Tuesday, June 9, 2020

Extended wetting events and accumulated wetting hours favor early rot development

Two extended wetting events at Winchester last week favored secondary apple scab and cedar-apple rust infection and early rot development.  These occurred June 4-5: 17 hr of wetting at 69° with 0.83 in. rain and June 5-6: 18 hr of wetting at 69° with 0.42 in. rain. Wetting was variable across other commercial fruit production areas of VirginiaApple powdery mildew infection occurs on days without rainfall above 53°, and the Winchester area we have had 33 days favorable for infection since spores were available on Mar 29.

To predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall or dew, starting 10 days after petal fall. The action threshold for SBFS development is 250 ACW. Here is a listing of selected petal fall date and total wetting hour accumulation (ACW) for selected locations as of June 9: Winchester, May 5, 112 ACW; Staunton, May 5, 217 ACW; Roanoke, Apr 18, 205 ACW; Floyd, Apr 28, 188 ACW; Manassas, Apr 28, 182 ACW; Sperryville, Apr 28, 146 ACW; Batesville, Apr 18, 208 ACW; Crozet, Apr 18, 178 ACW; Carter Mountain, Apr 18, 314 ACW; Red Hill, Apr 18, 189 ACW; Lynchburg, Apr 13, 243 ACW; Rustburg, Apr 13, 247 ACW; Danville, Apr 13, 331 ACW. 

ACW is related to wetting from rainfall, fog, or dew. The action threshold of 250 ACW signals that the SBFS fungi would be present on unprotected fruit when the threshold is reached and symptoms would appear with further incubation. Carter Mountain and Danville have passed the threshold, and Lynchburg and Rustburg are close to the threshold.