Transition Zone Fungicide Resistance Management Strategies
Fungicide resistance occurs when an active ingredient that was once effective on a disease is unable to stop pathogen growth and symptoms. Fungicide resistance has been documented in five turfgrass pathogens with suspected cases reported every season. Fungicide resistance occurs usually due to a natural mutation in a single individual pathogen allowing it to tolerate an active ingredient. It would be common to find a mutation like this in almost any population. However, what allows that resistant pathogen to thrive is the repeated applications of the same fungicide (or the same mode of action) applying selection pressure to remove the other competing susceptible pathogens. Eventually the resistant population can grow unchecked to the point that the fungicide will no longer control it and symptoms appear. The causes and science behind resistance are thoroughly discussed in article by Dr. Rick Latin (Understanding Fungicide Resistance. Green Section Record: 55(13), July 7, 2017).
Only a few diseases are likely to develop resistance
• Pathogens that reproduce prolifically, have a short life cycle, and have extended periods of favorable conditions for disease development, all of which create the potential for high inoculum levels in the turfgrass system • Diseases that require numerous fungicide applications throughout the season • The diseases that fit these characteristics are dollar spot, anthracnose, gray leaf spot, Pythium and Microdochium patch (only in the Pacific Northwest) • These five diseases are the most important to manage wisely to limit the chances of resistance • For the five disease most susceptible to resistance, avoid repeated use of fungicides with the same mode of action/FRAC code (Fungicide Resistance Action)
Minimize inoculum levels
• This is a numbers game where the larger the inoculum level, the better the chances of a genetic alteration allowing resistance to develop • Using only curative applications or long intervals between preventative applications allows tremendous build-up of inoculum • Make preventative applications for the five primary diseases most likely to develop resistance • Alternate between fungicides with different modes of action (FRAC codes), not just different brand names. • Include broad spectrum contact fungicides like Daconil or Secure in tank-mixes
Reduce the selection pressure
• Maximize cultural practices to limit disease pressure – dew management, air movement, regular mowing, rolling, etc • Maximize application technique to improve control – increasing spray volume and decreasing droplet size will improve control of most fungicides • Apply fungicides at the appropriate rate and application interval to keep inoculum levels low • Increase frequency of applications and/or rates during high disease pressure
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