Why Is My Potato Foliage Turning Black and Dying?

Blackening, collapsing potato foliage — leaves turning dark, stems going limp, the whole plant appearing to melt in place — is one of the most dramatic and worrying sights in the vegetable garden. In the vast majority of cases this is late blight, caused by the water mould Phytophthora infestans. This is the same organism responsible for the Irish potato famine of the 1840s and it remains the most destructive disease of potatoes in cool, wet climates today. Speed matters enormously: acting within 24 hours of spotting the first signs can mean the difference between saving your harvest and losing everything.

How to identify late blight

Late blight starts as pale greenish-brown water-soaked patches, usually on the tips or edges of leaves, that quickly turn dark brown to black. In humid conditions — which is when blight thrives — you will see a white or grey furry ring of fungal sporulation around the edges of the lesion on the underside of the leaf. The smell is distinctive: a heavy, musty, rotting smell from infected foliage. Once the haulm is heavily infected, stems blacken and collapse and the whole plant dies. In warm, wet weather (above 10°C with persistent humidity or rain) blight can destroy a plant in three to five days.

What to do immediately

Cut off all affected foliage at soil level immediately and remove it from the garden — do not compost it. If the infection is already heavy, remove all the haulm entirely. The goal is to prevent blight spores from washing down through the soil to infect your tubers. Leave the tubers in the ground for at least two weeks after removing the haulm: the skin will harden (set) and residual spores on the soil surface will die off. Then harvest carefully. Any tubers with brown rot inside are infected — remove them. Firm, clean tubers are safe to store.

Treating neighbouring plants

Spray all unaffected plants immediately with a copper-based fungicide such as Bordeaux mixture or a copper oxychloride product. Copper is the only effective organic option and works preventatively — it cannot cure existing infections but can protect healthy foliage from new spore landings. Reapply every 7–10 days and after heavy rain. Remove lower leaves to improve airflow, and avoid overhead watering from this point on as wet foliage dramatically speeds spore germination.

Preventing blight in future seasons

Choose blight-resistant or blight-tolerant varieties for your main planting — Sarpo Mira and Orla are among the most resistant commercially available varieties. Plant certified disease-free seed. Do not leave infected tubers in the soil over winter. Rotate your potato bed every three to four years. Sign up for the Blightwatch forecast service (UK) to receive alerts when blight conditions are forecast in your area, so you can apply protective copper spray before the disease arrives rather than after.