Potato Blight and Diseases: Late Blight, Early Blight and Scab

Late blight (Phytophthora infestans), the disease behind the Irish famine, is still the most damaging potato disease. It spreads fast in warm, humid weather: dark patches on leaves with white mould underneath, stems blacken and the plant collapses within days. Early blight and common scab are the other common problems.

Late Blight

• Symptoms: brown-black leaf patches with pale halo and white fuzz underneath; brown rot in tubers.
• Risk: two days above 10 °C with high humidity ("Hutton/Smith periods").
• Control: resistant varieties ('Sarpo Mira', 'Defender'), wide spacing, protective fungicide or copper sprays in high-risk weather, cut and remove infected foliage 2–3 weeks before harvest.

Other Diseases

• Early blight: brown target-like spots on older leaves; rotate and feed well.
• Common scab: rough, corky patches on skins; keep pH below 6.5 and soil moist at tuber formation.
• Blackleg: black, slimy stem bases; use certified seed.
• Viruses: mottled, curled leaves; spread by aphids and infected seed.

Prevention

• Rotate potatoes and tomatoes on a 3–4 year cycle.
• Plant certified seed.
• Remove volunteer potatoes and cull piles.

More Potato Guides

• How to Grow Potatoes: The Complete Guide
• Colorado potato beetle and pests
• When to plant potatoes by zone
• Seed potatoes and chitting

Frequently Asked Questions

Can you eat potatoes from blighted plants?

Yes, if the tubers themselves are sound; discard any with brown rot.

What causes scabby potato skins?

Common scab, favoured by alkaline, dry soil.