By Paul Koch, Ph.D., University of Wisconsin – Madison

Fungicides remain the most effective tool to control snow molds but reports of disease breakthrough on treated areas have seemingly increased in recent years. This is mostly due to degradation of snow mold fungicides in response to more frequent winter rainfall and snowmelt events and decreasing plant hardiness in response to warm fall temperatures. Plants that lack proper hardening and fungicide protection are very susceptible to snow mold infection over the winter, and even brief periods of snow cover can lead to rapid and severe disease development.

Are fungicide reapplications necessary?

For most superintendents, a fungicide reapplication isn’t necessary following a winter rainfall or snowmelt event. The simple reason is that the majority of ‘work’ that a snow mold fungicide does is during the days and weeks immediately after the application when it’s absorbed by the fungus. The initial uptake of the fungicide stunts the fungus and, in most cases, inhibits enough growth to prevent disease development for the entire winter. However, cases where the rainfall event occurs very shortly after application or locations that typically experience severe snow mold pressure may warrant a reapplication.

We can’t control the weather, so what can we do?

Since nothing you can do will prevent winter fungicide degradation at most locales, the key is to optimize injury to the snow mold fungi in the fall through proper product selection and proper application timing. Mixtures of multiple active ingredients are crucial for success in areas of moderate to heavy snow mold pressure, and you can utilize a multitude of resources to select the right product for your facility.

But almost as important as product selection is accurate application timing. Our research has found that there is no decrease in control when products are applied as late as possible (even on frozen ground) and applying as late as possible allows you to avoid fall rain events that would quickly wash away your fungicide. However, our research has also shown that you don’t need to apply on the last day before the snow flies and that most applications applied within 2-3 weeks of permanent snow cover perform as well as products applied the day before.

In areas that experience heavy snow mold pressure we often recommend an ‘early’ or ‘primer’ fungicide application to further knock back the fungi, and timing of these applications are just as critical as the later application. My research program is working to develop a model that will identify when fungicides will be effective against snow mold based on environmental conditions. The early indications are that in most cases, snow mold fungicides will begin to be effective against snow mold three to five weeks prior to permanent snow cover.

These practices won’t guarantee you success every winter as Mother Nature remains undefeated; however, they will give you the best shot at success based on the most recent research available. If you have any questions or want to discuss our research in more detail please email me at plkoch@wisc.edu.

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