White mould mapping project underway

Until this past growing season, sclerotinia or white mould was third behind soybean cyst nematode and sudden death syndrome as the biggest soybean threats for most of Ontario.

It likely ranks higher in eastern areas of the province, where white mould is a near perennial threat to manage, but there are scattered hot spots in other regions.

In wet years, it’s impact can be more widespread. Most of the province in June through August saw higher than normal amounts of rainfall, prompting one certified crop advisor to suggest researchers and extension personnel simply “colour the entire province red” to represent its impact.

Why it matters: White mould can cause significant losses in soybean and pulse crops.

The timing is impeccable for a collaborative project started by Valerio Hoyos-Villegas of McGill University and Tanya Copley of the Centre de recherche sur les grain near Montreal. Their goal is to examine the phenotypic and genotypic variation of Sclerotinia sclerotiorum, white mould’s pathogen, improve sclerotinia nurseries and get a better understanding of the disease.

According to Copley, western Canadian researchers have been gathering isolates since the early 2000s, so the database is large. But the same can’t be said for Eastern Canada, and especially Ontario. Quebec researchers and some with Agriculture and Agri-Food Canada on Prince Edward Island have been funneling samples to Hoyos-Villegas and Copley, but the numbers from Ontario are insufficient to help with the project.

That’s changing slowly via calls to agronomists and crop advisors from across the province. Once the two researchers have the requisite database of isolates, they can assess the aggressiveness of various genotypes and phenotypes of white mould. The goal is to help plant breeders develop more resistant and resilient cultivars.

“It’s definitely an aggressive disease. There is no cultivar that is completely resistant,” says Copley, a plant pathology researcher.

“If there’s an increase in pressure, even a resistant cultivar can end up dying off, depending on the amount of pressure of spores in a year.”

She cites a U.S. study (with a limited number of isolates) that found a complete inversion in resistance: a susceptible cultivar could become resistant and a resistant one could suddenly become susceptible to a given isolate.

Copley has also had calls from growers who say they’ve planted a cultivar labelled resistant (by either a seed company or in public performance trials), only to have a severe case of sclerotinia.

More than soybeans

Part of the challenge in assessing white mould’s aggression and potential for inflicting yield loss is the spectrum of crops it infects. In Eastern Canada, soybeans are the primary concern, yet the cropping landscape still counts edible dry beans as a host, as well as canola, thanks mostly to the recent arrival of a newer winter hybrid.

In the West, soybean production has fluctuated, with Manitoba and Saskatchewan leading the way. But sclerotinia is also third among canola diseases, behind clubroot and blackleg. And Western Canada also sees higher production of dry beans, lentils and chickpeas, all of which are hosts for white mould.

Copley also says some cropping practices may have an effect.  

“I’m seeing a lot more rotations that are shortened. We have producers here (in Quebec) who’ve been in soybean for 20 years who are having white mould issues.

“There’s diversification in cropping practices where we’re seeing legumes implemented in the east and we’re seeing more soybean moving out west, and we’re using more cover crops or green mulches, and some of those are susceptible to white mould.”

Shortened rotations that introduce pulses and soybeans –and possibly canola –increase the risk of inoculum load in a field, which will raise disease pressure in a favourable year, as has been the case in 2023.

If there are more sclerotia produced on a yearly basis due to susceptible crops, there’s a greater likelihood of damage in an outbreak year.

“We’re gaining a better comprehension of this disease that was often looked at from an ‘either resistant or not’ perspective,” adds Copley. “Now it’s a question of whether white mould is more complex than previously thought or are there isolates with genetic sequencing where we can get more information and be able to map things better with the technologies that are available now?”

Researchers have cast a fairly wide net, with contributions from Lone Buchwaldt at AAFC Saskatoon, Syama Chatterton at AAFC Lethbridge and Adam Foster at AAFC Charlottetown.

“We’re aiming nationwide with this, wherever there are problems with white mould, from any culture,” says Copley. “We’re not limiting ourselves, saying we only want isolates from pulses or soybean. We’re going to try to have a good bank. If someone needs an isolate, let’s say just for potato, we’ll hopefully eventually have it, to be able to improve the industry.”