Potato late blight is caused by the fungus Phytophthora infestans. It infects potato leaves, stems and tubers and can cause devastating crop losses. Tomato blight is also caused by the same fungus (see factsheet DC20 – Tomato Blight).
Leaves and stems: Dark brown/ blackish round patches, often surrounded by a pale halo. In warm, damp weather the patches quickly spread to rot the whole leaf. Stems can also be infected. The underside of the infected leaf develops a downy white coating of spores in moist conditions, particularly noticeable early in the morning. If weather remains warm and damp, the disease spreads rapidly, reducing the foliage to a rotting mass within a few days.
Tubers: Dark, sunken areas on the surface, which may extend to cover the whole potato, giving a dry firm rot. Cutting the potato in half will show patches of chestnut-coloured rot starting just under the skin. Other fungi and bacteria may follow, invading the tuber to produce a wet, foul smelling soft-rot. Seemingly healthy tubers may rot later when in store.
Another blight: Potato early blight (Target spot) symptoms may be mistaken for late blight. Early blight, Alternaria solani, however generally occurs earlier in the season (July) and spreads under warmer and drier conditions than late blight. The distinctive smaller dark brown spots, somewhat angular with concentric rings, are bounded by the leaf veins. Early blight rarely causes significant loss of yield and no treatment is necessary.