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Are plant infection rates increasing for crops?

Prairie farmers are spending more time and money on disease prevention than ever before

Sclerotinia infection in a canola stem.

Farmers are staring down a legion of diseases these days. Whether it’s scouting for this or spraying for that, they are scrambling like never before to keep them under control.

On the cereal front, says Tim Gardner, senior market development specialist with Bayer CropScience, leaf diseases have always been around but now fusarium head blight is the biggest issue. Because it doesn’t just affect quality or quantity of your crop; it will determine if you can even sell your product at the end of the year. With canola, Gardner identifies the main challenge as sclerotinia. “We’re just getting our heads around how we manage that.”

Jay Schultz farms 6,000 acres of canola, wheat, and yellow peas 100 km east of Calgary near Standard, Alberta. He’d love to spend his Canada Day long weekend at the lake but now he spends it spraying for sclerotinia. After many years of no issues with sclerotinia in canola, in 2012 it infested about 50 per cent of his crop. Other diseases he’s struggled with recently include septoria leaf blotch and tan spot. He also suffered heavy losses a couple of years ago when one variety of HRSW got “smoked with rust” brought in on winds from the U.S. He started growing peas a year ago and spraying fungicide was built into the plan from the get-go. Schultz wonders if in the years ahead he’ll be spraying for blackleg in canola too. “I really would prefer not to have to do it. I’d prefer to be at the lake!”

Farmers used to enjoy a break in the summer but it’s become harder to take time off. What’s creating such an intense and stressful disease situation?

For a disease to succeed, it needs all three parts of the disease triangle: the pathogen, the host, and the right conditions to flourish. So which part of the triangle is selling us out? The two main factors are, it appears, several years of unusually wet weather and the tight rotations between canola and cereal grains. Additional factors are at play, including whether or not producers are planting susceptible varieties. The problems are not simple and, as it turns out, the solutions aren’t either.

Five years or more of wet weather

Faye Dokken-Bouchard, provincial specialist in plant disease with the Saskatchewan Ministry of Agriculture says five years of unusually moderate temperatures and frequent moisture across much of the Prairies have provided the ideal environment for pathogens.

Anita Brulé-Babel, a plant scientist who works on developing disease-resistant wheat at the University of Manitoba reminds us, “Pathogens are always there. The conditions just have to be right.”

Producers in parts of Saskatchewan and Alberta have had to watch for — and treat — diseases they’ve never worried about before. In Manitoba, where it’s more humid, farmers are already old hands. Fusarium head blight (FHB) is a given in Manitoba and Ontario and even in some parts of Saskatchewan. Farmers in the rest of Saskatchewan and Alberta are now watching closely for it. Schultz says FHB is on his radar especially because there’s a zero tolerance for FHB in Alberta seed cleaning plants: “If we ever got it, we wouldn’t be able to use our own seed.”

Thicker plant stands mean more moist conditions

Recently, Schultz is “paying more attention to plant stands.” Higher density means better yield as well as less reliance on herbicides. But with a thicker crop, disease spreads more easily because the canopy dries out more slowly after a rainfall or even a dewfall.

“Dense plant stands are not a bad thing,” says Gardner. “Farmers spend a lot of time and money and effort to create a great stand. They are aiming for higher yields and they are doing everything right. But they are also creating an environment for disease. They have to accept that risk.”

Is going back to thinner plant stands the answer? More aeration could help, says Dokken-Bouchard, but only if there’s a relatively low rate of infection in the first place. Thin stands have disease disadvantages, too.

Gardner points out that in a thin stand, “the plants are flowering all over the place so the window for FHB infection slides open. A good solid stand means uniform flowering and the window for infection is narrower.”

Tight rotations mean higher pathogen levels

In addition to the wet weather, disease rates are a reflection of current tight rotations. According to Kelly Turkington, research scientist with Agriculture and Agri-Food Canada at Lacombe, Alta., up to 60 per cent of the acres on the Prairies alternate yearly between cereals and canola. If the weather is conducive, every year farmers are either dealing with a wheat disease or a canola disease. There is no break. When producers ran a solid four-year rotation, Gardner believes “things took more care of themselves.” That rotation included summer fallow, forage legumes, and diverse cereals — wheat, barley and oats.

The tighter rotations in the last 10 to 15 years are largely driven by commodity prices. Sabine Banniza, pulse specialist with the Crop Development Centre at the University of Saskatchewan, acknowledges that farmers “want to capture the high value of certain crops like canola.” Unless they’re close to a market or have a cattle operation, they’ve left summer fallow and long-term perennial crops, which both break up the disease cycle, behind.

In longer rotations, farmers take the host out of the disease triangle for a period of time, allowing pathogens to breakdown. With a short rotation, the pathogen population builds up over time, waiting for the right host and the right conditions to come around again. Turkington admits, “I know we need longer rotations because of my work as a pathologist. But if I were farming with my dad, we’d grow canola-wheat-canola-wheat because of economics.”

Pathogens are always evolving

Canola-cereal-canola-cereal is a “vulnerable rotation” because the farmer is simply selecting for resistant individuals of the pathogen population. You can be assured, says Turkington, that problems will develop in two, three or five years. Depending on the disease, resistant varieties aren’t always available. But even if they are, Brulé-Babel says, “If the pathogen shifts, then it may take plant breeders five to 10 years to shift the breeding programs to varieties that have new sources of resistance. Turkington warns, “You can go from a resistant crop to a crop with high disease susceptibility in an instant.”

Breeding for host resistance is difficult when it comes to residue-borne diseases, says Brulé-Babel. “When you build in resistance for one, you make room for another one.” In her work with leaf-spot pathogens, it was possible to find “good sources of host resistance.” But building resistance to FHB is a challenge because, she says, “We’ve got a large number of genes involved each with small effects.” The resistance is not complete but in combination with a fungicide, “you get some pretty good results, especially in a low infection year.”

Fungicides are now part of the plan

From now on “fungicide is built into our crop plan,” says Shultz. But, he acknowledges it can’t be our “one big hammer for dealing with all of these issues, otherwise we’ll create problems.” Schultz cites an example from Australia, where scientists were able to develop good resistant genes to blackleg in canola. But after producers overused a particular fungicide, a different string of blackleg emerged. Turkington agrees. “We don’t have a continuous supply of new active ingredients. The goal is “to add diversity into that system as best you can,” and farmers won’t achieve that by using the same product repeatedly in the same year and between fields.

The learning curve is steep for producers, says Banniza. They are working out the best timing for an effective application. Pulse growers across the Prairies and cereal growers in Ontario and Manitoba where FHB is predominant, have far more experience than western grain growers. Schultz uses what is happening in other parts of the world as a model: “As compared to Europe, we’re just at the cusp of using fungicide. We can learn from other people’s mistakes.”

Wrestling with the disease triangle

All points of the disease triangle are at play when it comes to the development of current disease conditions. Gardner says, “The bed is made. It’s not going to disappear overnight.” At the same time, the industry strives for solutions with the hope that they won’t just be short-term.

Individual management options won’t fully protect crops, says Dokken-Bouchard; multiple strategies, including varietal resistance, rotation, and fungicide application, will provide the best means of grappling with that dreaded triangle.

It’s not tillage

With no-till acres on the rise, it’s tempting to pin current disease issues on that change. University of Manitoba plant scientist Anita Brulé-Babel says crop residues left on the surface can take two to four years to decompose leading to a build-up of residue-born pathogens including tan spot, scald, fusarium head blight (FHB), sclerotinia and leaf spot diseases. When a susceptible crop reappears in a tight rotation, and suitable environmental conditions emerge, the pathogens are there, ready and waiting.

According to Kelly Turkington, Agriculture and Agri-Food research scientist, tillage is not a big factor in preventing disease, even FHB. For instance, aggressive tillage in northeast Saskatchewan in the 1990s didn’t prevent the spread of blackleg in canola.

Certain diseases, says Saskatchewan provincial plant disease specialist Faye Dokken-Bouchard, occur less frequently under reduced tillage such as root rots in cereals. The effects change according to the disease and the pathogen. For instance, for a disease like anthracnose of lentil, no-till is clearly beneficial because the pathogen deteriorates more quickly on the soil surface, says Sabine Bannisa, pulse crop pathologist at the Crop Development Centre in Saskatchewan. For other diseases, it’s the opposite.

Few long-term studies have focused on the impact of no-till on disease rates, notes Bannisa. Some research has looked at mold-board ploughing, where the plough completely inverts the first eight to 12 inches of soil creating a physical barrier. But, Turkington says, then the soil must be tilled to prepare the seed bed and pathogens may be pulled back up to the surface. Brule-Babel and Dokken-Bouchard agree that no-till benefits outweigh the disadvantages.

Patty Milligan is a freelance writer based at Bon Accord, Alta.

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Patty Milligan is a freelance writer based at Bon Accord, Alta.

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