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WSU Tree Fruit

Watch for Fire Blight

Tianna DuPont, Tree Fruit Extension Specialist

Warm temperatures and moisture during bloom made fire blight pressure high this year. Even where preventative control strategies were employed we are seeing damage in some fields. This is not surprising since fire blight conditions occurred during full bloom. Many flowers equals many infection points. 95% control of many many flowers still leaves a significant number effected. Keep your eye out for blight and make summer cuts at least 12” below the edge of the visible canker. The blight you see has usually traveled much further into the plant than the observable damage. Disinfest tools between cuts with bleach or alcohol. New research has shown that treatment of trees with the chemical, Acibenzolar-S-methyl (ASM, Actigard 50 WG), may reduce re-occurrence of blight after cutting out infected strikes see below. If there is still bloom in your orchard don’t forget to keep protecting it.

Re-occurrence happens when the act of cutting out the disease does not completely remove the pathogen cells that have moved ahead of the expanding canker. This is generally more of a problem in younger trees (3 to 10 years old) where the residual pathogen cells left in the tree can re-initiate disease development.

Plants have defense systems. If something stimulates the plant’s defense response before the symptoms develop (or re-develop), the plant will be in an active defense mode and will be less affected by disease when it occurs (or re-occurs). Actigard is a compound that has been found to trigger induced resistance. Its mode of action is to mimic the plant hormone, salicylic acid, which is responsible for priming the plant’s defense system.

For five years, Dr. Ken Johnson of Oregon State University has found that painting a concentrated solution of Actigard on trees after cutting out infection reduced the severity of re-occurring fire blight cankers in pears. For example, he found that without treatment after cutting out fire blight cankers in young Bosc pear trees, the disease came back 50% of the time and began to run through the tree. With Actigard applications, both the proportion of trees in which fire blight came back and the rate of canker expansion was reduced (Johnson and Temple 2016).

First cut out strikes with a cut at least 12” below the edge of the visible symptom. ‘Paint’ (generally with a small sprayer) concentrated Actigard with an up and down motion to a ½ meter length of the central leader or major scaffold near where the blight was. Use the labeled rate of 1 oz/ 1 quart with 1% silicone based penetrant. One quart will treat approximately 500 cuts.