Simultaneous use of infection criteria for three apple diseases for timing of fungicide sprays

The feasibility of using criteria for infection by Botryosphaeria obtusa, Venturia inaequalis, and Gymnosporangium juniperi-virginianae on foliage of apple (Malus X domestica) in a combined weather-based forecasting system for frogeye leafspot, apple scab, and cedar-apple rust was evaluated through computer simulation and in a field study. Ten sets of historical weather data from two locations in North Carolina were analyzed. Using a 7-day minimum waiting period between eradicant sprays, eight to 15 fungicide applications per season were required. More applications were required with the forecaster than with a typical calendar-based spray program for five data sets; the same number of sprays were advised for one data set; and less spraying was advised with the forecaster for four data sets. With a 14-day minimum waiting period between fungicide applications, six to nine sprays per season were advised. In a field trial, weather-based eradicant sprays of penconazole or tebuconazole resulted in similar levels of frogeye leafspot and lower levels of scab and rust as compared to the levels resulting from the standard calendar-based protectant program (mancozeb + benomyl at 2-wk intervals). However, more sprays were required in the weather-based program using a 7-day minimum waiting period between eradicant sprays than for the calendar-based program. Levels of all three diseases were similar in a 14-day protectant program using either tebuconazole or penconazole as compared to eradicant programs of the same fungicides. Apple seedlings were set outdoors and exposed to natural inoculum of B. obtusa, G. juniperi-virginianae, and V. inaequalis for 18 individual wetting periods to evaluate the effect of eradicant sprays on subsequent disease development. In all cases in which infection occurred, application of an eradicant spray of tebuconazole resulted in reduction of the three diseases as compared to that on a nonsprayed control.