Late blight, a fungal disease of potatoes, has been confirmed in Ontario’s Dufferin County. The news was posted by Dennis Van Dyk, OMAFA vegetable crops specialist, late evening of July 17th.

”The identified area was small but risk of late blight should be considered high,” stated Van Dyk. 

Samples have been submitted for strain ID and results will be shared when available. The late blight strain the last few years has been US-23 which is susceptible to Ridomil.

“If you haven’t already applied a late blight-specific fungicide, you should do so now,” advised Van Dyk. “Ensure new growth remains protected.”

Visit the Ontario Crop Protection Hub to view registered late blight specific products on potatoes and tomatoes.

Review the potato late blight entry in Ontario CropIPM to refresh yourself on what late blight infection looks like. The tomato late blight entry can be found at this link

Late blight is caused by the fungal-like oomycete pathogen Phytophthora infestans. The first signs of late blight are usually dark, water-soaked spots on the leaves and stems. On leaves, these spots often have a pale green or yellow border. A white, fuzzy mould which contains the pathogen’s spores, may appear on the underside of the leaves, especially in humid weather. 

Source:  ON Vegetable News July 17, 2025 posting