Potato Late Blight
Potato late blight will appear on leaves as a pale green irregular spot. These areas will enlarge rapidly and will look like a water soaked "grease spot", often these areas will have a yellow halo around the infected area. A white mildew ring will often form around the necrotic area.

Under humid conditions a white mycelium can be seen around the necrotic areas.

Stems and petioles will turn brown when infected and will be very brittle when infected with late blight; white mycelium may be present under moist conditions.

In moist weather, late blight spreads rapidly and all plants in a field can become infected. Diseased and decaying plants give off a fetid odor.

Tubers are readily infested in the soil by water movement of spores. Initial tuber infection will have a brownish discoloration of the skin. Under the skin, the flesh will appear reddish brown with a granular texture.