Heavy scab infection period May 10-12.

We recorded another heavy combined apple scab infection period at Winchester over this past weekend: May 10-13 (47 combined hr wet at 53° with 1.44 inches of rain). Scab lesions have been observed on unprotected trees in the Winchester area. A follow-up fungicide application with after-infection scab activity is suggested. Cedar-apple rust gall inoculum is now mostly depleted.

Also, we have had 20 days favorable for powdery mildew infection since spores were available at Winchester Apr 6. 

Heavy scab and rust infection, May 3-6

We recorded two heavy apple scab and rust infection periods at Winchester over the weekend: May 3-4 (19 hr wet at 64° with 0.4 inches of rain) and May 4-5 (more than 21 hr wet at 62° with 2.18 inches of rain). The last wetting period, with heavy rainfall, is still in progress. This amount of rainfall depleted any fungicide residue applied last week, resulting in potentially heavy rust and scab infection to foliage and fruit. A follow-up fungicide application with after-infection activity is suggested.

Also, we have had 18 days favorable for powdery mildew infection since spores were available at Winchester Apr 6. The fire blight outlook for Winchester remains much as indicated in the post on May 2: the risk of infection remains high wherever there is late bloom present.

Fire blight outlook into next week

CAUTION: The observations, conditions, and recommendations reported for Winchester, VA are provided as a guide to fire blight risk assessment only for the immediate area of the Virginia Tech AREC located six miles southwest of Winchester. Fruit producers in other areas are encouraged to consult their state extension specialists for information similar to that provided here.

Graphic from Maryblyt 7, May 1, 2019. Click to enlarge.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. We have moved through bloom in a timely fashion in the Winchester area, but many apple blocks still have some susceptible bloom. The temperature and rainfall data are current through Wednesday evening, May 1. Predicted weather conditions are shown for May 2-6. 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 Apr 19 and 25, and was possible with wetting Apr 18, 23 and 24, and will again be possible with wetting May 2-4. The temperature is marginally close for infection May 5. The extended outlook May 6 through May 10 (not shown) indicates infection is possible wherever there is bloom and wetting through May 10.

Streptomycin application is recommended for tomorrow, May 2, to protect any late bloom into the weekend. Also, remember to protect any young and recently planted trees that have flowers. Predicted temperatures into next week (not shown) indicate that infection will remain possible wherever there is late bloom and wetting every day next week. Note that "wetting" can occur with maintenance and thinning applications, so streptomycin should be included in such applications where there is late bloom. 
The BBS column tracks the appearance of blossom blight symptoms from infection that occurred Apr 19, which is now predicted for May 3. The CBS column is tracking canker blight symptom appearance and predicts canker margin symptoms (CMS, expansion of overwintered cankers) is predicted May 1. Further tracking in the CBS column will predict canker blight symptoms when that value reaches 100.

Below is the graphic from the Cougarblight model as shown on our NEWA site. with “Fire blight occurred in your neighborhood last year” selected as the orchard blight history option and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessments as “Cougarblight 4-Day DH” and infection potential EIP value. This risk is "extreme" for May 2-6, with infection possible if wetting occurs May 2-4. The average temperature of 60 F is lacking for May 5.

Graphic from the NEWA/Cougarblight model, May 1, 2019. Click to enlarge.

Fire blight risks can change quickly with warmer than predicted temperatures and wetting; in high-risk situations, a streptomycin application is more effective when applied ahead of predicted infection. Unless unexpected weather conditions arise, this will be the last fire blight update for this season.