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Fire Blight – Remember last year’s infections are this year’s innocula

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With the large number of fire blight infections last year it is important to be on top of our management this year and assume fire blight is in the neighborhood.

Many of the young trees planted recently are susceptible to fire blight.

With more of the trees planted in the last five years being fire blight susceptible varieties, we have more fire blight around. You can see for example that the majority of trees less than five years old are susceptible varieties according to the National Ag Statistics Service Tree Survey compared to more than half less susceptible red delicious and granny smith in 2000.






We have had more days with exceptional risk in 2015, 2016 and 2017 = more fire blight = more inoculum.

Take a quick glance at the graph to the right compared to the graph to the left. Overall there were many more days with high risk in 2014 to 2017 compared to past years indicated by the peaks over the horizontal lines. That meant more fire blight then and means more inocula now.

Cougar Blight – Assume fire blight is in your neighborhood.

With the large number of infected blocks last year it is best to assume fire blight is in your neighborhood. If you are using the Cougar Blight model there are two settings. If you set it to say fire blight is not in your neighborhood it will use a higher risk threshold (500 TRV- see the orange line in the graph below).  If you set the model to say that fire blight is in the area the threshold for high pressure is 200 temperature risk values (TRV) (red line). You don’t want to assume you don’t have pressure when you do!


Tianna DuPont

WSU Tree Fruit Extension Specialist

(509) 663-8181


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