beyond a 20 m distance from the source. The number of trapped ascospores observed in 2004 in the leaf removal treatment was reduced by 95%. Results from this 2-year experiment showed that leaf litter management by leaf removal allowed a reduction in apple scab inoculum and development and demonstrated the benefit of a complete removal of the leaf litter, when combining leaf sweeping from the alleys with leaf ploughing in within the row.