NOTE: This year for fire blight risk assessment, we are again comparing posted graphics from Maryblyt 7, as in previous years, with those from the Cougarblight model as shown on our NEWA site. We will be using the same recorded weather data from our NEWA station to make these comparisons. We thank Dr. Mizuho Nita for hosting the Maryblyt 7.1 download site at: http://grapepathology.org/maryblyt
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.
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| Graphic from Maryblyt 7, April 17, 2019. Click to enlarge. |
FIRE BLIGHT: Above is a cropped graphic from the Maryblyt 7 program. Most cultivars now have some bloom open in the Winchester area. The temperature and rainfall data are current through Wednesday evening, April 17. Predicted weather conditions are shown for April 18-22. 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). For infection to be predicted, wetting must occur after the EIP (epiphytic infection potential) reaches 100 or higher, and this must coincide with an average daily temperature of 60°F or more. Based on recorded temperatures and wetting, the risk column shows high risk for Apr 13 and 16, but with an EIP less than 100; by Apr 18 the EIP reaches 100, but wetting is lacking. Apr 19 shows a significant risk of infection with all infection criteria met. For such situations a spray of streptomycin is recommended on Apr 18 or 19. Cooler temperatures Apr 20-22 should reduce fire blight risk.
Below is the graphic from the Cougarblight model as shown on our NEWA site. We selected orchard blight history option as “Fire blight occurred in your neighborhood last year” and first blossom open date as 4/13/2019. Cougarblight shows color-coded risk assessment as “Cougarblight 4-Day DH” risk is "caution" for Apr 18 and "high" for Apr 19-22. This is a rather important difference between the two predictive models. The apparent unusual difference in amount of risk predicted by Maryblyt and Cougarblight is because the EIP does not reach 100 (only 85) in Cougarblight but in Maryblyt it is 103 on Apr 18 and 116 on Apr 19 due to higher daily average temperatures, especially for Apr 18.
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| Graphic from Cougarblight, April 17, 2019. Click to enlarge. |
Risk can change quickly with warmer than predicted temperatures and wetting. In high-risk situations, a protective streptomycin application is recommended ahead of predicted infection. The fire blight outlook will be updated Apr 19.
