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Decision time on sclerotinia control in canola not far off

There’s lots to consider, including the value of the crop and the disease risk

In Manitoba the potential for major canola yield losses due to sclerotinia exists every year with weather being the main driver.

Farmers will soon have to decide whether to apply a prophylactic fungicide or not.

“The (sclerotinia) inoculum is always there (in Manitoba) so the part of the disease triangle to consider is the environment and if it’s going to be right,” Manitoba Agriculture plant pathologist Holly Derksen said in an interview.

The disease triangle refers to the three factors required for a crop disease to exist. Besides the right environment, the disease itself needs to be present and there must be a host. Because so much canola has, and continues to be grown in Manitoba, the host is as ubiquitous as the sclerotinia inoculum.

Fungicide timing

Several fungicides are registered to control sclerotinia, a fungal disease that affects several other crops, including sunflowers (head and stem rot), edible beans (white mould) and soybeans.

Among them are Proline (prothioconazole, a Group 3), Lance (boscalid, a Group 7), and BASF’s newly registered product Cotegra, which is a combination of both active ingredients.

Most sclerotinia fungicides should be applied when 20 to 30 per cent of the crop’s flowers are blooming, Derksen said. Very often that’s at the end of June or first week of July. But it could be earlier this season following dry, warm conditions, which has hastened maturity in some fields, she said.

Pant leg test

Assessing the sclerotinia risk can be tricky. If conditions have been dry and the crop is thin there’s less risk as high humidity is required to get the disease going.

“You have to have a crop that is thick enough to create a canopy to create a humid environment,” Derksen said. “We talk about the pant leg test. Are your pant legs coming out dry (after walking in a canola field) at 10 a.m.? Then you probably don’t have enough moisture. If it’s 1 p.m. and you walk through and your plant legs are wet it’s an indication there is more than enough moisture.”

Many Red River Valley farmers tend to automatically spray for sclerotinia, while farmers elsewhere often weigh their decision based on conditions, she said.

The Canola Council of Canada’s website says farmers should consider factors such as potential yield and crop price, fungicide cost, current and predicted rainfall, the presence and level of disease inoculum and host susceptibility.

Assessment checklist

A checklist developed in Sweden can help farmers and is availble for download from the canola council (http://tinyurl.com/ydcehveq).

Farmers should start assessing after first flower when 75 per cent of the canola plants have at least three open flowers, the website says.

The total of the risk points from all questions answered represents the overall risk for sclerotinia. Assess early so if spraying is required it’s done at the right crop stage.

Continue monitoring up to full bloom. The risk can change from early flower to full bloom.

The greater the risk score, the higher the probability of a positive economic return to a fungicide application.

Swedish researchers found with a total of less than 40 risk points, the risk of heavy infection was low. A threshold value of 40 risk points accurately identified 75 per cent of the fields that needed spraying for sclerotinia, but also 16 per cent of the fields that did not need a fungicide.

A threshold of 35 risk points increased the accuracy to almost 90 per cent of the fields that needed spraying, but also included more than 20 per cent of the fields that didn’t.

“They concluded that under their conditions if the risk points were 40 or higher, it was likely worth spraying,” the site says. “If the risk points were less than 40, it was not likely worth spraying. This provides a good guideline for use of the checklist in Canada, but growers should keep in mind that it may vary a bit under our field conditions, and also depending on fungicide cost and commodity price.”

Using the checklist effectively requires scouting for apothecia that are usually in nearby cereal crops following canola or other host crops.

Soil conditions right for apothecia production also favour the development of other types of mushrooms or fruiting bodies.

Apothecia are small, golf tee-shaped structures that produce sclerotinia spores.

The site says accurate identification of apothecia is critical to determining the sclerotinia risk.

No guarantees

Sclerotinia can still hurt canola yields even after a fungicide is applied, Derksen said.

“I think our biggest problem, and why we don’t get the control that we expect in canola when we use a fungicide, are timing issues and when we have a stagey crop,” she said. “I think it is less to do with how the products actually work.”

The potential risk and severity of a sclerotinia infection is difficult to forecast, Keith Gabert, the Canola Council of Canada’s agronomy specialist, central Alberta south, said in an interview.

If farmers believe their canola might be at some risk, but can’t decide about spraying they should consider using one of the less expensive products, he said.

According to BASF’s tests Cotegra boosted average yields by two bushels an acre versus competing fungicides.

Yields averaged seven bushels more when compared to untreated fields.

“I don’t know if it (Cotegra) will look any different than other products in canola necessarily,” Derksen said. “There are two modes of action. That’s always good for (preventing) things like (developing disease) resistance (to fungicides.)

“I think sclerotinia is rated overall as a low risk (for fungicide resistance).”

Cotegra is also registered to control white mould in soybeans, bringing a new active ingredient (boscalid) to western Canadian soybean growers.

The product also controls Asian soybean rust, frog eye leaf spot and septoria brown spot in soybeans, BASF’s website says, as well as white mould in chickpeas, field peas, lentils and edible beans.

About the author

Reporter

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.

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