Fungicide Spray Volume and Pruning Impact Performance of a Warning System for Sooty Blotch and Flyspeck on Apple

The sooty blotch and flyspeck (SBFS) fungal complex degrades apples by blemishing the surface, reducing the fresh market value up to 90%. In the Upper Midwest United States, growers apply 3 to 8 fungicide sprays starting shortly after petal fall until harvest. The Brown/Sutton/Hartman SBFS warning system uses the duration of daily wet periods to extend the interval between the fungicide spray that is applied 10 days after petal fall (“first cover spray”) and the subsequent spray (“second cover spray”) to until a 175-h cumulative threshold is reached. When we tested this warning system in the Upper Midwest, SBFS incidence was significantly higher in the warning system treatments than in the calendar-based controls in 12 of 28 demonstration trials in commercial orchards. The causes for failure of the warning system were uncertain, but may have been related to pruning and spray volume practices by individual growers. In this study, we compared the effects of pruning and spray volume on the performance of the Brown/Sutton/Hartman warning system in Wisconsin and Iowa. Results of six orchard-year replicated trials and nine non-replicated demonstrations in commercial orchards suggested that both annual pruning and spray volume of at least 100 gal/acre enhance the success of this SBFS warning system and these practices were more beneficial with semi-dwarf than fully dwarf trees. Accepted for publication 17 June 2013. Published 30 September 2013.