Fire blight a concern as bloom approaches in the north

If forecasts are correct, the epiphytic infection potential is predicted to climb to over 100 by this weekend in northern Michigan orchards where bloom began May 13-21.

As we approach bloom, it will be important to keep an eye on the fire blight model at Enviro-weather to determine if antibiotic applications are needed. When the epiphytic infection potential (EIP) reaches 100 (or is forecast to do so) and the average temperature is greater than or equal to 60°F, the corresponding boxes on the model will turn red, indicating that rain or trauma such as high winds or hail is all that is needed for infection to occur (see picture of model below). At this time, the EIP is predicted to climb to over 100 by this weekend in orchards where bloom began May 13-21. Similar conditions are present at all northern Michigan weather stations.

Traverse City Fire Blight Assist Chart 

Locate the Biofix Date (the date bloom opened or the date a spray was applied to control fire blight) on the top row. Follow that column down to determine EIP for that block on each date in the left column. If this number is greater than 100, and the average temperature is greater than or equal to 60°F, this area will be shaded and rain or trauma (high winds or hail) is all that is needed for infection. Repeat for additional blocks that bloomed or were sprayed on a different date.

At this time, streptomycin remains the bactericide of choice for controlling fire blight, however, if you are located in Grand Traverse County and have streptomycin resistance, Kasumin may be applied. See MSU’s Plant Pathology professor George Sundin’s article, Section 18 Special Exemption Lable for Kasumin for Fire Blight Control In 2011, for more information on Kasumin use.

As temperatures fluctuate and scattered rainfall occurs around the region, it is difficult to gauge fire blight infection potential, but we don’t want to waste an antibiotic spray if it is not truly necessary. If the fire blight model’s EIP is close to but not at 100, there are a few rules of thumb to determine if an antibiotic application is warranted:

  • A block with a history of fire blight,
  • Susceptible varieties, or
  • Visible cankers are all good reasons to go into a rainy period with an antibiotic spray.

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