If you haven’t followed a one-in-four rotation, variety selection and disease scouting are even more important

Thursday, June 01, 2017

Several years ago, after a crash in resistance to blackleg in the popular variety Westar, canola breeders got to work and introduced a gene that gave good protection against the disease. But as usual, Mother Nature responded and new blackleg strains have evolved, making the gene less effective.

“A lot of research has been done by the University of Manitoba and Agriculture and Agri-Food Canada (AAFC) in the last few years looking at blackleg and they have confirmed that we are starting to see a shift to more of these virulent strains of the disease,” says Coreen Franke, canola pathology research manager for Crop Production Services (CPS). “The genetics that we have been relying on to manage the disease are not as effective as they once were in parts of the country.”

Blackleg was a big problem across the Prairies last year, and if Statistics Canada’s forecast of 22 million acres for this year is correct, there will be a lot of fields in fewer than the one-in-four-year rotation recommended to control blackleg and other diseases. Wet conditions last year and this spring will add to the disease pressure. As growers head into what is likely to be a challenging season, there are a few things they can do to manage the risk.

What’s the big deal about blackleg?

The inoculum load (the number of residual blackleg spores in the stubble that can re-infect the next year’s crop) is at its highest the year canola is grown. The spores can remain viable on canola stubble for several years until the stubble decomposes.

If a non-host crop, such as wheat or peas is grown next, the inoculum load decreases. Because non-canola crops are not susceptible, the spores cannot thrive and they decrease in number each year they are denied a host.

However, challenging weather conditions and good canola prices have encouraged tighter rotations and researchers are seeing the results.

“In the last 10 years, the acreage has gone up and rotations have tightened. In the past, there was one predominant strain of blackleg and genetic resistance worked quite well to manage the disease. But now, we are seeing an increase in other strains across most of the Prairies. These newer strains can overcome traditional resistance genetics and they are becoming more prevalent” says Franke. These are the blackleg strains that are virulent on some of the previously R-rated canola varieties, and will produce lesions on the plants and result in increased inoculum loads in the fields..

Researchers are working to stay ahead of the changes, but development of new genetics takes at least four years. Breeders are looking to incorporate appropriate resistance genetics, selected to be effective against more strains and in more regions of the Prairies.

“They need to be durable. We aren’t using single genes anymore but combining resistance genetics, making it much harder for the pathogen to adapt,” says Franke.

Rotate fields and varieties

Last year’s late — and in some cases missed — harvest across Western Canada may make her first recommendation difficult, but Twyla Jones, manager of agronomic services for CPS, says a blackleg management plan should start in the fall, right after harvest.

“Getting into the field right after harvest, taking stem snips just above the root and looking for blackleg pressure in the pith of the stem, is the best way to scout for this disease. It can be harder to identify in the spring once the stems have started to degrade.”

In fields where blackleg is observed, canola should not be grown again until the third, or even better, fourth year. This allows time for a sufficient reduction of the inoculum load.

The second pillar in a blackleg management plan is variety selection. “Growers should be thinking about switching up their canola varieties and specifically looking for recently registered R- and MR- rated varieties,” says Jones. This is even more important in tight rotations as it can aid in reducing blackleg resistance.

There are two types of resistance genetics to fight blackleg in canola. Quantitative resistance is like a jack of all trades. While this resistance has a way to fight multiple strains, it will only lessen the impact of the disease instead of stopping it altogether.

Qualitative resistance will be more targeted and effective, where a plant with a specific R gene will react to a specific strain of blackleg. If that strain changes though, the R gene will no longer be effective.

“The best and most durable blackleg resistance is derived from a combination of both solid quantitative resistance plus strategic use of effective major R gene resistance,” says Franke.

But if you’ve already seeded canola

“I know due to environmental and economic conditions some growers will be tempted to tighten canola rotations this spring,” says Jones.   If that does happen, proper scouting and dealing with issues quickly will benefit growers.

Blackleg is also an opportunist and takes advantage of wounds. If there is damage from hail or insects, the disease may take hold as a result. “Frequent scouting helps to catch it early,” says Franke.

Once you have seeded the crop, scout early and often, says Jones. “If there are  early symptoms  on the leaves, growers may want to consider a fungicide application— an agronomist can help with disease identification, proper treatment and timing.”

The Canola Council of Canada says that propiconazole (Bumper, Pivot, Propel, Tilt), azoxystrobin (Quadris, Exempla) and pyraclostrobin (Headline, Priaxor, Quilt) are registered to control blackleg.

“They only have protectant activity and little or no eradication activity, so ideally fungicides should be applied before blackleg symptoms are present. Quadris and Headline can be applied from the two- to six-leaf stage, while the propiconazoles can be applied from two-leaf to just prior to bolting,” the CCC says on its website.

It says that in a AAFC rotation study at Melfort and Scott, Saskatchewan, generally there was little or no reduction in blackleg severity or improved yield when applying a fungicide on R-rated varieties.

“Even on highly susceptible varieties, fungicides typically only depressed incidence by 20 per cent and severity ratings by about one severity point, which often did not translate into a significant or economical yield benefit.”

The Canola Council’s website has a separate section on blackleg management (www.blackleg.ca) and both Franke and Jones recommend it as a resource to help growers identify and manage the disease.

They also say agronomists can offer an important set of eyes and second opinion on variety selection and fungicide application.

 

Featured in Grainnews Jun. 1st, 2017 by Annemarie Pedersen

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