Fire blight infection conditions on late bloom May 15; scab and rust infection May 14-15.

Fire blight infection conditions finally occurred at Winchester on late bloom May 15. Below is a cropped graphic from the Maryblyt 7 program.

Graphic from Maryblyt 7, May 18, 2020. Click to enlarge.

While most apple blocks were past bloom in the Winchester area, a few still had some susceptible bloom. The temperature and rainfall data are current through Monday evening, May 18. Predicted weather conditions are shown for May 19-31. The components of fire blight risk are indicated in the columns labeled B (blossoms open), H (degree hours for epiphytic bacterial populations), W (wetting by rain or dew), and T (average daily temperature 60 F or above). For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows the infection was possible wherever bloom was present May 15. Infection aslo would have been possible with wetting May 16-17. The extended outlook through May 31 is shown primarily to track infection development from May 15, but indicates that infection would be possible wherever there is bloom and wetting May 27-31.

The BBS column tracks the appearance of blossom blight symptoms from infection that occurred May 15, which is now predicted for May 27. The CBS column tracks canker blight symptom appearance and predicted canker margin symptoms (CMS, expansion of overwintered cankers) May 16. Further tracking in the CBS column predicts canker blight symptoms when that value reaches 100 May 28.

Under these conditions a Streptomycin application would have been recommended for May 14-15, to protect any late bloom through the infective period. This would particularly include any young and recently planted trees with flowers. Cooler predicted temperatures the next week indicate a decline in risk, but warmer temperatures will again bring the EIP to an infective level May 26. Note that "wetting" can occur with maintenance and thinning applications, so streptomycin should be included in such applications if there is late bloom. Also note that, while bloom may have escaped infection during this unusually low fire blight pressure year at Winchester, expect canker blight and shoot blight symptoms if cankers were allowed to overwinter in trees that had infection last year.

At the AREC last week an apple scab and cedar-apple rust infection period occurred May 14-15: 8 hr wet at 62° with 0.04 in. rain. Similar conditions also extended south to Staunton and east of the Blue Ridge from Manassas to Sperryville. Staunton and Sperryville to the Charlottesville area also had an infection period May 17. At Winchester, cedar-apple rust galls remain active after this wetting event with only 0.04 inches of rain.

Apple powdery mildew infection occurs on days without rainfall above 53°, and in the Winchester area we have had 16 days favorable for infection since spores were available on Mar 29. Secondary mildew symptoms are now quite common on unprotected trees, as are secondary scab and cedar-apple rust lesions.

Extended wetting is predicted for all major fruit production areas in Virginia May 18-22. This wetting will be highly favorable to secondary scab and late rust infection on foliage and will contribute to recorded accumulated wetting hours for development of sooty blotch and flyspeck (SBFS). For purposes of predicting the development of the SBFS fungal complex, we record accumulated wetting hours from rainfall or dew, starting 10 days after petal fall. This year we will use May 5 as the petal fall date for Winchester, so the start of wetting hour accumulation (ACW) will be from May 15. For the Roanoke area and areas east of the Blue Ridge, we will consider petal fall to have been one week earlier than Winchester, and accumulation of wetting hours will be from May 8. The action threshold for SBFS development is 250 ACW.