Fire blight update and scab infection, Apr. 20

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. Earlier apple cultivars are approaching full bloom open, late ones have some bloom open, and more bloom will open over the several days. The temperature and rainfall data are current through Wednesday morning, April 20. Predicted weather conditions are shown for April 20-24. 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 if it rains later today, Apr 20 (note red color). (UPDATE Apr 21: We did not have the wetting from rainfall required for fire blight infection at the AREC yesterday; however, on a day when all the other requirements are met, wetting by a spray application can initiate infection). For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. Following today’s potential infection, risk will subside with cooler temperatures but will again reach an infective level with rain Sunday, Apr 24. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. An application today should protect for both of these high-risk periods. This graphic will be updated April 22, or sooner if significant change warrants it. The BBS and CBS columns are tracking predicted blossom blight symptom (from infection Apr 11) and canker blight symptom appearance, respectively. The numbers in these columns represent percent of required degree hours for symptom appearance. 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.

Length of wetting from 9 pm Apr 19- 8 am Apr 20 was just long enough for a scab infection period: 12 hr at 50-54F. There had been wetting earlier with 0.4 inches of rainfall on Apr 19 but that was not included because of 5-hr drying in the interim. Rusts, and especially quince rust, are still a threat, but rust-prone blocks covered with an SI fungicide would have been well-protected for this event.