Fire blight and fungal disease update, April 25

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: The fire blight outlook has varied with a changing forecast the past several days. Above is a cropped graphic from the Maryblyt 7 program. Most apple cultivars at our AREC have bloom open, some are nearing full bloom and many flowers are expected to open over the next several days. This prediction is for trees with first bloom open April 20. The temperature and rainfall data are current through this morning,  April 25. Predicted weather conditions are shown for April 25-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). Based on current predicted temperatures and wetting, the risk column shows that fire blight infection will remain moderate until May 1. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. Following briefly higher risk on May 1, the current forecast shows cooler predicted temperatures which hold the risk at a low or moderate rating through May 4. These cool temperatures will also prolong the bloom period and increase the possibility of fire blight infection with warmer temperatures on late bloom.

This graphic will be updated April 28. 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.

Other diseases: Extended wetting at relatively warm temperatures tonight will likely result in apple scab and rust infection, and blossoms are now very susceptible to quince rust. Rain and shower activity next week Apr 28-30 will also favor rust and scab infection. Expect scab lesions from April 11-15 infection periods to be present during Apple Blossom Festival weekend.

Powdery mildew spores were available under favorable dry weather infection conditions by Apr 9, and we have had 12 dry weather “mildew infection days” since Apr 9.