Monday, April 29, 2019

Fire blight outlook Apr 29-May 4; secondary scab infection Apr 27-28.

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, April 29, 2019. Click to enlarge.
FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most apple trees still have susceptible bloom in the Winchester area. The temperature and rainfall data are current through Sunday evening, April 28. Predicted weather conditions are shown for April 29-May 4. 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 would be possible with wetting Apr 18, 23 and 24, and will again be possible with wetting May 1-4. 
Streptomycin application is recommended for tomorrow, Apr 30 or Wednesday, May 1 to protect 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 while late bloom is present. The BBS column tracks the appearance of blossom blight symptoms from infection that occurred Apr 19, which is predicted for May 2. The CBS column is tracking canker blight symptom appearance and predicts canker margin symptoms (CMS, expansion of overwintered cankers) is predicted Apr 30.

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 1-4 with infection possible if wetting occurs. 
Graphic from the NEWA/Cougarblight model, April 29, 2019. Click to enlarge.
Note that real fire blight risks can change quickly with warmer than predicted temperatures and wetting and, in high-risk situations, a streptomycin application is more effective when applied ahead of predicted infection.
We recorded a secondary apple scab infection period at Winchester Apr 27-28: 13 hr wet at 50° with 0.16 inches of rain. This would have been a secondary infection period where scab was not well controlled during the Apr 5-6 infection, resulting in scab lesions with spores now available for secondary infection.