Pilot trial to evaluate the effect of alternative control measures to enhance leaf degradation, thereby reducing Venturia inaequalis inoculum levels in apple orchards

Apple scab caused by Venturia inaequalis is responsible for commercial losses on apples. The South African pome fruit industry relies on chemical treatments to control apple scab, some already phased out in certain countries. This study aims to determine the efficacy of various products to accelerate leaf breakdown, an environmentally friendly option to reduce apple scab inoculum in orchards.

A commercial Braeburn orchard in Villiersdorp was selected based on the presence of apple scab. Eight treatments were applied: untreated control, shredding, yeast extract, decomposing with biological agent, decomposing agent and a combination of these. Grids with yellow leaves and grids with symptomatic leaves were treated and evaluated for leaf degradation and ascospore quantification. Nursery trees were placed in the orchard as early indicators of symptoms for evaluation.

T8 (the combination of a decomposing & biological agent with another decomposing agent) was the only treatment that exhibited significantly more leaf degradation than the UTC on leaf litter left for 10 weeks in the orchard, whereas T8 and T5 (the combination of shredding and yeast extract) displayed significantly more degradation after 19 weeks. Although not significant, T8 had the lowest number of ascospores. No apple scab symptoms were noted from nursery trees but symptoms that appeared like frogeye leaf spot caused by B. obtuse or Glomerella leaf spot caused by Colletotrichum spp. were observed. Though not significant, T2 (shredding only), T3 (urea) and T8 exhibited low numbers of symptoms, with T8 being the lowest.

Based on the evaluations, T5 and T8 were the most effective treatments. Future studies should consider these treatments for further investigation. The trial should be repeated and evaluations conducted over multiple seasons to observe the long-term impact of the treatments on soil health and natural decomposition

Potential alternative methods to control apple scab and other preharvest diseases were identified. Further trials including the most effective treatments are advised.