UMass Extension Turf Program

Effect of Rainfall on Fungicide Efficacy

May 17, 2011
Effect of Rainfall on Fungicide Efficacy

Fungicide application is a crucial component to successful disease control on golf courses. The success of a fungicide application is significantly influenced by many factors before, during, and after application. One of the most important factors that golf course superintendents must monitor is the weather and more specifically rainfall. Avoiding fungicide applications before rainfall events can improve fungicide efficacy and lead to longer disease control. We have listed some crucial points to remember when scheduling fungicide applications.

  1. Avoid making an application when rain is expected later in the day.
  2. Contact fungicides (for example, chlorothalonil, mancozeb) are always more sensitive to wash-off by rainfall or irrigation than penetrant fungicides, because their topical mode of action relies on drying on the leaf surface. Failure of a contact fungicide to dry on the leaf surface may result in a loss in efficacy.
  3. Penetrant fungicides (for example, thiophanate-methyl, boscalid, iprodione, propiconazole, fluoxastrobin) are less sensitive than contact fungicides, but still need a minimum of 3 hours of drying time and fully absorbed by plants prior to rainfall or irrigation.
  4. Fungicide formulations containing different sticking agents may vary in the effect of post-spray rainfall. A minimum of three hours (to allow for proper fungicide drying and absorption) should be the worst-case scenario goal.
  5. Do not apply fungicides when the soil is saturated with rainfall/irrigation. This can lead to pesticide run-off, especially if an unexpected rainfall event follows the application.

Overall, the best recommendation is to avoid fungicide application if rain is expected later in the day. Since this is not always a practical solution, selecting penetrant fungicides instead of contact fungicides when afternoon rain is expected is the best solution when a fungicide application is absolutely necessary. Remember to avoid applications that will not allow the fungicide 3 hours to dry and absorb into turfgrass tissue.

If you have questions about fungicide applications and rainfall, please contact the Turfgrass Pathology lab ( or 413-577-3303) and we would be happy to discuss your inquiries further.


Submitted by: Dr. Geunhwa Jung


Archived Turf Management Updates contain information that is presented as it originally appeared, in an effort to preserve useful information and to illustrate changing management techniques and pest patterns over time. Some text may contain references to specific pesticide or fertilizer products. Due to the continually changing nature of the industry and pesticide regulations, some messages may contain references to products that are no longer available and/or are no longer registered for use.
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