This symptom is characteristic of late blight.  It is very important to be able to recognize late blight, so that control measures can be implemented as soon as it is detected.  Although is not a frequent problem in the Ontario tomato industry, it progresses and spreads very quickly through an area and can cause major crop losses.

A grey-to-white moldy growth may develop on the underside of infected leaves, especially under conditions of high humidity.  This is a distinguishing feature that you can see in the photo.

Verticillium wilt symptoms do not exhibit this sort of fungal growth and tend to have a V-shape, following leaf veins.