Examining Resistance to DMI Fungicides: History of resistance in turf pathogens

With repeated application of fungicides with the same mode of action, the sensitive ones are constantly knocked back, leaving the resistant ones to reproduce. Since resistant ones often do not grow as well as their sensitive counterparts because of resistance-related fitness costs, the latter can constantly attempt to re-invade when the pressure from fungicide application is eased up. Photo courtesy Tom Hsiang

By Tom Hsiang, PhD

There are several fungicides to which no field resistance has developed. These contain active ingredients such as thiram, chlorothalonil, and pentachloronitrobenzene. These types of fungicides are usually protectants, and act by inhibiting a wide range of metabolic processes within the fungus. The turf fungicides to which resistance has developed in the past are systemic fungicides. Before the introduction of systemics, the most commonly used fungicides were probably dithiocarbamates, such as thiram. There have been no major concerns about field resistance to these older compounds.

Benzimidazole fungicides such as benomyl were introduced in the late 1960s. They provided excellent control of many turfgrass diseases, and their use was very widespread. Soon after, there were reports of disease control failure for dollar spot in Pennsylvania. By the late 1980s, disease control failure with benomyl for anthracnose was reported in Michigan and Ohio.

Around 1980, iprodione—a dicarboximide fungicide—was registered for turfgrass diseases. Shortly after, there were reports of field fungicide resistance for Fusarium patch in Washington and for dollar spot in Michigan. There were also cases of multiple resistance to benomyl and iprodione involved in disease control failure for dollar spot in Michigan. Additionally, there have been reports of resistance to metalaxyl, which is used to control Pythium diseases. There have also been several cases where dollar spot was not controlled by DMI fungicides and where anthracnose blight was not properly controlled by regular rates of strobilurin fungicides. These reports have come from several U.S. states, but no verified report of fungicide resistance in turf pathogen has come from Canada.

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