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Don't call it a comeback, but late blight disease is on the rise in Central and Western Canada. The fungal disease that causes mold symptoms on potato and tomato crops is potentially devastating when left uncontrolled in fields. However, producers shouldn't panic - Agriculture and Agri-Food Canada (AAFC) research scientists in Prince Edward Island and British Columbia are helping to fend off late blight with new disease identification techniques, advanced preparation, and education to home gardeners.
Weathering the shift in disease targets
Evidence from late blight outbreaks around the world suggests that shifts in the genetic diversity of the pathogen (the organism responsible for causing disease) could be at fault. This is because new or newly introduced genotypes of the pathogen could have increased its ability to cause a higher level of infection and increased resistance to commonly used fungicides, both of which pose challenges for disease management.
"In the last number of years, we have discovered some new strains of the late blight pathogen that affect tomatoes more than potatoes. It's evolving so we've had a coordinated approach among researchers, producers, and home gardeners to stay a few steps ahead."
- Dr. Rick Peters, research scientist, Agriculture and Agri-Food Canada
The existence of late blight has always been very dependent on favourable weather conditions. It thrives in cool, moist environments and is far less present when weather is hot and dry. As climates change across Canada, new areas are affected like British Columbia, Ontario, and Quebec, while others, like Atlantic Canada, are being spared for now, explaining why potato producers are less affected.
Once late blight is visible on your crop, it's too late for excellent control of the disease. This makes prevention key to a well-rounded disease management plan. One way to prevent late blight is through disease forecasting and studying weather patterns – both historical and future. This data, like rainfall, temperature, and humidity, can provide producers with advance warning that future conditions might be ripe for late blight.
In various regions of Canada, the use of spore aerobiology, or spore traps, where producers can monitor the capture of late blight spores, can help prepare for a potential outbreak. Another AAFC research scientist on Prince Edward Island, Dr. Bourlaye Fofana, recently discovered that spraying selenium, a micronutrient or mineral, onto potato plants can also help prevent late blight disease.
New late blight genotypes crop up in Western Canada
To prevent or reduce future outbreaks, you must first know what you're dealing with, which is what led Dr. Rishi Burlakoti at the Agassiz Research and Development Centre to undertake a study on the genetic diversity of Phytophthora infestans (P. infestans), the causal agent of late blight.
During the 3-year study (2019-2021), Dr. Burlakoti and his team, with help from Dr. Peters, collected samples of tomato and potato infected with late blight from commercial fields, small farms, research plots, and community gardens in different regions of British Columbia and Eastern Canada. The samples were taken back to Dr. Burlakoti's lab for identification of P. infestans strains using simple sequence repeat (SSR, genetic marker) genotyping – a technique that helps identify differences in genetic composition of the pathogen populations. He uncovered the first occurrence of the genotype US17 in Canada and 25 new genotypes, now known as CAC1-CAC25. Previously existing genotypes US8 and US11 were also found, with US11 accounting for about 60% of the total population in British Columbia. US23 was the only genotype found among the strains collected from Eastern Canada.
While many of the newly discovered genotypes came from tomato samples found in community gardens, a diverse population of P. infestans was found on all sites, emphasizing the need for home and commercial growers alike to keep their guard up when it comes to late blight.
More importantly, 95% of P. infestans strains showed resistance to metalaxyl-m, a common fungicide ingredient to control late blight of potato and tomato in Canada. This finding suggests that future research is required to identify new groups of fungicides in managing this devastating disease.
"Biovigilance on population dynamics of the late blight pathogen can help in mitigating the future challenges that could be posed by this devastating disease on potato and tomato."
- Dr. Rishi Burlakoti, research scientist, Agriculture and Agri-Food Canada
Neighbours helping neighbours
Several years ago, a conscious effort was made by Dr. Peters to educate home gardeners in Atlantic Canada about how they can help control late blight disease.
"Late blight can spread to producer fields from a nearby home garden by spores that can travel on the wind or be splashed onto plants by rainwater. Gardeners can do their part by planting blight-resistant tomato seeds or plants from local seed and garden centres."
- Dr. Rick Peters, research scientist, Agriculture and Agri-Food Canada
This campaign and recent hot and dry summers in Atlantic Canada have substantially decreased instances of late blight to little more than a handful of known occurrences.
"Currently, with the way the late blight disease dynamics are working, home gardeners can play a huge role now that we never used to think about. There is a need for more education on that, especially in some provinces where Dr. Burlakoti discovered new genotypes."
Dr. Peters hopes that home gardeners across other provinces will start the trend. He and Dr. Burlakoti are working with various producer groups in Ontario and Quebec to help make this a reality.
Key benefits and discoveries
- Shifts in weather patterns across Canada over the past 20 years has caused late blight disease in Central and Western Canada while Eastern Canada has been mostly spared from the disease. The disease also shifted to affect more tomatoes than potatoes.
- New or newly introduced genotypes of late blight disease discovered by AAFC scientists can increase the disease's ability to cause a higher amount of infection and increased resistance to commonly used fungicides, both of which pose challenges for disease management.
- Management tools such as weather forecasting, spore capture, and selenium can help producers prevent late blight in their crops.
- AAFC scientists believe that home gardeners can play a role in supporting local producers by planting late blight resistant tomato varieties in their home gardens.
Photo gallery
Dr. Rick Peters has spent more than 25 years researching late blight disease in potatoes and tomatoes.
Dr. Rishi Burlakoti recently uncovered several new late blight genotypes affecting tomato and potato crops in Canada.
Disease dynamics have shifted over the past number of years where more tomatoes are being impacted by late blight compared to potatoes.
Severe late blight infection in field grown tomato.
Severe late blight infection in field grown potato.