Trauma blight control in Michigan

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

It appears that most areas of the state did not experience weather during bloom conducive to blossom blight infection by the fire blight pathogen. However, temperatures will eventually warm and fire blight cankers will begin to ooze. Thus, inoculum will be present throughout the state.

Weather events leading to potential trauma events in apple orchards are inevitable. Strong storms accompanied by high winds and/or hail can injure trees and deliver inoculum that can colonize and cause shoot blight infections. Shoots are susceptible to infection any time that they are actively growing.

The only effective control measure for trauma blight available currently is streptomycin. Streptomycin is best applied as early as possible after the trauma event has occurred to maximize control. This is especially important if there are active fire blight cankers or strikes in the orchard or if the trauma event resulted in extensive foliage damage. We used to think that there was up to a 24 hour window of time after the trauma event to get a spray on. New data coming out suggests that the timing should be much sooner, within 4 to 12 hours. The sooner the material can be applied after the event enables targeting of bacteria prior to their internalization and establishment of infection.

Streptomycin should not be sprayed as a preventive measure for shoot blight. This is not an effective use of this material and only promotes the development of streptomycin-resistant strains.

Unfortunately, in areas where streptomycin resistance occurs, Mycoshield will not be a useful substitute for trauma blight control. Mycoshield is bacteriostatic and inhibits the growth of the fire blight pathogen, but does not kill the pathogen. This works as a control for blossom blight because blossoms are only susceptible to infection for a short time period. Inhibiting the growth of the pathogen during this time period will enable blossoms to escape infection. This strategy doesn't work on shoots because the growth inhibition is not permanent. As the material becomes naturally degraded, the pathogen will begin growing again and cause shoot infection. The reason that streptomycin works is that the material kills the pathogen which will prevent shoot infection.

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