Classification of apple cultivar reactions to scab in integrated and organic production systems

Abstract

In a 7-year Hungarian study, 27 apple cultivars (9 scab resistant, 9 commercial, and 9 old) were evaluated for scab resistance in high-density integrated and organic apple orchards; classified into five susceptibility categories (low, low to middle, middle, middle to high, and high) based on their season-long leaf and fruit scab development; and the temporal progress of scab in each classification category was analyzed. Mean final scab incidence was considerably higher in the organic than in the integrated production system except for the resistant cultivars, which had no fruit scab symptoms. Fruit of all cultivars was protected in the integrated production system (final fruit scab was below 1.1%), but most commercial cultivars (‘Idared’, ‘Jonagold’, ‘Jonathan’, ‘Jonica’, ‘Mutsu’, and ‘Royal Gala’) were moderately or highly infected by harvest in the organic production system. Categorization of cultivar susceptibility indicated that season-long leaf and fruit scab development was low for the majority of resistant cultivars and moderate to high or high for most commercial cultivars. The resistance of old cultivars to scab ranged from low to moderate to high. Temporal scab progress of each classification category was analyzed by (i) calculating standardized area under the disease progress curves (AUDPCS); (ii) fitting a three-parameter logistic function to the disease progress data; and (iii) estimating upper asymptote (y f), relative rate parameter, and the inflection point from the fitted function. Results demonstrated that y f and AUDPCS showed significant differences (P < 0.05) among each scab classification category of leaf and fruit in both production systems; therefore, y f and AUDPCS can be used to differentiate the relative resistance of apple genotypes. Classification categories can assist in the selection of the most appropriate cultivars for planting in integrated or organic apple orchards.