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Analysis of Incidence–Severity Relationships for Strawberry Powdery Mildew as Influenced by Cultivar, Cultivar Type, and Production Systems

The relationships between strawberry powdery mildew incidence (I) and severity (S) were investigated for various cultivars, for June-bearing and day-neutral cultivars, and for production systems (open-field and plastic-tunnel) with the objective of deriving a simple relationship for predicting severity (proportion of leaf area diseased [PLAD]) from incidence (proportion of diseased leaves). Data were collected from 2006 to 2011 at 11 commercial and experimental sites, for a total of 2,326 observations (n). For the cultivars grown in open fields, higher severity was observed on ‘Seascape’, with mean PLAD of 0.299 (n = 427); followed by ‘Chambly’, with 0.133 (n = 334); ‘Cavendish’, with 0.115 (n = 250); ‘Darselect’, with 0.111 (n = 321); and ‘Jewel’, with 0.105 (n = 276). In general, mean severity was higher when the strawberry plants were grown in plastic tunnels, with PLAD of 0.204, 0.199, and 0.181 for Chambly (n = 204), Darselect (n = 261), and Jewel (n = 253), respectively. A linear model based on complementary log-log transformation of I and S provided a good fit of the data (coefficient of determination [R²] adjusted for degrees of freedom from 0.82 to 0.96). A covariance analysis indicated that the sampling year and site of sampling did not significantly influence the estimated slope of the I–S relationship, nor did the specific cultivar among the June-bearing ones, whereas the production system (open-field versus plastic-tunnel) and the cultivar type (June-bearing versus day-neutral) significantly influenced the estimated slope. From this analysis, we were able to develop three specific models for open-field-grown June-bearing cultivars (R² = 0.90), for the open-field-grown day-neutral cultivar (Seascape, R²= 0.91), and for June-bearing cultivars grown in plastic tunnels (R² = 0.92). From these results, it was concluded that strawberry powdery mildew leaf severity can be accurately estimated from incidence of diseased leaves. The I–S relationships developed in the present study may be used in making practical disease management decisions, especially for management programs that use information on disease level in the field to initiate fungicide spraying programs or to time the interval between sprays.