Effect of Powdery Mildew on Apple Yield and Economic Benefits of Its Management in Virginia.

Plots were established in a 'Ginger Gold' apple orchard to test the effects of different powdery mildew levels on cumulative yields. Disease pressure was heaviest in 1997 but yield effects were detected each year. Primary infection in 1997 and 1998 was correlated to previous foliar mildew incidence. Five myclobutanil applications followed by three of triadimefon (M5-T3) reduced primary infections in 1997 to 19% of those on trees not receiving a mildewcide (Ck). M5-T3 gave the most reduction in mildew incidence and severity but a schedule with two less applications (M3-T3) than M5-T3 gave the highest yield. Yields were lowest on trees not receiving a mildewcide. In 1998, the weakest and the strongest treatments had two and four times more flower clusters per tree, respectively, than Ck trees. Yield of the weakest mildew treatment (six applications of sulfur, S6), was about twice that of the Ck trees; several treatments yielded more than 2× S6. Increasing the number of sulfur applications from six to eight increased mildew control and yield. Most of the treatments resulted in yields higher than the nontreated control over a 3-year period. The M3-T3 treatment regime resulted in the highest 3-year yield, whereas a slightly more effective M5-T3 regime yielded slightly less. An economic analysis of cumulative treatment benefits, using local price inputs, is included.