Fungal disease and fire blight update

Primary apple scab lesions were observed today on unprotected trees at our AREC. Size and positions of infected leaves and appearance of lesions suggest that some could have been from infection Mar 30-Apr 1 and some from Apr 8-9. It is likely that they were present during infection periods Apr 22-23 and Apr 24-25 and, as such, could have provided secondary inoculum for heavy fruit infection in poorly protected orchards. Last night's wetting with 1.7 inches of rain was not quite long enough for scab or rust infection, but the 11-hr wetting event at 60-69° F Apr 24-25 favored quince rust of blossoms and cedar-apple rust infection of leaves and fruit, as well as scab. Quince and rust gall inoculum is still available on the cedar trees in our area.

Since primary powdery mildew inoculum was first observed Mar 23, eleven dry weather days have been favorable for mildew infection.

FIRE BLIGHT 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 our intent. Fruit producers outside of that area are encouraged to consult their state extension specialists for information similar to that provided here.

FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Some apple trees have reached petal fall but others still have much bloom still open and susceptible. Late cultivars such as Rome and York may be near full bloom. The temperature and wetting data are current through Thursday morning, April 28. Predicted weather conditions are shown for April 28-May 2. 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 predicted temperatures and wetting, the risk column shows that fire blight infection is possible with rainfall today, Apr 28 (note red color) and would again be possible if wetting were to occur Apr 29-30. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. This year’s bloom season has seen considerable sustained fire blight pressure and some infection will be expected in commercial orchards. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The BBS and CBS columns are tracking predicted blossom blight symptom (from infection Apr 11 and later ones as a, c and d) and canker blight symptom appearance, respectively. Numbers in these columns represent percent of required degree hours for early blossom symptom appearance and canker margin symptoms Apr 26. Today, early symptoms are evident from experimental inoculations done Apr 23. Today’s update was a result of changes in the weather forecast from yesterday. RISKS CAN INCREASE QUICKLY WITH WETTING FOLLOWING WARMER THAN PREDICTED TEMPERATURES. Growers are advised to be alert to changes in forecasts on a day to day basis and protect high-risk blocks through late bloom as needed. This graphic will not be updated until May 2 unless some significant change warrants it. The weather conditions used in the predictive part of this graphic come from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.