OMAFRA specialists Amanda Tracey and Katie Goldenhar are reporting late blight in Ontario as of July 21, 2023.

Sporangia of Phytophthora infestans have been detected via spore trap in tomato fields in Elgin and Essex counties, Ontario. There has also been a report of late blight symptoms on potato in Simcoe County, Ontario

Commercial growers should scout often and ensure they are using fungicides with good late blight activity in their fungicide program.  When late blight is in the area, spray intervals should be shortened.

In situations where there is continued high disease pressure, growers should consider adding a targeted late blight fungicide to the spray program.  If late blight has been identified in a field, use a fungicide with curative and antisporulent activity. A summary of fungicides for late blight management in tomato is available here.

Cloudy and high humidity or wet conditions are favourable for late blight.  The pathogen prefers cool temperatures.  The disease is suppressed by hot, dry weather, but it can continue developing and spreading when suitable conditions return.

Scout fields often.  Know the symptoms. Refer to the Tomato Late Blight Photo Gallery and Late Blight Look-Alikes for photos of late blight and possible look-alikes on tomato.

This is a community disease and control relies on everyone scouting and reporting any occurrences. If you suspect late blight in your crop, please reach out to an OMAFRA specialist to confirm the diagnosis quickly.

Source:  ON Vegetable News July 21, 2023