FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most apple cultivars are past petal fall but some still have scattered late bloom. The temperature and wetting data are current through Monday morning, May 6. Predicted weather conditions are shown for May 9-14. 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 could have occurred last Friday, May 6, and again where late bloom persists, May 13. Actually, after tomorrow, infection could occur anywhere that late bloom remains and wetting occurs each day this week. Be aware that wetting by a fruit thinning or maintenance spray can result in infection if susceptible blossoms are available. Streptomycin should be included in such applications if all infection criteria are met except wetting. For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher. With five consecutive days possible for infection Apr 24-28, we have seen the most sustained fire blight pressure since 2004, and some infection will be expected in commercial orchards. The BBS column is tracking predicted blossom blight symptoms from the first ten infection periods starting Apr 11 as “a”, then Apr 14, 20 and 24-28 as b-h. Some blossom cluster symptoms might be appearing now in local orchards. Growers are advised to protect high-risk blocks through late bloom as needed. The CBS column predicts that canker blight symptoms on new growth, due to extension of overwintering cankers from last year, will begin to appear May 11. Canker advancement cannot be prevented by chemical treatment at this time and the presence of symptoms indicates a build-up of inoculum which could become a factor in the event of a trauma blight situation due to hail injury, etc. Fire blight cankers should also be recognized as a warning for potential fruit rot problems later in the season; bitter rot (Colletotrichum spp.) spores can be produced on blight-killed twigs and spurs within six weeks after the initial fire blight infection.
With only late bloom remaining, this will be the last Maryblyt graphic update for this season. The weather conditions used in the predictive part of this graphic came from the Weather Channel for Winchester, supplemented by site-specific data from SkyBit Inc.
