Spore traps monitoring for causal agents of tomato late blight and cucurbit downy mildew in Kent, Elgin and Norfolk Counties

Joseph Tomecek (Tomecek Agronomic Services/M.Sc. candidate, Dept. of Plant Agriculture, Univ. of Guelph); Elaine Roddy (OMAFRA), Amanda Tracey (OMAFRA), Dr. Cheryl Trueman (Ridgetown Campus, Univ. of Guelph)

Late blight, caused by the Oomycete pathogen Phythophthora infestans, is a devastating disease of tomato and potato. Another oomycete pathogen, Pseudoperonospora cubensis, causes cucurbit downy mildew, a significant disease of cucumbers and cantaloupe.  The appearance of both of these diseases in Ontario can be 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 these oomycete diseases.

To better understand if spore trapping is a useful tool to better predict risk of these diseases, spore trapping networks have been deployed in Kent and Norfolk counties this year.  A network of spore traps has been set up at 15 locations across the tomato and cucumber growing regions of Kent, East Elgin and Norfolk counties. In Kent County, two types of traps are being tested. The first is a Spornado, which passively captures are particles through a mesh lined cassette placed inside a horizontal funnel. The second trap, called a Rotorod, spins silicone-coated plastic rods in the air to actively capture air particles. The cassettes and rods are collected twice-weekly for testing of pathogen DNA. In Norfolk County, only the Spornado trap is being deployed.

Late blight (Phytopthora infestans)

This is the 2^nd year of a 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, including a comparison of trap height at four of these location (Fig. 1). Three of these locations include unsprayed sentinel plots of tomatoes that are being monitored for first appearance of late blight. The BliteCast forecasting model, first developed in New York State is also being evaluated to indicate late blight risk based on environmental factors.

To determine if there is a benefit of spore traps or BliteCast, we are comparing fungicide program modifications based on the current high-risk trigger (late blight reported in the Great Lakes Region) to modifications based on detections in a spore trap, the BliteCast threshold, or both, in field trials at Ridgetown Campus and the Cedar Springs Research Station.

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

• No positive detections of P. infestans spores in Spornado or rotorod traps, which were installed June 7 in either Kent County or Norfolk County.
• As of June 18, the BliteCast forecasting model has hit the threshold (18 DSV) for the first fungicide application at Cedar Springs but not yet at Ridgetown Campus (16 DSV as of June 28).
• There are no reports of late blight on tomato or potato in Ontario or anywhere in the Great Lakes Region.
• 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, 226-971-0654) to confirm the diagnosis.

Cucurbit downy mildew (Pseudoperonospora cubensis)

If left untreated, cucurbit downy mildew can cause yield losses of up to 100% in cucumbers.  The current management strategy involves a broad-spectrum fungicide program during the early season, switching to targeted downy mildew fungicides as soon as downy mildew is identified anywhere in the Great Lakes basin. 

We are testing for downy mildew in the 3m tall Spornado traps at all 15 locations.  In Elgin and Norfolk, five traps are located in commercial cucumber fields, one trap is in a commercial tomato field and the final trap in located at the University of Guelph, Simcoe research station adjacent to a sentinel plot of unsprayed cucurbit plants.

Current Risk of Cucurbit Downy Mildew

• Symptomatic plants were identified, and downy mildew was confirmed in a commercial cucumber field in Kent County on June 17, 2021.  The current level of infection in the field is very low.
• As of June 25^th, there have been no positive detections of downy mildew spores in any of the Spornado traps within our network.
• The weather conditions over the past 10 days have been windy and overcast, with substantial rainfall in some regions, providing significant periods of potential infection.
• Due to the presence of CDM in the field, all growers in the region are encouraged to maintain a preventative fungicide program using targeted downy mildew products.
• For the most current Ontario downy mildew control strategy, visit Cucurbit Downy Mildew – get out and scout! – ONvegetables

Project collaborators: Yaima Arocha Rosete (Sporometrics), Hervé van der Heyden (Phytodata), Rene van Acker & Rachel Riddle & John O’Sullivan (University of Guelph), and Genevieve Marchand (AAFC).