Strategies for resistance management

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.

Pathogen resistance is a real threat to the most successful of disease management programs. In the laboratory, pathogen resistance can be seen on Petri plates when Botrytis or Phytophthora grows despite the addition of a fungicide. In the greenhouse, pathogen resistance can be seen when disease becomes uncontrollable despite the use of fungicides that have been previously proven to work. Another term for fungicide resistance is control failure. Control failure is the result of the fungicide no longer working and the resulting plant loss from disease.

To avoid control failure and keep effective fungicide tools working, keep the following in mind:

  • When is the pathogen most vulnerable? Control should be aimed at the pathogen’s “weak link.”
  • How is the pathogen being introduced to the growing area? If the pathogen is coming in as a hitchhiker, how is the pathogen being managed before you receive it? There are lots of examples of a grower unknowingly receiving a pathogen on a crop and then that pathogen is already resistant to the key fungicides.
  • What fungicides have been proven to be effective in controlled studies? Sometimes growers experience control failure and think they have a pathogen resistance problem when the real problem is that they aren’t using a proven effective fungicide.
  • If a fungicide isn’t working, the problem may not be fungicide resistance but could be the wrong spray interval. Fungicides suppress the pathogens for a period of time, after which the pathogen will resume its destructive activity. If the interval between fungicide sprays is too long, then the pathogen will not be limited and will continue to cause problems.
  • Alternate fungicide sprays making sure that the active ingredients in each spray differ enough so that the pathogen cannot readily adjust and overcome one particular fungicide type. Relying only on your “favorite fungicide” may give the pathogen ample opportunity to mutate resulting in a pathogen that no longer responds to your fungicide program.
  • Use the labeled rates of fungicides. Using less than the labeled amount of fungicide results in a sub-lethal dose that is thought by some to condition a pathogen to tolerate the active ingredient that fungicide.
  • Do not over-apply fungicides. When disease is rampant and a crop is at stake, it is tempting to overdo fungicide rates and the number of applications. This tactic rarely works and tends to cause further problems, including plant burn.
  • Keep up-to-date on the newest fungicides as they become registered. If they’ve been proven to be effective, incorporate them into your fungicide rotation program if they offer a new and unique means of managing your pathogen problems.

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