Blight of Tomatoes and Potatoes in the Home Garden Blight of Tomatoes and Potatoes in the Home Garden Late blight of potatoes and tomatoes is caused by a fungus called Phytophthora infestans . It is not a bacterium or a virus. Late blight also occurs on eggplant, nightshade and occasionally on peppers. It can occur in the interior of BC, but is more common in coastal areas. The first symptom is often a brown to black spot on a stem, leaf or petiole.
Leaves develop large brown or black blotches, often starting at leaf margins.
In humid weather and in early mornings, a fuzzy mould can often be seen on the underside of the blotches or on the stem lesions. This fuzzy growth contains spores of the fungus. The spores spread in wind and water and cause new infections in wet weather. They can spread 20 kilometers or more. Rain-spread spores can cause infection even in a garden where tomatoes or potatoes have not been grown before.