Long-term disease management crucial to continuing success of canola

Seed companies are constantly working to develop and test new canola hybrids to improve yields and provide effective disease resistance. However, to ensure that this lucrative crop remains profitable and sustainable for the future, canola growers also need to play a very important role in combatting diseases that threaten their crops.

The greatest threats

While developing new and more complex canola varieties to battle diseases like blackleg and clubroot is a major priority for people like Bruce Harrison, director of Research, Development and Innovation for Crop Production Services in Canada, getting growers engaged is just as important.

“We need to do a better job of educating growers on proper stewardship and agronomy practices. Management practices like responsible crop rotations will help to protect the longevity of the resistance genetics we are developing,” says Harrison.

Plant pathologist Coreen Franke agrees. While she sees diseases like blackleg and clubroot as a threat to future canola production because of their high potential for adaptation, she believes canola growers can play a key role through good disease management practices.

“Tightened crop rotations and the practice of consecutively growing a single canola variety, especially if the variety doesn’t have the most current resistance genetics, are serious risk factors,” adds Franke, RD&I pathology research manager for CPS in Saskatoon. “Growers should always choose resistant varieties, and grow them within a responsible crop rotation in order to manage pathogen spore levels and preserve the resistance genetics available.”

Blackleg and clubroot have two things in common; they are virulent canola diseases that have serious yield implications, and they can adapt quickly to overcome current resistance genetics.

“A shift towards new virulent strains of the blackleg fungus has resulted in the breakdown of resistance in many traditional canola varieties. Regarding clubroot, resistance in most clubroot-resistant hybrids available on the market has also become ineffective in some fields due to a shift in virulence of the pathogen,” says Franke. “Plant breeders are in a race to stay ahead of these diseases.”

Blackleg virulence growing

Blackleg, a serious disease of canola and oilseed rape in Canada and worldwide, is caused by a stubble-borne fungus and can result in significant yield losses in susceptible varieties. It progressively damages the crop throughout the season by producing cankers which girdle stems and restrict moisture and nutrient uptake, leading to yield loss. Blackleg was first detected in northeast Saskatchewan in 1975, but has now become a threat in most canola growing regions throughout Canada.

Blackleg resistance has been available in commercial canola varieties since the early 1990s. In recent years, resistance in many traditional canola varieties has become ineffective. Canola breeders are challenged to identify and integrate new effective blackleg resistance genetics into the latest varieties to address the concern.

This is no easy feat, as the blackleg pathosystem is complex and requires complex resistance genetics. Breeders have to consider two types of resistance when developing a blackleg resistant variety: single major R genes which are race-specific (qualitative resistance), and a multiplex of minor resistance genes, which are non race-specific (quantitative resistance). The particular combination of these resistance genetics is what differentiates the level of protection and determines how a variety performs across a range of environments and against a range of blackleg strains.

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

Global experience

The CPS Research, Development and Innovation team accesses strong blackleg genetics from around the world, and especially from Australia where they have a dedicated breeding effort and the devastating effects of blackleg have been seen first-hand. Genetics that are effective against blackleg in Australia have a definite benefit for canola growers in Canada.

Blackleg in Western Canada is becoming more complex and by understanding the evolution of the disease and also what genetics are effective, the team has established a breeding strategy to develop solid blackleg resistant hybrids using multiple resistance genes and attempt to stay ahead of the ongoing problem of adaptation by this disease.

“All new hybrids are tested in a rigorous multi-location blackleg field screening program, which evaluates over 100,000 plants annually, to determine performance and resistance ratings,” says Harrison.

The resistance ratings currently available on commercial varieties are based on variety performance in terms of disease severity relative to a susceptible check. “It isn’t a perfect system. An R rating doesn’t mean the variety is immune to blackleg or that it would be resistant to blackleg in all geographies but in most instances will provide good protection against blackleg. The true test of a variety is how it performs across a range of environments and against a range of blackleg strains,” says Harrison.

Local solutions

Harrison believes that blackleg is a disease that can be managed effectively with a four-year canola rotation. However not all producers are prepared or willing to execute this practice on their farm. With current trends for tighter rotations and recent environmental conditions that are favourable for disease development, blackleg is on the rise in many parts of the Prairies.

The only way to stay ahead of blackleg in today’s agricultural climate is to grow varieties with effective blackleg resistance genetics. “Genetics will help growers manage the disease. We are developing canola hybrids with exceptional blackleg resistance in addition to having the high yield and desirable agronomic characteristics that our growers demand,” says Harrison.

Franke shares these same disease management recommendations. “For blackleg, grow varieties that have received an R or MR (moderately resistant) rating in recent years. Older varieties may no longer be resistant to newer blackleg strains, even if they carry an R rating.”

The CPS canola breeding program keeps the Proven Seed pipeline full. “Our newest canola variety PV 540 G is a Genuity Roundup Ready hybrid that is the first of our next generation of hybrids,” adds Ryan McCann, director of seed for CPS. “It boasts high yields and world-class blackleg protection. We’re seeing excellent field results and its performance in the blackleg nurseries this summer provides us a lot of confidence that we’ve got a strong performer for growers.”

Clubroot – the new threat

ClubrootCompared to blackleg, clubroot is fairly new to the field in Canada. The first report of this disease in a commercial canola field in Western Canada occurred near Edmonton, Alberta in 2003. Since then, the disease has spread quickly and thousands more infested fields have been identified in Alberta to date. In recent years, the clubroot pathogen has been detected at low levels in fields in other provinces as well.

In just over a decade, clubroot has become a serious disease of canola on the Prairies. The soil-borne disease causes swelling or galls to form on the roots, which ultimately causes premature death of the plant. It is caused by a fungus-like protist called Plasmodiophora brassicae.

Not long after clubroot was first identified, canola varieties carrying a single gene conferring resistance to the predominant clubroot strain (‘pathotype 3’) in Alberta became commercially available. However, the repeated use of these varieties in shortened crop rotations resulted in a shift towards new virulent strains, and consequently the loss of effectiveness of resistance in some areas within a short period of time.

There is hope on the horizon. CPS introduced the very first true multigenic clubroot resistant variety (two gene source) that was commercially available for 2016. Proven Seed PV 580 GC affords resistance to the predominant pathotype 3 as well as some additional newly emerging virulent strains, offering an alternative clubroot genetics package for growers in high risk areas.

“Our novel clubroot resistance genes have been introduced from plant species related to canola through traditional plant breeding methods. What makes our genetics different from others is that we have been the first to incorporate more than one effective resistance gene into a single hybrid variety. Multiple effective resistance genes should go a long way to slowing down the evolution of clubroot – and having the resistance remain durable for a longer period of time,” says Harrison.

“No variety will be resistant to all the clubroot strains out there but what we are offering for growers is an alternative resistance genetics package to use as part of their rotation management strategy,” says Harrison.

“Rotation is especially important for managing clubroot, a disease which has already proven a menacing capacity for adaptation. Canola varieties with multiple effective resistance genes are the first line of defense and can offer additional protection and durability, but crop rotation is central to long-term disease management,” added Franke.

Strong agronomic practices critical

As fungicides aren’t an option to manage clubroot, good stewardship practices are a farmer’s only defense. Preventing infestation by cleaning and disinfecting equipment, responsible rotations to help manage spore loads and planting clubroot-resistant varieties are the only viable means of fighting the disease.

According to Coreen Franke, there are three crucial components for disease to develop in a crop: susceptible host plants, the presence of the pathogen and favorable environmental conditions. “This is what plant pathologists refer to as the ‘disease triangle’, and all three components must be present in order for disease to develop,” said Franke.

While the weather is out of anyone’s control, the other two components of the disease triangle can be managed. “Breeders work to develop varieties with new and effective disease resistance genes in order to defend against evolving pathogens and eliminate susceptible hosts,” said Franke. The ongoing development of disease resistant canola varieties provides growers with excellent tools to help reduce the risk of devastating losses due to disease.

In addition to growing resistant varieties, crop and variety rotation will help manage spore loads and is equally important in the disease battle. Growers should avoid seeding any canola on clubroot confirmed fields within a minimum rotation of three to four years. If that cannot be adhered to, and breakdown has occurred, the only variety selection to make is a hybrid carrying multiple clubroot genes, PV 580 GC.

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