Late blight in Ontario

Late blight is considered a community disease. Once it is found in a region, all susceptible crops nearby are threatened. It is important that tomato and potato growers, particularly in Simcoe County, apply an effective fungicide program on a regular basis to protect their crops. The disease is best kept under control when fungicides are applied prior to the arrival of the pathogen and infection. Conventional tomato growers, who are applying fungicide on a regular schedule to protect their crops from anthracnose, early blight and bacterial disease, are also protecting the crop from late blight infection however more targeted late blight fungicides may also be required (see OMAFRA Publication 838).

Scouting fields for diseased plants is also recommended at this time particularly after wet weather. If a few infected plants in a patch are found, pull them out and place them in garbage bag and dispose of them away from the field prior to spraying an effective fungicide. Do not transport infected plants through the field without placing them in a garbage bag first.

Symptoms first appear as water soaked light green to grey lesions on leaves and stems of tomato and potato plants. During cool (18-25°C), wet or humid conditions the lesions expand rapidly and often appear as a brown necrotic spot surrounded by a light green or yellow halo (Figure 1). On stems and leaf petioles, late blight lesions appear chocolate brown (Figure 2). Water soaked greasy blotches develop on the top or along the sides of infected tomato fruit eventually becoming sunken and developing into a dry brown rot (Figure 3).

Figure 1. Late blight lesion on a tomato leaf appears as a brown patch surrounded by a light green or yellow halo.

Figure 1. Late blight lesion on a tomato leaf appears as a brown patch surrounded by a light green or yellow halo.

Figure 2. Lesions on tomato stems caused by the late blight pathogen appear as dark brown patches that eventually expand and girdle the stem.

Figure 2. Lesions on tomato stems caused by the late blight pathogen appear as dark brown patches that eventually expand and girdle the stem.

Figure 3. Lesions on tomato fruit first appear as a grey, water soaked greasy patch that turns brown and sunken.

Figure 3. Lesions on tomato fruit first appear as a grey, water soaked greasy patch that turns brown and sunken.

Over the past decade, scientists in North America have been studying and monitoring the different strains that have developed through sexual recombination and spreading throughout North America. There are several strains of the pathogen, some that infect primarily potatoes, some that infect tomatoes primarily and many that infect both crops. The strain US-23 has been the most common strain found this year in the Northeastern states and can infect both tomato and potato.


For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca