Late Blight
Disease · Commonly affects: Tomatoes, Potatoes, Petunias, Hairy nightshade
Late blight is a fast-moving plant disease caused by the water mold Phytophthora infestans, the same organism behind the historic Irish potato famine. It matters to home gardeners because, once cool and wet weather arrives, it can destroy a healthy tomato or potato planting within just a few days.
How to identify it
Look for pale green to olive, water-soaked blotches on leaves that quickly turn brown or black, often edged with a downy white growth on the underside during humid mornings. Stems develop dark brown to nearly black lesions, and tomato fruit show firm, greasy-looking brown patches that spread from the shoulders. Unlike early blight, these spots lack concentric rings and race across the entire plant rather than staying near the older lower leaves.
Damage
The pathogen collapses foliage, stems, and fruit, turning vigorous plants into blackened, rotting tissue in a matter of days when conditions favor it. Infected potato tubers rot in the ground or later in storage, so the disease can ruin the entire crop before or after harvest.
Prevention
Rotate tomatoes and potatoes away from ground where any nightshade crop grew the previous season, and space plants generously so leaves dry quickly after rain or dew. Start with certified disease-free seed potatoes and healthy transplants, and choose varieties bred for late blight resistance when they are offered. Clear out cull piles, dropped tubers, and volunteer potato and tomato seedlings, since these overwintering survivors are a common starting point for outbreaks.
How to manage it
There is no cure for an infected plant, so promptly pull, bag, and discard heavily affected plants to keep spores from drifting to neighbors. Prune away lower and crowded foliage to improve airflow, water at the soil line instead of overhead, and handle plants only when the leaves are dry so you do not carry spores from plant to plant. Dig up and destroy infected tubers rather than composting them, and scout the remaining plants often because the disease advances rapidly during cool, damp spells.
General guidance for US home gardens, focused on prevention and non-chemical control. For a confirmed diagnosis and options specific to your area, contact your local Cooperative Extension office. Page updated Aug 4, 2026.