Categorization of apple cultivars based on seasonal powdery mildew disease progression in two disease management systems over 12 years

Abstract

Five cultivar classification categories were determined from season-long powdery mildew disease progression in integrated and organic production systems. Parameters derived from the progress curve categories were suggested for disease warning improvements. The goal of this 12-year study was to evaluate the powdery mildew infection rates of nine resistant, nine commercial, and nine old apple cultivars in two production systems: integrated and organic. Cultivars were classified into five categories, from low to high, based on their shoot and fruit disease progress curves assessed during the season with the disease progress analysed separately in all five classification categories. Powdery mildew incidence at harvest had increased considerably more for both fruit and shoots in the organic than in the integrated plots, except for the shoots of cultivars ‘Reglindis’, ‘Reka’, ‘Rewena’, ‘Parker Pepin’, and ‘Kings of the Pippin’. Control efficacy was acceptable for all resistant and most old cultivars in both production systems (final disease incidence was below 7.5%), but the level of powdery mildew incidence at harvest was middle-to-high or high for cultivars ‘Idared’, ‘Jonagold’, ‘Jonathan’, ‘Jonica’ and ‘Royal Gala’ from the commercial cultivar group in both production systems. Cultivar categorization for powdery mildew showed that season-long disease progress was low for the old; low-to-middle for the resistant; and middle-to-high for the commercial cultivars. A three-parameter logistic function was fitted to the temporal progress data of each classification category. Then the upper asymptote (Y f), rate parameter (β), and inflection point (M) were estimated from the fitted function and the standardized area under the disease progress curves (AUDPCS) were also calculated. Results demonstrated that AUDPCS and partially Y f were able to differentiate the five mildew classification categories for both shoot and fruit in both production systems. Therefore, AUDPCS and Y f were suggested as input parameters in disease warning systems for measuring host resistance. Yields in the integrated plots were higher compared with those in the organic ones. Powdery mildew classification categories, together with yield, can contribute to a more appropriate cultivar selection for establishing orchards managed by environmentally friendly approaches.