Increasing summer disease pressures

At Winchester last week we had two significant extended wetting events favorable for SBFS and fruit rot development: June 15-16: 11 hr at 76-72°, with 0.46 in. rain, and June 16-17: 15 hr at 69-63° with 1.95 in. rain. Much of this rain came at the beginning of the event, and this volume of rainfall would have depleted much of the protective fungicide residue, leaving the fruit exposed to potential rot infection.

For purposes of predicting the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we began wetting hour accumulation at Winchester from May 14. As of Monday, June 20, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 257 hr (with the 250-hr threshold reached June 5); at 952 ft elevation, 248 hr (near the threshold); and at the 983 ft elevation, 218 hr ACW.

At Tyro, VA we started wetting hour accumulation May 4, and as of June 20, a sensor placed at the 941 ft elevation had accumulated 315 hr (with the 250-hr threshold reached June 6); the one at 1165 ft. elevation had accumulated 188 wetting hours, and the one 1465 ft. elevation had accumulated 213 wetting hours. 

At both Winchester and Tyro, most of the wetting hour accumulation at the higher elevations has been only with rain events, while accumulation at the lowest elevation has been with dew as well as rain events.

Summer disease management should be directed toward the array of potential problems in specific orchard blocks, utilizing protective fungicides for general purposes, but supplementing them with more specifically active fungicides directed at higher disease pressures and special control problems. Expect higher fruit rot pressures where fire blight killed shoots and fruit mummies are present in the trees. For more information on the effectiveness of apple fungicides for management of sooty blotch/flyspeck, Brooks fruit spot, black rot, white rot and bitter rot consult Table 6 (pp. 53-54) in the 2016 Spray Bulletin for Commercial Tree Fruit Growers.