Late Blight
Act nowFungal disease · spreads to other plants · also called potato blight, Phytophthora infestans
Late blight is the disease that caused the Irish potato famine, and it is the one garden disease genuinely worth acting on immediately. Unlike the leaf spots it does not creep up from the bottom — it appears anywhere, spreads across a plant in days, and its spores travel miles on wind to infect a neighbour's garden.
What it looks like
Large irregular blotches that look greasy or water-soaked, grey-green at first and turning brown-black. In humid weather a fine white fuzz appears on the underside at the edge of each lesion. Fruit develops firm, greasy brown patches.
How to confirm it
- Lesions look wet or greasy rather than dry and crisp, even in dry weather.
- White fuzzy growth on the leaf underside at the lesion margin in humid conditions.
- Progresses in days, not weeks. A plant can collapse within a week.
- Firm brown greasy patches on green fruit, unlike the soft sunken rot of other problems.
What to do
- 1Act the same day. Pull affected plants entirely, bag them, and put them in the bin — not the compost.
- 2Do not leave pulled plants lying in the garden; they keep producing spores.
- 3Report it if your region has a late blight tracking programme. Spores travel, so your neighbours are affected by your decision.
- 4Harvest any unaffected mature fruit before removing the plant.
What it gets confused with
One observation settles each of these.
Early blight is dry, ringed, and starts low and climbs slowly. Late blight is greasy, unringed, and moves in days.
Bacterial lesions stay small and speckled; late blight lesions become large irregular blotches.
Preventing it next time
- Buy certified disease-free seed potatoes and healthy transplants.
- Never save or replant potatoes from an infected crop, and remove volunteers.
- Choose resistant varieties in regions where late blight recurs.
Plants affected
Check these two things first
- Push a finger into the soil, two knuckles deep. Wet soil plus a wilting plant means too much water, not too little — and it is the commonest reason a plant is dying. Dry soil plus crisp leaves means the opposite. The leaves look nearly identical either way, so the soil is what tells you.
- Turn a damaged leaf over. Half of all diagnoses live on the underside: fuzz means downy mildew, fine webbing means spider mites, clusters of insects mean aphids. Nothing on the top of a leaf is as informative.
Most garden problems are cosmetic and resolve without treatment. Reach for a spray last, not first.