How to control crown rust in oats

Something Prairie oat producers need to keep a sharp eye out for this summer is crown rust.

James Menzies, a plant pathologist with Agriculture and Agri-Food Canada (AAFC) in Morden, Man., says you’ll find crown rust wherever oats are grown, except in very arid climates. It’s more damaging than any other oat disease, causing yield losses of up to 40 per cent in severe cases.

“It can also affect the quality of the grain that is produced, so it is an issue for producers,” says Menzies. Infected plants produce a higher percentage of thins and lower test weights, which limit farmers’ abilities to get the highest grade for their crops.

Crown rust, also known as leaf rust, is caused by the fungus Puccinia coronata. The pathogen favours warm, humid weather. Crown rust can really take off in fields with high disease pressure during mild to warm days, with temperatures from 20 to 25 C, and mild nights, with temperatures in the range of 15 to 20 C, which produce lots of dew (crown rust requires dew to infect plants).

Disease symptoms include tiny, orange pustules on the oat leaves. Each pustule contains thousands of spores that can travel long distances when released.

Crown rust spores are typically blown into Canada each summer from oat-growing areas in the United States. Saskatchewan and Manitoba are the primary areas where oats are farmed in Canada, producing 90 per cent of the country’s crop. The rest is grown mainly in Ontario and Quebec.

Menzies conducts annual surveys to assess crown rust incidence in Prairie oat fields. He says in his experience, crown rust is probably most prevalent in areas along the Saskatchewan-Manitoba border.

Menzies notes in 2019 and 2021, crown rust levels were relatively low since both were generally hot, dry years in the Prairies. He says 2022 was a different story, with moisture and temperature conditions driving disease incidence higher.

Menzies says if conditions are right, crown rust will typically start appearing by early July in the Prairies. Fortunately, there are a number of steps you can take to help prevent the disease from taking a big bite out of your oat crop.

Grow resistant varieties

One of the main ways to control crown rust is to plant rust-resistant oat varieties. The problem with the disease, however, is it is highly genetically variable, making it difficult for plant breeders to keep up with the rapidly evolving pathogen.

“It’s a constant effort to keep up producing oat cultivars with highly effective resistance to crown rust because the pathogen keeps changing,” says Menzies. “The average crown rust resistance gene remains viable as an effective gene for resistance, on average, less than five years.”

Menzies maintains using resistant oat varieties is still a good idea. He recommends farmers consult their provincial seed guides every year to check variety ratings for crown rust resistance.

He adds if a variety has a good resistance rating, “I would use it anyway because even if the pathogen population has changed, lines that had good resistance genes tend to do better than lines that never did have good resistance, even if the resistance is overcome.”

Seed early

Menzies also advises Prairie farmers to seed their oats early if they can. By doing so, the crop should be advanced enough that it won’t suffer significant yield or quality losses by the time crown rust spores arrive in the Prairie provinces.

“Crown rust tends to be blown in from the United States in late June or July, so the sooner (the oat crop) matures, the fewer cycles of disease it will be exposed to,” says Menzies.

“It will have lower disease levels simply because it’s just maturing early and avoiding inoculum buildup,” he adds. “The later the crop grows, the more chance there is for the epidemic to build up and the greater the chance of disease on your crop.”

Avoid buckthorn

Another useful tip for Prairie farmers is to avoid planting oats in fields where buckthorn is present or close by.

Buckthorn is an invasive shrub/tree that can grow up to six metres tall, which you can find close to rivers, creeks, ponds and lakes. It is an alternative host for crown rust, so if you can avoid it, this lowers the chances of crop infection and also reduces the risk of new crown rust races developing.

“If you have a field near a waterway and there’s buckthorn there, it’s probably not a great idea to plant oats there,” Menzies says. “Where buckthorn is present, farmers will see crown rust more often and perhaps earlier in their oats.”

On infected buckthorn plants, sexual reproduction of crown rust will occur all year, explains Menzies.

“What this means is in the spring, when the spores are starting to be produced on the buckthorn, it will spread that much earlier to your oat crop and there’ll be more chance for the disease to build up,” says Menzies.

Farmers could try removing buckthorn in or near oat fields, but it’s not the easiest thing to do, especially with larger plants, he adds.

Apply fungicides

Foliar fungicides are an important tool for managing crown rust in oats. Menzies says they can be very effective in reducing disease severity if crown rust is present in a field. However, decisions to spray or not depend on the time of year.

Menzies recommends farmers scout their oat fields in July for signs of the disease. The ideal timing for fungicide application is to spray at flag leaf emergence to protect the flag leaf, where most of the nutrients required for seed development come from. Once the flag leaf is covered with pustules, it is too late to apply fungicide.

If it’s late July and you haven’t spotted any crown rust in your oat fields, chances are there isn’t much of it in the crop and a fungicide application likely isn’t warranted, says Menzies.

If you do decide to spray, Menzies says you must ensure the fungicide you’re using is registered for crown rust. It’s also wise to rotate different modes of action to prevent fungicide resistance from developing in the pathogen population.

“If you put a resistance gene out on a huge acreage, the pathogen will evolve to overcome that resistance. If you use the same fungicide over a large acreage, the pathogen will evolve to overcome that fungicide.”

– This article was originally published at Grainews.