Fire Blight (not yet!)

Thursday noon, May 8, 2014: With apples and pears in bloom and several days of intermittent rainfall, many folks are probably wondering about the risk of fire blight.  I just finished running the MaryBlyt model using weather data from the Highland-HVL NEWA weather station along with forecasted temperatures from Accuweather for the next 7 days.

The MaryBlyt model indicates that we will not accumulate enough degree hours (DH) to trigger blossom blight until Sunday, May 11, if current forecasts hold.  Even on Sunday, the accumulated DH will barely reach threshold levels required for infection.  HOWEVER, if forecasts hold, then by next Tuesday we will have accumulated enough heat to be at 2.5 times threshold levels, and that level of risk has the potential to result in extremely serious blossom blight infections. (Click to see the MaryBlyt output, including forecasted temperatures through May 15).

Note that MaryBlyt is a different model than Cougar Blight.  The NEWA website uses Cougar Blight and therefore may differ slightly from the projections provided by MarbyBlyt.  I prefer results from MaryBlyt which was developed using more east coast data, but either model is acceptable.  For those who wish to run MaryBlyt using their own weather data, you can access the free software at

Streptomycin protects only those blossoms that are open at the time of application.  As a result, there is no benefit from applying strep sprays today or tomorrow so as to get the jump on protection for Sunday. Strep sprays applied today or tomorrow will be a total waste of money and WILL NOT be adequate to protect trees on Sunday because a large number of new flowers will be opening over the next few days.  Thus, if current forecasts hold, the best time to spray strep (weather conditions permitting) will be Saturday evening or Sunday morning.  Even applications made within 24 hr after infection event are better than those made two or three days too early because strep has about 24 hr of post-infection activity and all flowers open during the infection period are accessible for spray coverage in post-infection sprays.  However, we generally prefer sprays applied just ahead of infection periods to those applied after infection periods because bad weather after an infection period may limit spray coverage.

Within the MaryBlyt model, infection can occur only when four criteria are all fulfilled simultaneously:
1. There must be open flowers.
2. Accumulated degree hours (base 65 F.) since the flowers opened must total almost 200.
3. Mean temperature for the day must be at least 60 F.
4. There must be moisture (rain, dew, sprays) on the day of infection or the prior day to allow the bacteria to move from the stigma down into the nectaries at the base of the flower where infection occurs.

The model incorporates other complex factors to account for the effective “lifetime” of a flower after it opens along with some other details, but the four items listed above are the key factors.  Given our current temperatures, Items 2 and/or 3 are missing because temperatures are too cold.

Organic growers have have a new option for controlling blossom blight this year:  Blossom Protect (BP) is a biocontrol fungicide that has worked pretty well in tests on the west coast.  Unlike the case with streptomycin, however, BP absolutely must be applied as suggested on the label, with the first application at about 10% bloom and additional applications at roughly 40, 70, and 90% bloom.  The biocontrol organisms in BP require time to become established on the flowers before the fire blight pathogen arrives.  Thus, it cannot be applied at the last minute prior to potential infection periods as we attempt to do with streptomycin.  As noted on the label, application of BP after mid-bloom may, in some years, contribute to fruit russetting on russet-susceptible cultivars such as Golden Delicious.

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