Late blight is being found in many locations in Ontario. To help you identify it, here is a gallery of photos of the symptoms on tomato. Click on any photo to see a larger version.
If you notice it early enough, you might see symptoms like these.
Within a short time, with favourable conditions, you may see symptoms like those pictured below. In wet conditions, you may see the white powdery growth on the underside of the lesions. This is the pathogen producing spores.
The disease can progress quickly, resulting in complete crop loss if uncontrolled.
Fruit symptoms are shown in the photos below.
Other related plants (potato, hairy nightshade, climbing nightshade, petunia), can be susceptible to late blight. The photo below shows late blight symptoms on hairy nightshade. Photo credit: Eugenia Banks, OMAF&MRA.
eOrganic (http://www.eXtension.org) has an excellent video on understanding, identifying, and scouting for late blight, presented by the Reducing Losses to Late Blight Project, funded by USDA-NIFA: Identifying and Scouting for Late Blight on Farms video (eOrganic)