Timing of first and final sprays against apple scab combined with leaf removal and pruning in organic apple production

The aim of this two-crop season study was to evaluate scab development and scab control efficacy in integrated approaches of (i) three sanitation treatments (fallen leaf removal combined with winter pruning and non-sanitized control), (ii) three onsets of first fungicide sprays (dormant bud, early tight cluster and pink bud stage) and (iii) three final dates for finishing fungicide programmes (mid-July, mid-August and mid-September). The study was performed in a Hungarian commercial organic apple orchard on a scab-resistant cultivar Prima and a moderately scab-susceptible apple cultivar, Jonathan. Final scab incidence of cultivar Prima was low on both leaves (below 3%) and fruits (below 1%) at harvest in all treatments, and delaying the first spray and/or omitting final sprays did not significantly affect fruit scab development on this cultivar. Results on cultivar Prima indicated that sprays against scab could be omitted before pink bud stage and after mid-July if the cultivar is not susceptible to summer fruit diseases and fruit scab incidence remains below 1% by harvest. On cultivar Jonathan, a delay in the onset of the first spray until pink bud stage resulted in higher scab incidences on both leaves (16.6–20.6%) and fruits (13.1–15.3%) compared with the non-delayed (dormant bud) spray treatments (5.2–8.3% and 6.7–9.4%, respectively). Final leaf and fruit scab incidence of cultivar Jonathan increased significantly when sprays were omitted after mid-July compared with spray treatments finished at mid-August or mid-September. Treatments of combining leaf removal with winter pruning resulted in lower scab incidence (5.2–11.8%) compared with treatments of non-sanitized plots (6.7–14.7%) when spray treatments were finished at mid-August or mid-September. Results on cultivar Jonathan suggested that sprays against scab could only be omitted before early tight cluster stage and after mid-August if removal of fallen leaves combined with pruning was applied and if the cultivar expressed <15% leaf scab incidence at harvest.