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Effect of Pruning on Apple Scab in Organic Apple Production

In a 3-year Hungarian study, the effectiveness of apple tree pruning as a management tactic for control of apple scab (Venturia inaequalis) was investigated on two susceptible (cvs. Jonagold and Mutsu), two moderately susceptible (cvs. Elstar and Idared), and two V_f resistant apple cultivars (cvs. Liberty and Prima) in three high-density organic apple orchards treated with copperand sulfur-based fungicide programs. The area under the disease progress curve (AUDPC) for leaf and fruit incidences was calculated to evaluate three winter pruning treatments (unpruned, weakly pruned, and strongly pruned). Strong pruning significantly decreased leaf scab on the susceptible and the moderately susceptible cultivars at all sites compared with unpruned ones. Moreover, for the cultivar Jonagold in all years and for the cultivar Mutsu in 2001, AUDPC values of strongly pruned trees were significantly lower at P = 0.05 compared with the weaklypruned trees. The effect of pruning on development of fruit scab was weaker than on leaf scab in all years and at all sites. For fruit scab incidence on susceptible cultivars, only strong pruning treatments decreased AUDPC significantly compared with unpruned treatments. The effects of pruning treatments on moderately susceptible cultivars were significant only in 2001 at all sites. Resistant cultivars showed no significant effect of pruning treatments on development of either fruit or leaf scab except for cultivar Liberty in 2001 at one site. Pruning resulted in no consistent difference in the apple tree canopy microclimate. However, strong pruning resulted in improved spray deposition in the tree canopy when applications were made with an airblast sprayer at midsummer. Results are compared with similar studies, and their biological interpretation is discussed.