Apple Alert: Blossom Blight Infection: April 14, 2021

— Written By

The risk of a blossom blight infection event is HIGH for Henderson County (NC) for Wednesday, April 14. The risk of a blossom blight infection event is EXTREME for Polk County (NC) for Wednesday, April 14. The risk of a blossom blight infection event for Moravian Falls area is HIGH for Wednesday, April 14.

blossom blight

  • High Risk: If there are open blossoms in your block, a high risk of a blossom blight infection event means that 2 of 3 requirements (see Bloom Disease Update ) are satisfied for blossom blight to occur. A preventative streptomycin or other bactericide application should be considered prior to a wetting period (dew or precipitation).
  • Extreme (“Infection”) Risk: If there are open blossoms in your block, an extreme risk of a blossom blight infection event means that 3 of 3 requirements (see Bloom Disease Update) are satisfied for blossom blight to occur. A preventative streptomycin or alternative bactericide application should be made prior to a wetting period (dew or precipitation).

Additional Information on Blossom Blight Forecasting

All unprotected, open, and intact blossoms are susceptible to fire blight caused by the bacterial pathogen, Erwinia amylovora. Blossom blight is initiated during the spring as the pathogen, overwintering in cankers, begins to ooze and is disseminated to open flowers, primarily by rain splash or by insects. Upon arrival to an open blossom, bacterial multiplication occurs on the stigmatic surface of the flower, provided that enough heat units have accumulated to support reproduction. At this point the flower is not considered infected and the presence of the bacteria on the stigmatic surface presents an ideal time for agrichemical management. Even the infection of a few blossoms can set the stage for catastrophic losses later in the season, due to the systemic movement of the pathogen through xylem tissue.

Fortunately, there are two models that stakeholders can rely on to predict blossom blight infection: Cougar Blight and Maryblyt. Both programs can be downloaded from the internet, although Maryblyt is only available for Windows systems. Both models will require weather data inputs (temperature and precipitation/dew). Alternatively logic from both models is available on the NEWA Apple Disease Models website. While anyone can access the NEWA site, blossom blight infection predictions will only be provided for those orchards with a Rainwise Weather Station or Onset Weather Station linked to the NEWA system. Most years, blossom blight infections occur less frequently than one might think. According to the Maryblyt blossom blight infection model, four conditions must be met in order for blossom blight to occur:
  1. Flowers must be open and intact: Once the flower is in petal fall it becomes resistant.
  2. The epiphytic inoculum potential (EIP) must be reached: The accumulation of a certain number of heat units during bloom must be reached to support bacterial multiplication of the flower’s stigmatic surface. For MaryBlyt, any EIP over 100 satisfies this requirement.
  3. A wetting event such as dew or rainfall greater than 0.01 inches: This requirement enables the movement of the bacteria from the stigmatic surface to the flower nectarthodes where infection occurs.
  4. Average daily temperature must be above 60F: Increases bacterial multiplication rate on stigma as well as migration rate to the nectarthodes.

Written By

Sara Villani, N.C. Cooperative ExtensionDr. Sara VillaniExtension Specialist (Apple and Ornamental Plant Pathology) Call Dr. Sara Email Dr. Sara Entomology & Plant Pathology
NC State Extension, NC State University
Posted on Apr 13, 2021
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