Why Are My Potato Plants Collapsing with Blight?

Late blight — caused by Phytophthora infestans — is the most feared disease in the potato garden. It can reduce a healthy, vigorous crop to a field of black, collapsed stems within a week. This is not an exaggeration: under ideal blight conditions of warm temperatures (above 10°C) and persistent humidity or wet weather, a single infected plant can spread spores to the entire patch in days. Understanding the disease cycle and knowing how to respond immediately is essential for every potato grower.

The blight cycle and conditions

Blight survives winter in infected tubers left in the soil — the so-called "volunteer potatoes" that come up from last year's crop are a major source of new infections. In spring and early summer, spores spread through the air from infected volunteers or neighbouring gardens. The disease needs two specific conditions to take hold: temperatures above 10°C and relative humidity above 90% for at least 12 hours. Weather forecasters and specialist services like Blightwatch (UK) issue warnings when these conditions are met. In the UK, blight season typically runs from July through September, peaking in warm, wet, foggy periods.

Identifying a blight attack in progress

The first sign of blight is usually a pale greenish-brown water-soaked patch at the tip or edge of a leaf, often with a faint yellowish halo. Within 24–48 hours in humid conditions, the patch darkens, the leaf dies, and a white or grey furry sporulation appears on the underside of the dead area. The smell is characteristic — heavy and musty. As the disease progresses, stems blacken and collapse and the whole plant dies. Once a stem is blackening it is too late to save that plant, but you can still protect the rest of the crop and the tubers.

Immediate response

Remove all affected foliage immediately, including stems. If more than half the plant is affected, remove the entire haulm. Take the material off the site — do not compost it. Spray all unaffected plants with a copper-based fungicide (Bordeaux mixture, copper oxychloride), which works preventatively to protect healthy tissue from new spore landings. Reapply every seven to ten days and after rain. Stop overhead watering immediately. If the haulm of the whole plot is heavily infected, cut it all off to prevent spores washing down to tubers — then leave tubers in the ground for two weeks for the skin to set before harvesting.

Long-term blight management

For gardens with annual blight pressure, growing blight-resistant varieties is the single most impactful change you can make. Sarpo Mira consistently outperforms most varieties in blight trials. Do not leave any infected tubers or volunteer potatoes in the ground over winter. Rotate potato ground on a four-year cycle. Consider starting copper spray applications in July as a preventive measure rather than waiting for the first signs of infection. Early varieties that are harvested before peak blight season naturally avoid most of the problem, which is another reason many gardeners rely on them.