Late Blight Update – July 10

Late Blight Update – July 10

on July 19 | in Ag News | by | with No Comments


By: Janice LeBoeuf, Vegetable Crops Specialist, OMAFRA, Ridgetown

To date, I know of no confirmed cases of late blight in Ontario, but it has been found on potato in Branch County, Michigan (south of Battle Creek). Nevertheless, our experience of recent years would indicate we are likely to see it in Ontario tomatoes at some point in the season.

Remember that conventional tomato growers using a recommended fungicide program for early blight, septoria leaf spot, and anthracnose, are also protecting the crop from late blight infection. 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.

If late blight is found in the area, tomato growers should:

  • Tighten up spray intervals – During wet cool periods, a fungicide should be applied every 5 – 7 days to protect against late blight. If the weather conditions become dry, the spray intervals may be extended. Rotate late blight fungicide groups to manage fungicide resistance.
  • 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.

Past OMAFRA articles on tomato late blight are found under the late blight tag.  Please report any occurrences so that we can keep the tomato community informed about its development and spread and so that we can collect samples to learn about the strains, fungicide sensitivity, and the biology of this evolving disease.

Contact OMAFRA at 519-674-1690 or if you suspect you have found late blight in Ontario. Those in the US can contact their state cooperative extension service or report online at This helps us alert others of late blight in the region and if possible, we will try to collect samples for late blight researchers studying the disease.

Note:  Organic producers may be interested in an article by Meg McGrath (Cornell University) on managing late blight in organically-produced tomato.  Note that crop protection products mentioned in her article relate to US registrations.  Consult the Canadian labels and your organic certifier for registered products that can be used in organic production.

Tomato Late Blight Photo Gallery

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