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Field Guide - Potato Late Blight

Potato Late Blight

Potato late blight on leavesPotato 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.

white mycelium on leaves Infected stems and petioles

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.

infected tuberIn 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.


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