Some products are site specific in their mode of action whereas others affect multiple sites. In general, contact fungicides have a very broad base of activity and use of these products will very rarely result in resistance because they possess multiple modes of action ((the one exception is fludioxanil (Medallion)). The chance of resistance is much greater with the systemic fungicides because these products only affect a single biochemical pathway (single mode of action). One of the best known cases is the resistance of dollar spot to the benzimidazole fungicides.
All active ingredients from all chemical manufacturers have now been assigned a “group number” based on their mode of action. Some fungicides are labeled "M," which means that the fungicide acts upon multiple sites and the risk of resistance is low. The numbering system allows for a quick comparison of modes of action between two or more different products (Picture to the right). For a complete list of FRAC codes see http://ipm.ifas.ufl.edu/resources/success_stories/T&PGuide/pdfs/Appendices/Appendix6-FRAC.pdf. Alternating between products that have different modes of action will help prevent resistance from developing.
Mode of action symbols should never be used alone when making resistance management decisions. Other guidelines to help prevent or delay resistance include:
- Use good cultural practices that will limit disease activity
- Choose resistant turfgrass cultivars
- Properly identify the pest
- Decide if it warrants treatments based upon established thresholds
- Apply according to label direction through properly calibrated equipment
- Know the characteristics of the product including the active ingredient and if repeated use is likely to lead to resistance
- Mix or alternate between products with single modes of action and those which posses multiple modes of action
Graduate Research Assistant