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by Dr. Tom Hsiang


Turfgrass managers are often concerned about the development of fungicide resistance, particularly if they have ever experienced it previously. In Canada, we have relatively few fungicides compared to the U.S. and we need to ensure that the activity of our registered products remain efficacious. In this article, we will look at what fungicide resistance is, how it develops, some history of its occurrence with turfgrass pathogens, and ways of managing fungicide resistance.


American statistics show that the turf fungicide market is larger than for any other single crop, and that golf course putting greens receive the most intensive use of fungicides. Similar statistics are not available for Canada, but they are likely to be similar for the intensity of fungicide use compared to adjacent American States.


Fungicides usually act by inhibiting important processes in fungi such as cell division (benzimidazoles), sterol formation (DMI fungicides), or other general metabolic activities in fungi (dicarboximides, chlorothalonil, quintozene, dithiocarbamates). Within populations of fungi, there may be particular individuals that are mutants and can tolerate a larger dose of the fungicide than their normal relatives. In these mutants, some process is altered so that the fungicide either does not penetrate to the site of action within the organism, or the site of action is altered so that the fungicide cannot fully exert its effect. An individual that is resistant to a particular fungicide generally will show resistance to another fungicide that has the same mode of action. For example, benomyl (Tersan 1991) and thiophanate-methyl (Easout) have the same mode of action, and organisms resistant to one will show cross-resistance to the other one. Another major group of fungicides which shows cross-resistance are the DMI fungicides such a propiconazole (Banner) and myclobutanil (Eagle). In laboratories we can often generate mutants that are resistant to a fungicide, but usually these mutants are weak-growing or lack some feature that would allow them to persist in the field. What we are really concerned with is the occurrence of resistance in the field leading to disease control failure when we make applications at normal rates and schedules.


When we use a fungicide, we impose a selection pressure on the fungi. Within fungal populations, individuals that are able to withstand higher amounts of fungicide can then survive and reproduce, while ones that are susceptible are killed off or inhibited. The progeny of the surviving resistant individuals are also able to inherit the resistance to the particular fungicide. What differs between fungicides is the range of sensitivity that may be found within populations. For some fungicides that are general metabolic inhibitors, the range between the most sensitive and the least sensitive is likely narrow. For other fungicides, there may be a wide range with the least sensitive isolates able to tolerate very high doses without adverse effect. It is this latter group of fungicides where resistance may develop.


There are several fungicides to which no field resistance has developed. These are fungicides that contain active ingredients such as thiram (Thiram, Arrest), chlorothalonil (Daconil), and quintozene (PCNB). These types of fungicides are usually protectants and act by inhibiting a wide range of metabolic processes within the fungus. These are older fungicides and were the most commonly used before the introduction of systemic fungicides, and there have been no major concerns about field resistance to these older compounds. The turf fungicides to which resistance has developed in the past are systemic fungicides.

Benzimidazole fungicides such as benomyl (Tersan) are systemics and were first introduced in the late 1960's. They provided excellent control of many turfgrass diseases, and their use was very widespread. Very soon afterwards, there were reports of disease control failure for dollarspot disease in Pennsylvania. By the late 1980's there were reports of disease control failure with benomyl for anthracnose in Michigan and Ohio. Iprodione (Rovral) is a dicarboximide fungicide which is locally systemic. It was registered around 1980 for turfgrass diseases. Very soon after that, there were reports of field 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. There have also been reports of resistance to metalaxyl, which is used to control Pythium diseases. More recently, there have been several cases where dollar spot was not control by DMI fungicides. These have been reported from Illinois, Kentucky, Michigan and Georgia.


A case study on the development of DMI fungicide resistance illustrates some of the conditions that may lead to disease control failure. At a golf course outside of Chicago where resistant isolates were found, an older variety of creeping bentgrass was used that was very susceptible to dollar spot. This likely lead to greater frequency of fungicide use. Secondly, nitrogen levels were purposely kept low to prevent the occurrence of other diseases such as Pythium blight, which again may have contributed to greater fungicide use because dollar spot disease is favoured by low nitrogen regimes. Thirdly, DMI fungicides were used extensively for several years.

Michigan researchers recommend that the use of DMI fungicides be limited when dollar spot disease is most severe to reduce selection pressures for resistant isolates. General recommendations to prevent DMI resistance problems in fungi include: 1) not using repeated applications of DMI alone; 2) using mixtures or alternating with non-DMI fungicides; 3) reserving DMI use for the critical part of the season; 4) using label rates rather than reduced doses; and 5) using other measures such as resistant varieties and cultural practices.

In recent conversations with the Pest Management Regulatory Agency in Ottawa, they’ve mentioned that the older fungicides are undergoing re-evaluation in the US and elsewhere, and there may be some that will no longer be registered for certain uses. This can pose a problem for turf managers because one of the major recommendations for resistance management is to mix or alternate a modern systemic fungicide with an older protectant fungicide. If older fungicides loose their registration, we will then have to guard against resistance to the newer fungicides even more carefully.


There have been few confirmed reports of fungicide resistance for turfgrass pathogens in Canada. In our annual field trials for fungicide control of dollar spot disease, we found that benomyl failed to control the disease after we switched testing for a year to another range of turfgrass although it worked well in previous years and in later years at our usual test site. Subsequently, we learned that benomyl resistance had been encountered in that field 10 years previously, but that no benomyl had been used on that range since then.

Propiconazole (Banner), a DMI fungicide became registered for use on turfgrass in 1994. Because of the recent discovery of DMI-resistant isolates of the dollar spot pathogen in the Great Lakes States, there was great concern that resistance would develop in Ontario. We started a study in 1994 to look at baseline sensitivity of the dollar spot pathogen to DMI fungicides. From eight locations throughout southern Ontario, we've found that most populations of the dollar spot pathogen are sensitive to DMI fungicides. However, there is one population near the American border that has reduced sensitivity to DMI fungicides and we suspect that there has been previous exposure to DMI fungicides in this population.

A recent graduate student from my lab, Wayne Barton, has tested several populations of the dollar spot fungus in 1995 and 1996. Some of these locations used DMI fungicides while others did not. He found that there were no detectable shifts in sensitivity to DMI fungicides during that time. Our studies are continuing by examining the effects of changing fungicide regimes on the development of DMI fungicide resistance, and continued monitoring of dollar spot populations at several locations across southern Ontario. We are also initiating a study looking at fungicide sensitivity of Fusarium patch and pink snow mould across the country.

Dr. Tom Hsiang is an Associate Professor of Plant Pathology, Department of Environmental Biology, University of Guelph, Guelph, Ontario, N1G 2W1 email:

Dr. Hsiang will be a special guest speaker at the 37th WCTA's 2000 Annual Conference and Show in Victoria.

Turf Line News October/November 1999

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