A point of constant consternation amongst many superintendents in the upper Midwest is when to apply snow mold fungicides. Waiting too long to apply leads to obvious problems (one facility was kind enough to show me the results of spraying part of their nursery prior to snow and the rest after it snows…Figure 1), while applying too early can lead to potential degradation of the fungicides prior to snow cover and defeat the whole purpose of applying them in the first place. Recent winters with prolonged stretches of snowless ground has led to worry that much or most of the fungicide applied in the fall was gone before snow cover could even arrive, and in some cases have been suspected of leading to disease breakthrough.
Is worrying about snow mold fungicide persistence worthwhile? What are some of the conditions that influence fungicide persistence during the winter months? Since very few agricultural or horticultural systems spray fungicides for winter disease protection, very little is known about the effects of winter on fungicide persistence. At Wisconsin we recently concluded a 4-year study investigating just this issue with iprodione (Chipco 26GT) and chlorothalonil (Daconil WeatherStik), with the primary objective of investigating whether both of these fungicides degrade faster under snow cover or in the absence of snow cover (Figure 2).
While I won’t recap the entire study here (a full recap will be published in Golf Course Management within the next several months), I can give a brief overview of the results to allow you a better idea what may be happening to your snow mold fungicides as you peer out the office window. First, the presence of snowfall (as long as it wasn’t melting) didn’t have any impact on the degradation of either fungicide. Second, significant rainfall or snowmelt events always led to a rapid degradation of both iprodione and chlorothalonil. Third, even in the absence of rain or melting snow, fungicide degradation markedly increased once temperatures rose significantly above freezing (>40°F).
So what’s the take-home message? Well it’s a two-parter;  whether you have snow cover or not your snow mold fungicides should persist if temperatures remain at or below freezing and there are no rain events (in other words, fungicide photodegradation doesn’t appear to be significant in the winter), and  don’t expect much fungicide protection to remain if you have repeated rainfall events or a significant snow melt event. While this probably won’t reduce your level of worrying this winter, at least you have some more information about what to worry about. I’m aware that this was a very brief overview of a long and complex study, so please free to contact me at 608-576-2673, firstname.lastname@example.org, or @uwpaul if you have any questions, comments, or concerns.