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The changing races of wheat leaf rust

Some resistance genes have lost their effectiveness against wheat leaf rust. Photo courtesy of Dr. Brent McCallum, Agriculture and Agri-Food Canada.

Wheat leaf rust (Puccinia triticina) is an ever-changing disease that requires continual attention. That is why Dr. Brent McCallum, pathologist at the Agriculture and Agri-Food Canada Cereal Research Centre in Winnipeg, Manitoba, performs an annual cross-Canada survey of the disease. The aim is to keep on top of which races of the fungus are increasing in prevalence so that breeders might incorporate resistance genes that are effective against the predominant races, allowing growers to plant tolerant cultivars if they so choose.

Because the fungus is an obligate parasite that requires living plant tissue to survive, the level of risk in Canada each year is highly dependent on environmental and agronomic conditions in the southeastern United States.

“We have control over the cultivars that we put in the field and the resistance genes that are incorporated into those cultivars,” says McCallum. “But the rust comes from the US and blows in on the wind, so the races that are developing depend on which cultivars they’re growing in the southern and Midwestern US states and which resistance genes they’re using.”

Although wheat leaf rust is a potential concern at any stage of development, it has the greatest potential to cause yield loss early in the season.

The races tend to show some geographic and crop-related differences, explains McCallum. Races that end up in Manitoba and Saskatchewan, where the main wheat crop is spring wheat, tend to blow up from Kansas through Minnesota, South Dakota and North Dakota. In Ontario and Quebec, where winter wheat is predominant, the races are more similar to those found in eastern states like Illinois and Ohio. 

Wheat leaf rust races are identified by purifying the samples and then growing them on a specific set of 16 host differential lines, each of which contains a different resistance gene. As the races change and certain races become more prevalent, varieties that once showed good resistance or tolerance to leaf rust may lose that tolerance. “That durable resistance in some varieties starts to decline over time as we see increases in races that can bypass that resistance,” says Albert Tenuta, plant pathologist with the Ontario Ministry of Agriculture, Food, and Rural Affairs (OMAFRA). “The survey for races is very important for the development of new varieties. Particularly, when you’re anticipating variety development four to five years down the road, breeders are starting to incorporate some of the newer genes that can handle some of these newer races that are starting to develop.”

One resistance gene that has recently lost some of its effectiveness is Lr24. “Lr24 is a race-specific gene, so it’s effective against some races — it controls them completely — but it’s ineffective against other races,” says McCallum. “We’ve seen an increase in virulence on Lr24 in the last two or three years, and that may help to explain why the leaf rust incidence increased in southern Ontario.” He notes, for example, that the winter wheat cultivar ‘Vienna,’ which is grown in Ontario, contains the Lr24 resistance gene.

In breeding for resistance, “a good strategy is to use genes that are not being used in the US, even if those genes are race specific. Races that are virulent on those genes don’t tend to be selected because they won’t be exposed to those genes in the US,” says McCallum. Another good strategy is to use a broadly effective resistance gene such as Lr34 to give broad-spectrum protection.It can be a bit confusing to translate the race codes to determine which resistance genes may be effective and then which wheat cultivars carry those genes. So McCallum recommends that growers in Western Canada consult the provincial seed guides when deciding which varieties to plant. “The seed guide doesn’t actually tell you about the pathogen surveys. It tells you how the cultivars are performing to various diseases,” he explains. The guides are updated annually according to how the varieties have been performing in the field, both agronomically and against specific diseases. In Eastern Canada, similar information is available from the Ontario Cereal Crops Committee variety trials at www.gocereals.ca and from Quebec Agri-Réseau, résultats et recommandations des Réseau Grandes Cultures du Québec, at www.agrireseau.qc.ca.

Control strategies
“Wheat leaf rust is a potential concern every year,” states Tenuta. It can affect the crop at any stage of development, but has the greatest potential to cause yield loss when infections begin early in the season. Tenuta recommends that growers start scouting their wheat soon after it begins to green up, in late April and early May. “In terms of the top two leaves, we’re generally looking at less than two to five percent pustules as the treatment threshold. Anything that challenges that flag leaf or the top two leaves is a concern.” Growers are advised to consult OMAFRA’s Agronomy Guide for Field Crops for further information. The guide is being updated for 2009 and is accessible online at www.omafra.gov.on.ca/english/crops/pub811/p811toc.html.

During the growing season, growers can access US survey information online to get an idea of how the rust epidemic is progressing. “There’s an informal rust e-mail list that sends out reports on the levels of rust, where it is, and how heavy it is, so you can get an idea, as the rust moves from Southern US to the Northern US and into Canada, just what to expect in terms of severity,” says McCallum. This information, as well as regularly updated cereal rust bulletins, can be accessed through the United States Department of Agriculture Agricultural Research Service’s Cereal Disease Laboratory at www.ars.usda.gov/mwa/cdl (click on Cereal Rust Bulletins).

McCallum suggests three control strategies for wheat leaf rust:  seeding spring wheat early, planting resistant or tolerant cultivars and spraying fungicides. “In spring wheat areas like Manitoba and Saskatchewan, we advise the farmers to seed as early as possible because the epidemic only starts in mid-June.” Early planting may allow for some grain filling before infection levels get high. The Agronomy Guide for Field Crops also recommends planting spring grains early.

“We always recommend planting resistant cultivars, whether you’re planning to use a fungicide or not, because it always will give you better protection than having a susceptible cultivar,” says McCallum. He says that cultivars that have an intermediate level of tolerance may do well if the epidemic is light. However, they may show some yield loss if the epidemic is severe. Therefore, growers may need to use a fungicide, depending on the cultivar, the level of infection, and cost/benefit factors. 

Fungicide considerations
A grower’s fungicide control strategy should consider the wheat variety planted and the timing of treatment. “In winter wheat in Ontario, it would be rare to see enough leaf rust that you would need to apply fungicide at the flag leaf stage or earlier,” says Peter Johnson, OMAFRA cereals specialist. “With spring wheat, on the other hand, it develops later, so rust is more likely to be an issue. Fortunately, there are no really susceptible varieties of spring wheat at this time, but we do have some extreme susceptibility in some of the winter wheat varieties.”

For varieties that are fairly tolerant of wheat leaf rust, the strategy is to wait until heading and apply a fungicide for Fusarium head blight control. The triazole fungicides Folicur and Proline are the only two that are currently registered for this purpose in Canada. However, two products are in the pipeline. 

“We anticipate having a new product registered prior to next season,” says Dr. Trevor Kraus, supervisor of BASF’s research and development group for Eastern Canada. He is referring to Caramba (with the active ingredient metconazole), which he says performed well against both Fusarium head blight and leaf diseases in federal research permit trials. Also under application for registration is Prosaro, which is basically a jug mix of Folicur and Proline.For susceptible wheat varieties, it is imperative to monitor them closely during flag leaf development, which is generally in late May, says Johnson. “At this earlier timing, you could use a triazole or strobilurin, but we generally save the triazoles for the Fusarium timing.” Research indicates that strobilurins should not be used after the full flag leaf stage because they can increase levels of deoxynivalenol (DON) toxin in the grain. Fungicides that are available for use at the earlier timing are Headline (strobilurin), Stratego or Quilt (both contain strobilurin plus triazole), or Tilt, which is a triazole that is not registered for use against Fusarium.

When selecting which product to use at the earlier timing, Johnson suggests that growers consider the cost of the product and the amount of active ingredient in the jug. “You tend to look at those products that have two modes of action in the jug, plus they also tend to have more active ingredient per dollar spent. They are probably the product that you would lean towards.” He also recommends, “To get good efficacy, it is good practice not to cut application rates.”

Fungicide resistance management

Minimizing the development of fungal resistance to fungicides is always something to keep in mind. “It has been reported from other parts of the world that rust races can develop some tolerance to the chemicals that are used to control them, but we haven’t seen that yet in Canada. As far as we know, the chemicals have been quite effective at controlling rusts to this point,” says Dr. Brent McCallum, pathologist at the Agriculture and Agri-Food Canada Cereal Research Centre in Winnipeg, Manitoba. But he notes, “It’s always a concern that resistance or tolerance in rusts will develop over time, especially if growers are using more fungicide applications on a regular basis.”

“We have to be aware of it and should try to manage for it, but we don’t anticipate seeing it develop nearly as quickly in Ontario as it did in Europe,” says Peter Johnson, cereals specialist for the Ontario Ministry of Agriculture, Food, and Rural Affairs. He cites the longer growing season and repetitive fungicide application up to five times per growing season in Europe as factors that place more selection pressure for resistance on rusts. “When the organism can develop on volunteer wheat in the fall or on its alternate host in fall and winter, when there’s no selection pressure for resistance, then we think it will take longer for resistance to develop.”