Blight, scab, slugs and the Colorado beetle
What goes wrong with potatoes, what it looks like, and what UK sources say to do.
Jump to: Blight | Scab | Blackleg | Slugs | Green potatoes | Colorado beetle
Our pictures are simple drawings to show the idea, not identification photos. For a firm diagnosis, use the RHS pages linked from each fact.
Blight
Potato blight (late blight) is caused by a fungus-like organism, Phytophthora infestans. It spreads rapidly through leaves and tubers in warm, wet weather. RHS: Potato and tomato blight The RHS calls blight the most serious and damaging disease of potatoes. It can also affect tomatoes. RHS: Potato and tomato blight
The first sign of blight is a fast-spreading, watery rot of the leaves, which collapse, shrivel and turn brown. Infected tubers have a reddish-brown decay under the skin. RHS: Potato and tomato blight
First earlies are more likely to escape blight, because the disease tends to increase as summer goes on. RHS: Potato and tomato blight Gardeners can check blight forecasts for their area on the BlightSpy website. RHS: Potato and tomato blight · checked 29 Oct 2026
When about 25 percent of leaves are affected, or stems show lesions, cut off the foliage (haulm). Dig the tubers about two weeks later, once the skins have hardened. RHS: Potato and tomato blight
Blighted plants should be deeply buried, put in council green waste or burned, not composted. RHS: Potato and tomato blight
Early blight is a different disease, common in North America but not a common problem in British gardens. RHS: Potato and tomato blight
The Great Famine in Ireland, 1845–49, happened when the potato crop failed in successive years because of late blight. Encyclopaedia Britannica: Great Famine (Irish history)
Common and powdery scab
Common scab and powdery scab cause rough, scabby patches on potato skins. Common scab is worse in dry soil when tubers form; powdery scab is worse in wet conditions. RHS: Potato scabs To reduce common scab, the RHS suggests watering from 2–3 weeks after emergence for about four weeks, at 20 litres per sq m. It is worse on alkaline soil. RHS: Potato scabs
Blackleg
Blackleg is a bacterial disease that causes black rotting at the stem base. It can show as early as June, especially in wet years, and only infects potatoes. RHS: Potato blackleg The most important blackleg control is to remove and destroy infected plants as soon as symptoms appear. There are no chemical controls for gardeners. RHS: Potato blackleg
Slugs
Slugs particularly like to tunnel into maincrop tubers during damp autumn weather. RHS: How to grow potatoes Slugs and snails also help recycle dead leaves in gardens and are food for birds, beetles and reptiles. The UK has over 44 species of slug and 90 of snail. RHS: Slugs and snails
Green potatoes and frost
Earthing up means mounding soil around the stems as they grow. It protects shoots from frost and keeps light off the developing potatoes, which would turn them green. RHS: How to grow potatoes The RHS says to discard any potatoes that are green. RHS: How to grow potatoes
If frost is forecast after shoots appear, cover them overnight with a cloche, hessian or straw, or with soil or compost. RHS: How to grow potatoes
Colorado beetle
Colorado beetle is orange-yellow with 10 black stripes. It can strip the leaves from potato plants. Suspected sightings should be reported to the Animal and Plant Health Agency with a photo and location. GOV.UK (Defra/APHA): Major potato pest eradicated from UK (11 December 2025)
Colorado beetle was declared eradicated from the UK on 11 December 2025, after it was found in Kent in July 2023 and none were found for two years. GOV.UK (Defra/APHA): Major potato pest eradicated from UK (11 December 2025) · checked 29 Oct 2026 Colorado potato beetle was first recorded in the USA in 1811 and first became established in Europe at Bordeaux, France, in 1921. GOV.UK (Defra/APHA): Colorado potato beetle confirmed in Hampshire
Next: potato history →