Category Archives: Pesticides/Fertilizers

Examining Resistance to DMI Fungicides: Research at the University of Guelph

Dollar spot disease on Kentucky bluegrass showing the tip yellowing and a dark band at the base of the lesion. Photo courtesy Tom Hsiang

By Tom Hsiang, PhD

Propiconazole, a DMI fungicide, became registered for use on turfgrass in 1994. Due to the recent discovery of DMI-resistant isolates of the dollar spot pathogen in the Great Lakes States, there was concern resistance would develop in Ontario. This author started a study in 1994 to look at baseline sensitivity of the dollar spot pathogen to DMI fungicides. A graduate student, Wayne Barton, worked on sensitivity to propiconazole, while other students and lab members worked on sensitivity to other DMI fungicides.

From eight locations throughout southern Ontario, the team found most populations of the dollar spot pathogen were sensitive to DMI fungicides. However, there was one population near the U.S. border with reduced sensitivity to DMI fungicides, and the team suspected there had been previous exposure to DMI fungicides in this population.

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Examining Resistance to DMI Fungicides: Anti-resistance strategies

Dollar spot disease on Kentucky bluegrass (left) and creeping bentgrass (right). Photo courtesy Tom Hsiang

By Tom Hsiang, PhD

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 led 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.

General recommendations to prevent DMI resistance problems in fungi include:
● not using repeated applications of DMI alone;
● using mixtures or alternating with non-DMI fungicides;
● reserving DMI use for the critical part of the season;
● using label rates rather than reduced doses; and
● using other measures such as resistant varieties and cultural practices.

Read the full article: Examining Resistance to DMI Fungicides

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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Surviving a Municipal Cosmetic Pesticide Ban: New products

Trichoderma—the greenish fungi growing in circles on the special selective pink agar—recovered from leaves of plants treated in a 2012 field trial. Photo courtesy Deborah Henderson

By Deborah Henderson, PhD

Some new fungicide products are becoming available to help control soil-borne diseases. One registered now in turf is a bacterium (Bacillus subtilis, Rhapsody) for controlling a broad spectrum of bacterial and fungal diseases of turf including Brown Patch, Anthracnose, and Dollar Spot. A second is a fungus (Trichoderma harzianum, Rootshield) that has not yet been registered for turf, but controls soil-borne diseases in ornamentals.

A reality check for all new biological and low-risk pesticides is they do not work like chemical pesticides. This author suggests paying more attention to the label instructions; do not wait until a pest or disease is epidemic, take a proactive approach. As a pest manager, you need to know more about your system as well as these new products. Small-scale ‘trials’ with any new product are a great way to get some experience with them.

Some new low-risk herbicides seem to work well and others are more fussy about how they are used. Get to know them. Even if a product does not work for you in some situations, it may work quite well in others, so it is important not to give up until it has been tried a few times in different conditions. To get the best out of new soft products, they need the best chance for success. These are your new tools and they will perform best when used within IPM strategies where you employ all available tactics to suppress weeds, pests, and diseases.

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Surviving a Municipal Cosmetic Pesticide Ban: Controlling pests with nematodes

Two Galleria melonella insect larva. The red one has been infected by the nematode Heterorhabditis bacteriophora, and infection caused the larva to turn this characteristic red colour. The white larva is uninfected. This insect is not a pest of turf, but a lab-reared insect commonly used to study insect pathogens. Photo courtesy Deborah Henderson

By Deborah Henderson, PhD

Nematodes are microscopic ‘worms’ that do not require registration in Canada as a pesticide, and control soil-dwelling insects. There are two groups (i.e. genera) available commercially that can be quite useful. Numerous suppliers provide nematodes of various species and some have pest specificity, so one should check the labels.

One genus, Heterorhabditis (i.e. H. bacteriophora, H. megidis, and H. marelatus), finds its prey in the soil by using a ‘seeking’ predator strategy, actively moving about in the soil searching out prey. The other genus, Steinernema, is a smaller nematode (i.e. S. carpocapsae, S. krausii, and S. feltiae) that uses an ‘ambush’ strategy, waiting for hosts to come along then latching on.

Nematodes find natural host orifices and burrow inside to grow, mate, release insect toxins and produce young. The host insect turns a characteristic colour and dies. The juvenile nematodes burst out of the dead insect and start their search in the soil to find another host. While an application of nematodes will result in some being present in the following year, the concentration is not usually high enough to provide a second year of control for a pest that annually invades turf or ornamentals.

Read the full article: Surviving a Municipal Cosmetic Pesticide Ban