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Plan rotations to avoid fungicide and herbicide resistance

Pathogens, organisms and plants will eventually find ways to adapt and develop immunity to the various methods used to control them. This has been shown clearly with the announcement of glyphosate- resistant kochia in Alberta.

Other recent challenges for farmers include increasing amounts of Group 2 herbicide-resistant cleavers and wild mustard in pulse crops. Group 1- and 2-resistant wild oats in cereal crops continue to have significant economic impacts for some growers.

Fungicide resistance

As fungicide costs have come down it’s more economical to use them. Fungicide use in wheat has been fairly high for a number of years, as farmers spray to control diseases like fusarium head blight and rust.

As the use of chemical controls increases, is resistance in crop varieties under greater pressure? “There are some concerns in using fungicides but variety resistance should not be overcome any quicker with fungicide use,” says Brent McCallum, a plant pathologist at Agriculture and Agri-Food Canada in Winnipeg. “The concerns with continual fungicide use are that the pathogens will develop resistance or tolerance to the fungicide and the environmental effects of fungicides, since they are broad spectrum and control all fungi (beneficial and harmful) in the crop, and to some extent the soil.”

Application of fungicides can, in some cases, hamper research work. As most wheat fields in Manitoba are sprayed with fungicides, there are limited samples that researchers can collect to test for virulence. Samples are more easily available in Saskatchewan, where fungicide use is lower and from other sources like nurseries and experimental trials. “Monitoring is very important,” says McCallum. “This year we found virulence to Lr21 for the first time in Canada. Lr21 is in many of our wheat cultivars and many lines in development, so knowing that it is less effective is important information for the future.”

Pre-mixed fungicides

New products are coming to the marketplace all the time, including pre-mixes, which promise better efficacy by combining different modes of action in one, ready-to-use product. But the theory behind pre-mixes, especially fungicides, is somewhat confused, says Bruce Gossen, a plant pathologist with AAFC at Saskatoon. “The accepted best practice for managing fungicide risk includes use of pre-mixes, which are expected to substantially reduce the risk of fungicide insensitivity,” he says. “But some of the modelling that is being done doesn’t actually substantiate that approach. How effective it’s going to be in the long term is still being assessed.”

In the field, pre-mixed fungicides offer advantages to growers in Western Canada, where farmers have not used a large amount of fungicides over many acres for a long time. These fungicides often mix one product, which may be at a high risk for insensitivity but that has a lot of activity against a particular pathogen, with another product that may not be as effective against that pathogen, but which has a much lower risk of insensitivity. This makes the odds of getting resistance to both products very low.

“From a grower’s standpoint, these fungicide mixes are a smart way to deal with the issue,” says Gossen. “We don’t recommend that growers apply the same mix year after year on a field. But if they are rotating fungicides and crops and including mixes as part of the approach, the odds of having trouble with fungicide sensitivity are much reduced.”

Farmers should, however, be cautious about seeing any one product as a solution in and of itself. “The lessons learned with herbicide-resistant weeds should be applicable to the evolution of fungicide-resistant pathogens,” says McCallum. “In some places, such as Europe, where fungicide use is high, they have seen the appearance of fungicide-resistant, or tolerant, or less-sensitive pathogens, so we should watch for that here as well.”

Farmers should not rely on just fungicides, but also make use of the genetic resistance in the host plant and McCallum advises farmers to regularly consult their provincial seed guides, which list the levels of resistance of each cultivar to various diseases. These can change over time if a pathogen population evolves increased virulence on a particular cultivar.

“The combination of improved genetic resistance and fungicide application has proven effective in controlling diseases such as fusarium head blight,” says McCallum. “We don’t want to get into a situation in which we rely on fungicides and neglect genetic resistance. In some cases in Europe the reliance on fungicides has lead to a decline in genetic resistance to diseases such as rust. Recent legislation in Europe has mandated a reduction in fungicide use and now they are trying to develop genetic resistance, which can be a hard task.”

Knowing what’s in the jug is just as important for pre-mixed herbicides, which aren’t always designed to manage resistance. “In many cases the pre-mixes are just to broaden the spectrum of weed control and it may not do anything for resistance management,” says Hugh Beckie, a research scientist with AAFC in Saskatoon. “You have to look at what are the active ingredients in these pre-mixes and what are your key target weeds that you want to try and protect against resistance and see if it’s going to be effective or not.”

Management practices

Good management practices are still essential to help maintain long term resistance in crop varieties. “I tell growers that 95 per cent of their disease management is done before they ever plant the crop,” says Gossen.

It all begins with rotation, of both crops and chemicals, to try and help keep resistance strong. “One of the things we are concerned about is the short crop rotations occurring over large parts of the Prairies, which is the wheat/canola rotation,” says Randy Kutcher, associate professor of cereal and flax pathology at the Crop Development Centre, University of Saskatchewan. “Instead of having a longer rotation with more species and more opportunities to use different products, we are targeting the same disease in the same crop perhaps every two years or in some cases every year, which increases the risk that pathogens will adapt to the fungicide and the variety, or both.”

A rotation of three to five different crops drastically reduces the risk of resistance developing to herbicides or fungicides. It also helps to break up the disease cycle, as most diseases (with the exception of sclerotinia and a few root diseases) are specific to one crop type. For example, rusts infect cereals but not pulses; blackleg will affect canola but not cereals or pulses. “If you can break up the cycle and let any diseased residue deteriorate before you plant the next crop it will certainly help prevent adaptation of the pathogen to particular fungicides or varieties. This applies to residue-borne diseases, such as blackleg of canola and many of the leaf spot diseases of cereals,” says Kutcher.

Canola, which has woody stems, does take a lot longer to break down, which is why a three-to five- year gap between canola plantings is advantageous. “We are growing close to 19 million acres of canola across the Prairies,” says Kutcher. “And although shorter rotations make total sense from a business case, from a biological perspective it’s a risky thing to do.”

No canola varieties are effectively resistant to sclerotinia, which also affects other broadleaf crops that may be used in rotation, making it a difficult disease to deal with. As a result, producers in the moister areas of the Prairies routinely spray for sclerotinia in every canola crop. Kutcher recommends that, in this case, producers use a different product each year, so that if they spray in year one, by the time they come back to use that product again, hopefully six or eight years later, most of the pathogen that was sprayed using the original product will be gone. “Sclerotinia bodies can last about five years in the soil, so if they are spraying for sclerotinia with the same product every second year there is much more risk that the fungus will become insensitive to that product,” says Kutcher. “Using a different product, and with two actives in the jug will help negate that somewhat.”

Farmers can also make sure they use clean seed and a seed treatment. “If someone else has a fungus in their field that is resistant to a fungicide and you buy that seed, you could be transferring the problem to your farm,” says Kutcher. “So that’s why it’s a good idea to start with good, clean, certified seed and use a seed treatment to try and get rid of the pathogen on the seed.”

Managing resistance

Sound agronomy and a long- term approach is always important in managing resistance, says Beckie, especially when it comes to weed populations. “Whatever farmers can do to give the crop a competitive advantage to suppress weeds will be helpful,” he says “Whether that be increasing your cereal crop seeding rate, banding or point injecting fertilizer or including competitive crops or varieties in your rotation.”

Farmers also tend to forget about trying to minimize weed seed spread during harvest, says Beckie. Cleaning equipment often will help reduce weed transmission from one field to another.

“All of these things together do have an impact but it does take a longer-term viewpoint,” says Beckie. “Unfortunately, with the lack of alternative herbicides, it will be inevitable that we will see more resistance in the future.” †

About the author


Angela Lovell

Angela Lovell is a freelance writer based in Manitou, Manitoba. Visit her website at or follow her on Twitter @angelalovell10.



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