Late blight on potatoes — symptoms and treatment

Severity: high

What is Late blight

Late blight is caused by Phytophthora infestans. It was responsible for the Irish Potato Famine and it remains a major threat — not a historical curiosity but the disease that still destroys plantings when conditions turn in its favour. That speed is the whole story. Most potato diseases give you weeks of warning; late blight can take a green, healthy-looking crop to collapse in a single humid week. This is why it rates high severity, and why the control strategy is built around acting before you see it.

Symptoms

The lesions start as water-soaked dark patches on the leaves, often at the tips and margins, with a greasy, translucent look quite unlike a dry fungal spot. They expand quickly. In humid conditions — early morning, after rain, in a dense canopy — a white fuzzy growth appears on the leaf undersides at the edge of the lesions. That is the confirmation: the pathogen is sporulating, releasing spores onto everything around it. From there the plant collapses rapidly, stems and foliage blackening, the crop going down far faster than the damage on any single leaf would suggest.

• Early: water-soaked dark lesions on leaves, expanding quickly.
• Confirming sign: white fuzzy growth on the leaf undersides in humid conditions.
• Advanced: rapid plant collapse.
• Confusable with: early blight — but that makes dry brown spots with concentric target rings on the lower leaves and progresses slowly.

Causes and conditions

Phytophthora infestans thrives in cool, wet, humid weather and needs free water on the leaf to infect, so prolonged rain, heavy dew, fog and dense canopies that never dry out set an outbreak going. Spores are produced on infected tissue and carried by wind and rain splash, which is why the disease can arrive from a neighbouring garden rather than your own soil. It survives between seasons in living plant material — infected tubers left in the ground, volunteers and cull piles — so the source is often a potato you did not lift. Rain also washes spores down into the soil, where they reach the tubers and rot them.

Treatment

Destroy infected plants — cultural

Timing: upon detection. Remove and destroy infected plants immediately. Do not compost. Hill soil around stems. Immediately means immediately — an infected plant is a spore factory aimed at the rest of your crop, and composting it keeps the pathogen alive on your own plot. Hilling soil around the stems puts a barrier between spores washing off the leaves and the tubers below.

Trichoderma harzianum — biological

Timing: preventive, throughout season. Apply as soil treatment and foliar spray for biological protection.

Metalaxyl + Mancozeb (Ridomil Gold) — chemical

Timing: before disease onset. Apply systemic fungicide preventively before disease onset. Switch to contact fungicides after. Read that timing literally: the systemic goes on while the crop is still clean. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Treat preventively — with late blight, a crop that already shows lesions is a crop you are salvaging, not saving.
• Lift every tuber and remove volunteers and cull piles; leftover potatoes are how the pathogen survives to next season.
• Water at the base, never over the canopy, and space plants so foliage dries quickly.
• Hill soil well around the stems to keep spores from washing down onto the tubers.
• Destroy infected plants immediately and never compost them.

Frequently asked questions

How fast does late blight kill a potato plant? Fast — rapid collapse is the defining feature, which is why the response is immediate removal rather than watching how it develops. In humid weather a crop can go down while you are still deciding.

Can I eat potatoes from an infected plant? Sound, unblemished tubers can be used, but tubers from an infected crop often rot in storage — use them promptly and store nothing that shows damage. Observe the pre-harvest interval of 14 days after spraying.

Can I compost the infected plants? No. Do not compost them. The pathogen survives in living plant tissue, so composting keeps it alive and on your plot, ready for next season's crop.