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Tackling tough canola diseases

The rotation study involves canola, wheat, corn and soybean in 16 different rotation sequences. Photo courtesy of Balakrishnan Prithiviraj, Dalhousie University.

Canola production in the Maritimes has good potential – canola grows well in the region’s climate and has a higher value than some of the small grain crops typically grown in the region. But one of the barriers to increasing canola production is disease. So Balakrishnan Prithiviraj, an associate professor at Dalhousie University’s agricultural campus in Truro, N.S., is working on two projects to assess integrated management strategies for two important canola diseases.

The diseases are sclerotinia stem rot (Sclerotinia sclerotiorum) and blackleg (caused primarily by Leptosphaeria maculans). Both these fungal diseases are found across Canada’s canola-growing regions, and both can have major yield impacts on canola. Sclerotinia sclerotiorum has a very wide host range including many broadleaf crops and weeds. Leptosphaeria maculans affects cruciferous plants like canola and cabbage.

From what Prithiviraj has seen, stem rot is a much more serious problem in the Maritimes than blackleg. “This region is the perfect recipe for stem rot because it has exactly what is required for the development of the disease,” he says. Weather conditions in the Maritimes often favour the stem rot, and other host crops, such as potatoes, are commonly grown in the region.

“The Annapolis Valley area, where we’re running our canola trials, is the vegetable belt. They grow a lot of cauliflower and broccoli, as well as soybean, [which are all susceptible to sclerotinia stem rot,] so the area has become a hot spot for this disease,” Prithiviraj notes.

He adds, “It was a disastrous year here for stem rot in 2015. The summer weather had been warm for a while, and then it was humid for a long period of time, which is perfect for development of stem rot.”

Rotation and fertilizer effects
One of Prithiviraj’s projects is evaluating the effects of crop rotation and nutrient management on the incidence and severity of stem rot and blackleg.

Initially, Prithiviraj conducted the project at the university’s research farm at Truro. But for the past two years he has piggybacked his project onto projects of his Dalhousie colleague, Claude Caldwell, being conducted in Canning, in the Annapolis Valley region. Caldwell is involved as a co-operator and leads projects that are assessing the effects of nutrient management and crop rotation on canola yields. Prithiviraj’s project fits really well with that work, plus the collaboration is lowering Prithiviraj’s project costs by almost 60 per cent.

The rotation study involves four crops (canola, wheat, corn and soybean) in 16 different rotation sequences. Canola follows each of the other crops every year in one of the rotations. The rotation study started five years ago, so the rotations have all gone through at least one complete cycle.

Although Prithiviraj’s project has three more years to go, he’s already getting some interesting preliminary results on the effects of rotation on disease. “Based on the two years of data, we had the least canola disease when corn preceded canola. Second best was when wheat came before canola. When soybean was before canola, we had a little more canola disease, which might be because sclerotinia is an issue in soybean. We had the most disease with the canola-canola rotation.” Corn and wheat are not hosts to stem rot or blackleg.

The rotational effects were easy to see just by looking at the plots. He says, “Where corn preceded canola, we had a beautiful canola crop. We could see a dramatic difference between the plots where corn preceded canola and the other rotation plots in terms of the height of the canola crop and the flowering intensity. However, this dramatic difference in appearance and disease level has not consistently translated into a significantly increased seed yield.”

The continuous canola plots were almost like a textbook illustration of just how bad disease levels can sometimes be in a monocropping system. “In 2015, to our surprise, there was so much disease in the continuous canola rotation that we couldn’t collect any data from it; it was completely devastated by disease. There was not enough canola remaining by the time plants had reached podset to make any assessment of sclerotinia or harvest any seed. There was stem rot in very early phases, but the main cause of the canola death was clubroot. About 80 per cent of the plants were infected severely in the continuous canola plots.”

The nutrient management project is evaluating the effects of sulphur, boron and nitrogen application rates.

“We’re looking at sulphur because, as we all know, sulphur nutrition is very important for canola,” Prithiviraj notes. The researchers are applying this nutrient as ammonium sulphate and comparing four rates (zero, 10, 20 and 40 kilograms per hectare), alone and in combination with different nitrogen rates. Based on the results so far, the sulphur applications aren’t having a big effect on disease levels.

Canola also has a higher need for boron than crops like wheat and barley. The project’s results to date show only a very small difference in disease levels with the different boron rates used in the treatments. “A higher boron level of two kilograms per hectare reduced the disease, but it was not statistically significant,” Prithiviraj says.

The nitrogen response trial treatments are having the greatest effects on disease levels in canola. “Higher nitrogen always leads to more disease,” he notes. Higher nitrogen rates result in denser crop canopies, which can trap moisture, favouring stem rot and blackleg.

This project will continue for the next three years.

Innovative disease-fighting option
Prithiviraj’s other project is focusing mainly on the use of seaweed bioproducts to manage stem rot and blackleg. “Another area of research in my laboratory is using marine bioproducts to improve plant health, for example the use of seaweed extracts – chemicals derived from seaweeds – to see if they can reduce the incidence and severity of disease. I’ve been working on this for 10 years, since I arrived in Nova Scotia. We have lots of seaweed here, and I work with a seaweed extract company here; it is one of the largest seaweed bioproduct companies in the world,” Prithiviraj explains.

In the stem rot and blackleg project, Prithiviraj’s lab is examining the possibility of reducing the use of fungicides by using them in combination with seaweed bioproducts. For the past two years, the researchers have been comparing the effects of various combinations of these products in greenhouse trials.

The results are very promising. “We’ve found that using a fungicide at around 10 per cent of the recommended dose along with the seaweed extract gives almost the same effect as using 100 per cent of the fungicide’s recommended dose. That could be a huge benefit, not only for farmers in terms of the cost but also for environmental stewardship,” he says.

“We are still studying how exactly [the seaweed extract] works, but our initial results show that it’s partly a surfactant effect, because the seaweed has very good surfactant properties. So when we use it in combination with fungicides, it improves the activity of the fungicide. Also the seaweed extract by itself works almost like a vaccine, inducing an immune response in the plant.”

He adds, “We’ve done this work in other crops also, and we’ve found similar effects, for example in potatoes. So my major focus at the moment is to see if we can fine-tune the technology so growers can adopt it.

“Although seaweed extract is not widely used in Canada, it is being used in other areas, like California and other parts of the western United States, especially on high-value crops like grapes, apples and vegetables. The company that I work with exports its products to 70 countries around the globe. Seaweed extract is mainly used, not for disease management, but especially for alleviating abiotic stresses in crops.”

This year, Prithiviraj’s lab will be starting field experiments with the seaweed/fungicide work in canola. They’ll be testing the most promising combinations from their greenhouse trials to see how those combinations affect disease management and abiotic stress response in canola.

Prithiviraj notes, “There are two more years left in this project, but we’d like to do three years of field trials. At the end of the third year, we hope we’ll be able to identify one or two combinations that growers could adopt.”

Potential benefits for growers
According to Prithiviraj, the results from his projects will help eastern Canadian canola growers by providing information on rotation and fertilizer practices for managing stem rot and could introduce some fungicide/seaweed bioproduct combinations that reduce the disease.

He adds, “Some of the results might benefit canola producers in both Eastern and Western Canada, especially the fungicide combinations. Canola is extremely prone to heat stress during the flowering and seed setting stages, which is the same time we have an increase in stem rot. So applying these seaweed/fungicide combinations might be helpful not only for reducing the use of fungicides and reducing the disease, but also for alleviating heat stress effects on the crop. That’s our long-term goal, but there is a lot of work that still needs to be done.”

Both of Prithiviraj’s projects are part of a series of canola and soybean projects being conducted under the Eastern Canada Oilseeds Development Alliance. Researchers in Ontario, Quebec, New Brunswick, Prince Edward Island and Nova Scotia are involved in this initiative. The projects are co-funded by the Growing Forward 2 program of Agriculture and Agri-Food Canada (AAFC) and industry. In the case of Prithiviraj’s projects, funding is from AAFC and TRT-ETGO, a major canola crushing facility at Becancour, Que. (The Canola Council of Canada is the industry leader for the Growing Forward 2 canola rotation study being conducted by Caldwell, where Prithiviraj is collecting disease data.) 

April 26, 2016  By Carolyn King