Current late blight risk in Ontario field tomatoes: June 20, 2019

By: Cheryl Trueman, Department of Plant Agriculture, Ridgetown Campus – University of Guelph

With the wet and generally cool weather conditions this Spring there is a lot of concern within the Ontario field tomato industry about diseases. One of these diseases is late blight, caused by the Oomycete pathogen Phythophthora infestans. In recent years, appearance of this disease in Ontario has been sporadic, with symptoms first reported anywhere from late June to late August. This has made it difficult for growers to know when to modify fungicide programs to account for increased risk of late blight.

This year, we started a new three-year project to assess the value of different spore traps and forecasting models to better predict late blight risk. We are comparing the Spornado and rotorod spore traps at eight sites in Kent County (Fig. 1). Three of these locations are research sites, where sentinel tomato plants will also be monitored for first appearance of late blight symptoms. We are also running the BliteCast forecasting model at Ridgetown Campus, first developed in New York State, to indicate the risk of late blight based on weather factors. To determine if there is a benefit of spore traps or BliteCast to predict risk, we are comparing fungicide program modifications based on current high-risk triggers (late blight reported in the Great Lakes Region) to modifications made based on positive detections in a spore trap, reaching the BliteCast threshold, or the combination of a positive detection in a spore trap AND reaching the BliteCast threshold in a field trial at Ridgetown Campus.

So, what is the risk of late blight so far this year?

• No positive detections of P. infestans spores in Spornado traps, which were installed June 10. Our rotorod traps are being installed today and will begin reporting early next week.
• As of today (June 20), the BliteCast forecasting model has hit the threshold for the first fungicide application at Ridgetown Campus.
• There are no reports of late blight on tomato or potato in Ontario or anywhere in the Great Lakes Region. The only report of late blight in the United States is in Florida.
• Taken together, the above points mean that the environment has been conducive for infection by P. infestans, but so far, we have no evidence that there is an active source of inoculum present in the growing region.

If you suspect late blight in your tomato (or potato) crop, please reach out to Amanda Tracey (Amanda.tracey@ontario.ca, 519-350-7134) or Cheryl Trueman (ctrueman@uoguelph.ca, 519-674-1500 x63646) to confirm the diagnosis.

Project collaborators: Tomecek Agronomic Services, Amanda Tracey (OMAFRA), Sporometrics, Phytodata, and Genevieve Marchand (AAFC).

Funding acknowledgement: Ontario Tomato Research Institute, Fresh Vegetable Growers of Ontario, and the Ontario Agri-Food Innovation Alliance.