Site-specific Late Blight Advisory for Ontario Tomato Growers

Dr. Rishi Burlakoti, Weather INnovations Consulting LP, Chatham ON

July 17, 2015

Weather INnovations Consulting LP has been deploying several site-specific decision support tools to advise Ontario Vegetable Growers on better fungicide timing to control fungal diseases of tomatoes, leafy vegetables, carrot, and asparagus at WIN has developed a late blight advisory tool, which calculates daily disease severity values (DSV), indicating the daily risk for late blight infection. Growers can sign up at, register their fields, and add 'tomato' crop to access site-specific late blight advisory. Growers will get accumulated DSV graph since the planting date. They can update the spray dates to recalculate the DSV graph since last spray. Management guidelines are also available on the website to guide fungicide timing using DSV threshold. Growers can apply first fungicide application when the DSV has reached 40-45 DSV accumulating since the planting date (Fig 1). The subsequent sprays can be made at 15-25 DSV depending on the crop stages, varietal resistance, and incidence of late blight in the region.

Fig.1. Comparison of late blight disease severity value (DSV) from three locations in Ontario in 2015 (Assuming that fungicides were applied at 40/20 DSV intervals).

The 2015 growing season late blight advisory model has shown very high risks of late blight infection across tomato growing regions in Ontario (Fig.2). High and frequent rainfall occurred in June and early July (Fig. 2) creating very favourable environment for late blight infection and the pathogen dispersal. The first spray threshold (40 to 45 DSV) was reached during mid to 3rd week of June depending on the locations (Fig. 1 and 2). Late blight in tomatoes was reported on July 10th in Essex County. The first spray threshold reached 3 to 4 weeks earlier than the first symptom of late blight reported in the province indicates that the model recommended the first spray at the appropriate time to protect tomato plants from late blight.

Fig. 2. Map showing accumulated rainfall and late blight DSV values across Ontario.

The thresholds for subsequent sprays have been reached around 7 to 13 days depending on the growing conditions and locations across the province. The 7-day DSV and rainfall for this week is also very high in most of the locations as conditions are still favourable for development (Fig. 3). WIN suggests applying fungicide at every 15 to 20 DSV since the late blight has already been reported in Ontario and neighboring states.

Fig. 3. Map showing 7-day rainfall and late blight DSV values across Ontario.

If you have any further questions on the late blight advisory and how to access other crop management tools at, feel free to contact
Dr. Rishi R. Burlakoti
(519-352-5334, x 225)

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