Fire blight and fungal disease update

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. Use of the information reported here for making orchard management decisions outside of that area is not my intent. Fruit producers outside of that area are encouraged to consult their state extension specialists for information similar to that provided here.

(Click image for larger view)

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most cultivars still have much susceptible bloom. Here is the prediction for trees with first bloom open Apr 17-18, 2015. The temperature and rainfall data are current through Monday morning, May 4. Predicted weather conditions are shown for May 4-9. 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). Based on recorded temperatures and wetting, the risk column shows that fire blight infection was possible Apr 20 and would have been possible with slightly warmer average temperatures for Apr 19 and 21. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. 

Based on predicted temperatures, EIP will remain at an infective level through this entire week, requiring only wetting to trigger infection conditions wherever susceptible bloom is present. Wetting from rain is shown as being predicted for May 5 and 6; other forms of wetting capable of triggering infection conditions include heavy dew, fog or routine spray applications-- any wetting event that will move the fire blight bacteria from the flower stigma into the nectaries in the base of the flower. It is recommended that streptomycin be included with any routine spray applications in high risk situations this week. Under the temperature conditions shown for this week, expect streptomycin to remain residually effective for only about 3 days.

The BBS column is tracking predicted appearance of blossom blight symptoms from infection that occurred Apr 20, and this is predicted for May 6. (Actually we are already seeing early symptoms from test inoculations Apr 20). The CBS column at the right in the graphic predicts the appearance of canker margin symptoms due to extension of overwintering cankers by Apr 30. Canker advancement cannot be prevented by treatment at this time and the presence of symptoms will signal build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc.

The weather conditions used in the predictive part of this graphic come from the WeatherChannel for Winchester, supplemented by site-specific data from SkyBit Inc. Be aware that risk can change quickly with unpredicted warmer temperatures and wetting.

FUNGAL DISEASES

We recorded our 7th apple scab infection period of the year May 1-2 (15 hr at 58-53° with 0.21 in. of rain). This also favored quince rust fruit infection and cedar-apple rust on foliage and flowers. Apple scab lesions were first observed at our AREC last week; now some leaves on unprotected trees have as many as six lesions per leaf. Also present are cedar-apple rust lesions, probably from infection that occurred April 14. 

Powdery mildew is our only "dry weather" fungal disease on apples. So far, we have had 13 “mildew infection days” since spores first became available Apr 6.