Ontario onion attacker is in the weeds

A stubborn new fungus is attacking Ontario onions. Luckily, it does not cause food-borne illness, but it could make your onions smaller and more likely to sprout in storage, leading to potential lost revenues for growers and lower-quality onions for consumers.

Researchers are a few steps closer to understanding the culprit, called Stemphylium leaf blight, thanks to research completed by PhD candidate Sara Stricker in the Department of Plant Agriculture.

Stricker’s research, completed over several years at the Bradford Muck Crops Research Station and partially funded by the Ontario Agri-Food Innovation Alliance, aims to unearth information about Stemphylium’s life cycle so farmers can better control the pathogen. “By understanding how the fungus lives, we can get to fewer sprays while still providing the same level of disease management,” says Stricker.

Stemphylium is a foliar fungus, meaning it affects the plant’s leaves. It has proven difficult to control since it was first spotted in Ontario onions in 2008.

“We don’t usually think about the leaves, but photosynthesis happens and impacts the size of the bulb. Therefore, we have smaller onions if it kills the leaves,” explains Stricker. “And if the pathogen kills the leaves, it means farmers can’t apply their typical sprout inhibitor, which needs to be taken up by the leaves to work.”

This two-pronged attack means growers must sell their onions sooner, onions will need to be kept colder and they will sprout sooner. 

To help mount a defence against Stemphylium, Stricker completed the first genomic confirmation of this pathogen species in Canada. Then she set out to discover where it overwinters, which fungicides could help control it and whether drought stress impacts the disease.

Stricker’s research — summarized in the video below and for which she won the University of Guelph 3-Minute Thesis competition — showed that knowledge of Stemphylium prevention is in the weeds, rhetorically at least. It overwinters in weed species, some of which do not show symptoms. It’s tricky to control for two reasons, she says. “It can infect over 300 different species, and fungicides aren’t working against it, even with five to seven sprays per season. There’s no disease suppression.”

During several years of spore collection and research, she worked closely with research technicians from the Bradford Muck Crops Research Centre. “The people who work there are immensely knowledgeable about carrots and onions. I learned a lot from them.”

Working at the research centre also allowed her to build relationships and speak directly with the local farmers and growers’ association. “They’re confident adopting recommendations from our work, because they see us every day.”

Although her PhD fieldwork is complete, Stricker hopes her research will ultimately contribute to the development of a disease forecasting model or tool such as a “calculator where you input weather variables for growers and it spits out whether you should spray or not.”

“I hope my research will help growers to apply fewer fungicides and select the ones that will work best, reducing the cost to farmers and the cost to the environment.”

This research is funded by the Fresh Vegetable Growers of Ontario, the Bradford Co-op and the Ontario Ministry of Agriculture, Food and Rural Affairs through the Ontario Agri-Food Innovation Alliance, a partnership between the Government of Ontario and the University of Guelph. The Bradford Muck Crops Research Station is one of 15 research stations owned by the Agricultural Research Institute of Ontario and managed by the University of Guelph through the Ontario Agri-Food Innovation Alliance.