Disease update May 23.

We have some continuing apple scab activity, with a secondary infection period across the region from Winchester southward to Roanoke and east of the Blue Ridge May 21-22. Length of wetting varied from 7-16 hr at temperatures of 58-70° and mostly less than an inch of rain. More extended wetting is forecast for May 23-24. Later this week and early next week we expect secondary scab lesions to appear from the heavy secondary infection period May 11-13.  

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 chose Apr 24 as our petal fall date for Winchester, so the start of wetting hour accumulation was from May 4. As of May 22, accumulated wetting hours (ACW) toward the 250 wetting hour threshold for specific treatment against the SBFS fungal complex were: at 909 ft elevation, 127 hr; at 932 ft (the AREC NEWA station), 109 hr; and at the 983 ft elevation, 84 hr ACW. In the past week some wetting hour accumulation occurred from both rainfall and dew. 

For central Virginia, we have selected Apr 20 as the petal fall date, and accumulation of wetting hours from Apr 30 is recorded by three weather stations at different elevations at Tyro. As of May 22, a sensor placed at 1465 ft. elevation had accumulated only 35 wetting hours from Apr 30, while the one at 1165 ft. had accumulated 83 wetting hours, and the one placed at 941 ft elevation had accumulated 115 hr.

Apple powdery mildew remains active on susceptible new growth. Mildew is a dry weather disease, and conidiospores were available for infection at Winchester as early as Mar 27. Since then we have had 23 days suitable for mildew infection through May 22. 

East of the Blue Ridge we have seen several examples of fire blight blossom infection.