Wednesday, July 22, 2020

Bitter rot; sooty blotch, flyspeck threshold and activity

Conditions have been dry at Winchester, with only three extended wetting events at Winchester in the past month:  June 27-28: 9 hr of wetting at 72° with 0.17 in. rain, July 10-11: 6 hr of wetting at 71° with 0.07 in. rain, and July 21-22: more than 10 hr of wetting at 72° with 0.28 in. rain. In spite of the rain received July 21, the Winchester area is still experiencing drought, with July rainfall total at Winchester AREC only 0.49 inches.

Bitter rot (shown below) was observed in a organic production block July 21. This indicates that, although weather has been dry the past month, there was ample opportunity earlier for bitter rot infection which is now evident. 
Bitter rot in Honey Crisp apples July 21, 2020. Likely inoculum source was a large overwintering fruit mummy (upper left).
Bitter rot in Honey Crisp apple July 21, 2020. Likely inoculum source was small, current season, fruit mummies (upper left).

Other areas of Virginia have received variable amounts of rainfall in the past month. Some areas have been dry like Winchester, while others have received adequate moisture. Few have had excessive rainfall, but all could have bitter rot conditions as favorable as those in Winchester.

To predict the development of the sooty blotch and flyspeck (SBFS) fungal complex, we record accumulated wetting hours (ACW) from rainfall, fog, or dew, starting 10 days after petal fall. The action threshold of 250 ACW predicts that the SBFS fungi are present on unprotected fruit when the threshold is reached, and symptoms will soon appear with further incubation. After several weeks delay, Winchester and Sperryville have now exceeded the 250-hr threshold. Here is a listing of selected petal fall dates and total wetting hour accumulation (ACW) for selected locations as of the night of July 21: Winchester, May 5, 271 ACW; Staunton, May 5, 591 ACW; Roanoke, Apr 18, 423 ACW; Floyd, Apr 28, 342 ACW; Manassas, Apr 28, 461 ACW; Sperryville, Apr 28, 272 ACW; Crozet, Apr 18, 332 ACW; Carter Mountain, Apr 18, 473 ACW; Red Hill, Apr 18, 375 ACW; Lynchburg, Apr 13, 488 ACW; Rustburg, Apr 13, 361 ACW; Danville, Apr 13, 667 ACW.