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Chemical Pest Control Products: Resistance Update

Signs of resistance should be answered with careful resistance management strategies.

Stakeholders in the potato industry, from members of the scientific community to manufacturers of crop protectants and growers, understand that undesirable crop pests can develop resistance to the chemical pest control products used to control them. Resistance is a phenomenon that occurs when pest populations—insects, diseases, weeds and nematodes—are repeatedly exposed to the same active ingredient.

As resistance develops in a particular pest population, a chemical pest control product becomes ineffective and provides little or no pest control. Once resistance is established, pest control products with particular chemistries are no longer effective.

Depending on the mode of action of the chemical and how often it is applied, resistance can develop rapidly, or over a long period of time. All pests have the potential to develop resistant populations.

Documenting Resistance

The development of resistance to chemical crop protectants used on Phytophthora infestans, the causal agent of late blight, is of great concern to the global potato industry. Most manufacturers of crop protectants are working to find novel compounds to keep late blight at bay. Millions of dollars are allocated each year to research aimed at late blight control strategies and novel products to combat the disease.

Resistance to chemical pest control products has also developed in Alternaria solani, causal agent of the early blight disease. Researchers Neil Gudmestad and Julie Pasche, who work at the Department of Plant Pathology, North Dakota State University, have discovered reduced sensitivity of Alternaria solani to fungicides.

The researchers state that reduced sensitivity to quinone outside inhibitor (QoI) fungicides, due to the presence of the F129L mutation, was identified in Alternaria solani isolates in the paper entitled Prevalence, Competitive Fitness and Impact of the F129L Mutation in Alternaria solani from the United States.

Gudmestad and Pasche reported that in addition to the Midwestern states of Minnesota, Nebraska and North Dakota, Alternaria solani isolates containing the F129L mutation were collected from Colorado, Michigan and Texas in 2003, Wisconsin in 2005, as well as from the Western United States, including Idaho, Washington and Wyoming in 2005, and Oregon in 2006. The detection of these isolates in areas outside of the Midwest suggests that the F129L mutation is stable and present under conditions that are less conducive to the pathogen and where it is under less selection pressure by QoI fungicides.

Gudmestad and his team have also detected the development of resistance of the early blight fungus to the active ingredient boscalid, a fungicide first registered in the United States in 2005. “Resistance has been confirmed in North Dakota, Minnesota, Nebraska, Texas and Idaho,” he said. “There are two resistance mutations, one conveying moderate resistance (or reduced sensitivity) and the other high resistance to boscalid. To date, 79 per cent of the isolates we have tested have resistance to boscalid from these four states. Of the isolates resistant to boscalid, more than 95 per cent also have resistance to the strobilurin fungicides azoxystrobin and pyraclostrobin (double fungicide resistance).”

Strategies for Control

The danger of the development of resistance to chemical pest control products can hardly be overemphasized. Resistance management is crucial to successful crop production now and in the future. Manufacturers of crop protection products are acutely aware of this issue.

According to Harold Wright, biological assessment manager for Syngenta in Eastern Canada, companies are constantly focusing the research and development of new products with resistance management in mind. When developing a new active ingredient, research is aimed at its specific mode of action, which influences how often it should be applied once it reaches commercial growers.

“Aside from this, there are also strategies around combination products, where we target two modes of action at once against a particular pest,” Wright says. “And that is very effective as long as there is no resistance against either of the two active ingredients in the mixture. Sometimes this strategy is provided in advance of any sign of resistance, but at other times it is recommended that growers start using a combination of products to provide them with tools to deal with a pest in the short term until another more effective product is available.”

When there were signs of shifts in the resistance of fusarium to the active ingredient fludioxonil in seed treatment products, says Wright, his company offered growers another choice of fungicide within the family of products, addressing the resistance concern. He says that while neonicotinoid insecticides are still performing well, Syngenta is working on combination products to combat resistance problems that may arise in the future.

Wright recommends that everyone in the industry be aware of what is going on in the field with different pest populations to be prepared for potential shifts in resistance. He says his company works with AAFC researchers, who regularly conduct grower surveys on the behaviour and population shifts of certain pest populations.

Mark Goettel and Robert Blackshaw at Manitoba Agriculture, Food and Rural Initiatives provide practical guidelines for growers to deal with pest resistance. In the publication Guide to Commercial Potato Production on the Canadian Prairies, they urge growers to monitor their fields regularly and apply pesticides only when necessary, and apply pesticides only to areas of the field where there is a pest problem. Growers should also employ agronomic practices that play a role in managing pest populations. For example, maintaining at least 200 metres between the previous and present years’ potato fields will reduce the survival of overwintering adult Colorado potato beetles.

Goettel and Blackshaw further advise growers to alternate between groups of pesticides, also considering the products they may wish to apply to control other pests later in the season. For instance, growers can use an alternative insecticide to control Colorado potato beetles early in the season, and another to control aphids later in the season. When adequate control is not obtained with a product, another important strategy is to change chemical groups. Growers must not repeat a particular treatment because this may be the first sign that resistance to that chemical group is high.

A thoughtful resistance management strategy will help maintain the commercial life of existing pest control products. Resistance management should be a broad-based effort and include multiple tactics—chemical, biological and cultural. However, growers should inform themselves about potential resistance problems in advance, and establish a resistance management strategy before problems develop in their fields.

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