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Rotation, resistance and fungicides to manage diseases of canola and field pea

Crop rotations are an important practice in all crop production systems. Research is showing just how effective rotations are for managing diseases in canola and field pea.

November 6, 2008  By Top Crop Manager

Rotations and genetic resistance effective tools to manage diseases. 

Three-year pea-canola-wheat rotation at AAFC Melfort Research Station.
Photos courtesy of Agriculture and Agri-Food Canada.

Crop rotations are an important practice in all crop production systems. Research is showing just how effective rotations are for managing diseases in canola and field pea. Results from a 10-year Agriculture and Agri-Food Canada (AAFC) research project show the consequences of intensifying rotations over the short and longer term.

Researchers compared different canola and field pea rotations at two locations in Saskatchewan, Scott and Melfort Research Stations, ranging from continuous to four-year rotations. Two canola varieties were compared, a blackleg resistant herbicide-tolerant Invigor hybrid, and Westar, an older, conventional variety with no blackleg resistance, with or without fungicides. “Comparing the two extremes in varieties allows us to make inferences about varieties that fall in between those extremes,” explains Dr. Randy Kutcher, Research Pathologist in Melfort.


The canola rotations included continuous canola, a two-year canola-wheat, a three-year pea-canola-wheat and a four-year canola-wheat-pea (or flax)-wheat. The similar pea rotations using one pea variety included continuous pea, a two-year pea-wheat and the same three- and four-year rotations.

Implications of intensive canola rotations
“Our research showed that in the short-term, growing blackleg resistant canola in a canola-wheat rotation didn’t result in much yield loss,” says Stewart Brandt, Crop Management Agronomist at Scott. Genetic resistance to blackleg is a very effective tool for blackleg management. “However, as we continued that rotation over several more cycles, there are indications there could be problems, particularly with blackleg disease.”

Researchers found that with shorter rotations and susceptible varieties, the incidence of blackleg disease increased. “When rotations are intensified from a four-year to a three- or two-year rotation, there is increasing evidence of the disease, with more infected stems, more infected residue left behind in the fields and lower yields in each of those rotations,” explains Kutcher. “With the resistant varieties, the decrease in yield is not as evident from the four-year to the two-year canola-wheat rotation.” However, the blackleg disease was still there even with the highly resistant variety, and over the years the inoculum load is increasing.

“The big concern is the more infected residue you leave, the greater the opportunity of a genetic recombination event or the development of a virulent strain of fungus that will overcome the resistance we are relying on,” explains Kutcher. “Generally, the resistance in our varieties has stood up fairly well in Canada, however in countries like Australia and France there are instances where the source of resistance doesn’t work anymore.” There have been some potential resistance problems and new fungus strains identified recently in southern Manitoba.

Fungicides reduced blackleg incidence and severity, but the magnitude of the reduction was generally low. “In some cases, we saw the best response to fungicide treatment in the three-year Invigor rotation on pulse crop stubble,” says Brandt. “However, in the continuous Westar rotations under high disease pressures, we saw the poorest fungicide responses, which was a surprise. We expected that under those high disease pressures is where fungicides would shine.” This suggests that in order for fungicides to be effective, a certain level of crop health must be maintained, but they will not correct for poor disease management overall.

“At this point, unless our blackleg resistance fails, growers don’t usually need to be spraying fungicides for blackleg,” says Kutcher. However, he reminds growers that may not be the case for sclerotinia problems. Researchers did not encounter high levels of sclerotinia in the project, and early in the study conditions were extremely unfavourable for sclerotinia due to a drought cycle. “However, if growers already have a sclerotinia problem then shortening rotations should be approached with additional caution,” says Brandt. “If sclerotinia is showing up in the pea phase, then consider inserting a wheat crop to extend the rotation.”

From an economic standpoint, there was a big advantage of using hybrids in rotation, regardless of the rotation practices. Even with intense rotations, there were no real concerns with weed control with the hybrids. “We were also surprised with the continuous Westar rotations, and how long it took to run into serious weed control problems,”says Brandt.

Well-managed three- and, over the longer term, four-year rotations, and genetic resistance remain the two most effective tools for managing blackleg in canola. The focus should be on using cultivars with the best disease resistance available. Where those fail, growers may be able to intervene with fungicides. Intensifying canola production will depend on maintaining a high level of genetic resistance to common canola diseases and development of new resistant cultivars.

Field plots flowering at AAFC Melfort Research Station.

Intensifying pea rotations
Mycosphaerella blight is the main disease pathogen of field pea, which is widely spread by air and is fairly long lived in the soil. “Rotations are important for field pea, with a significant difference between the continuous pea and the two-year pea-wheat rotation,” says Kutcher. “However, we didn’t see a huge difference in terms of disease severity between the two-year and four-year rotations.”

Under certain conditions fungicides can be an effective tool for control of mycosphaerella blight. However, researchers still have not been able to develop a reliable predictive capability. “In some years, we’ve seen as much as a 20 percent yield increase from a fungicide application, but in others we often didn’t get any response,” says Kutcher. “It’s not to say fungicides don’t work in field pea for mycosphaerella blight, it’s just the difficulty in predicting a consistent response from using fungicides.” In higher moisture growing conditions, fungicides may be warranted.

“Although the yield differences overall weren’t always significant between the rotations, there was a general tendency for yields to start to decline as we grew peas more frequently in rotation,” says Brandt.

“The yield reduction was substantially higher in Melfort than at Scott, particularly in the latter years of the study, because we ran into a very serious weed problem with cleavers.” In the longer term the four-year rotation is probably the most appropriate, and with careful management growers may be able to go to a three-year or a two-year for dealing with specific issues.

“Overall, the research suggests that for those growers who traditionally follow an appropriate rotation, there is flexibility to shorten up rotations to take advantage of high prices, or to deal with other agronomic issues or to change field sizes, without any yield penalty in the short term,” says Brandt. “However, over the long term the risk of disease buildup or overcoming disease resistance is a major consideration.”

The study was funded by the Saskatchewan Canola Development Commission and the Manitoba Canola Growers Associations through
the Canola Agronomic Research Program (CARP) of the Canola Council
of Canada.  This funding was matched with funding from the Matching Investment Initiative (MII) program of Agriculture and Agri-Food Canada.


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