The unseasonably warm winter has prompted many inquiries about the persistence of fungicides applied for snow mold suppression. We have an experiment that is evaluating the persistence of two commonly used snow mold fungicides, iprodione and chlorothalonil. Basically we applied the fungicides to plots thatwe keep free of snow cover and ones that are kept under snow cover for the duration of the experiment. We take two cup cutter samples from each plot. One is used to determine the concentration of the fungicides using commercially available ELISA kits for iprodione and chlorothalonil developed by Horiba LTD. The other core is used in a bioassay where we inoculate the plants with a solution of (Microdochium patch or pink snow mold) and measure disease development. From this research we have learned that as long as soil temperatures remain below 32 F, fungicide concentration does not seem to degrade regardless of snow cover. However, once soil temperatures consistently eclipsed 32 F then we observed a rapid decline in fungicide concentration and consequently a rapid increase in disease severity in our bioassay. The chlorothalonil kits have not performed as well as the iprodione kits, but we still have bioassay results. From the bioassay we observed that snow cover did not seem to affect chlorothalonil concentrations because disease severity was similar in the snow covered and non-snow covered plots treated with chlorothalonil. We do not have the exact concentrations for chlorothalonil, but we can infer that photo degradation does not play a major role in determining chlorothalonil persistence. We also think photo degradation plays a minor role in the persistence of iprodione.
From our research, temperaturesgovern degradation of these two fungicides. This likely means that these fungicides are degraded by microbes or through hydrolysis. Both of these processes are governed by temperature, but we do not know which process is more important. Knowing is not important right now as the question remains, do I re-treat or not. The answer to this question is complicated. Currently we are running our degradation experiment and we have observed some degradation, but the concentrations are still adequate to protect against Microdochium patch. At least in our climate I do not think it is necessary to re-treat. Moving farther south the temperatures have been warmer, and yet the answer is still complicated. I have heard that some golf course superintendents have had to mow tees, if that is the case then I was suggest a re-application. If mowing has not occurred, my suggestion is to apply if it makes you sleep better. Why do I say this? Well based on our inoculation work and work done by Phil Dwyer when he was a student of Joe Vargas, Microdochium patch takes a long time to develop when temperatures are below 40 F. However, once temperatures exceed 40 F consistently and humidity is high then the disease can develop quickly. The problem with re-applications is you’ll need access to water no matter what formulation of fungicide you choose. Our experience with granular fungicides is they work better when watered in.
Unfortunately we do not have a good answer concerning re-treatment. I am hesitant to suggest a re-application because I am afraid the weight of the sprayer would do more harm than good. This might be an unfounded concern and if it please tell me otherwise. What I am encouraging is to be ready once things warm up in the spring, whenever that maybe. I do think with the weather conditions experience throughout the Northern regions we will likely see outbreaks of Microdochium patch, so be ready to spray when those conditions come. I do know that Typhula blight (gray and speckled snow mold) will likely not be a big factor for many because these fungi need long lasting snow cover to cause severe damage. My participation with the blog has been terrible as of late, but I will be more active in 2012 and I will try to answer any comments about this post as well.