Early blight on potatoes — symptoms and treatment

Severity: medium

What is Early blight

Early blight of potato is caused by Alternaria solani, a fungus that produces distinctive concentric ring-patterned lesions on the leaves. It works from the bottom of the plant upward, taking the oldest foliage first and progressing through the canopy as the season goes on. It rarely collapses a crop the way late blight does — hence the medium severity — but it steadily removes the leaf area the plant needs to bulk up tubers, and it can mark the tubers themselves. It is a disease of gradual attrition, which is why growers tend to notice it too late.

Symptoms

Look at the lower leaves first, because that is where it starts. The lesions are dark brown spots carrying concentric rings — the "target" pattern that gives the disease away and the single most useful thing to look for. Around each lesion the leaf tissue yellows, and as spots multiply and merge the whole leaf yellows and dies. The damage climbs the plant over time, leaving a bare lower stem and a thinning canopy above it. Tubers can carry lesions too, which shows up at lifting or in storage.

• Early: dark brown spots with concentric rings (target pattern) on the lower, oldest leaves.
• Progressing: yellowing around lesions; spots merging; leaves dying and the damage moving up the plant.
• On the crop: tuber lesions.
• Confusable with: late blight — but late blight lesions are water-soaked and dark with white fuzzy growth underneath in humid weather, without the concentric target rings.

Causes and conditions

The fungus carries over on infected plant debris, volunteer potatoes and other solanaceous crops, which is why the disease returns to a bed that grew potatoes or tomatoes last year. Spores are moved by wind and rain splash onto the foliage, and infection needs the leaf surface to stay wet — so humid weather, heavy dew and overhead irrigation all favour it, while a dense canopy that dries slowly extends every wet period. Early blight also presses hardest on plants that are stressed or short of nutrition; a weak crop loses its older leaves faster, which is why nutrition is part of the control programme.

Treatment

Crop rotation and irrigation — cultural

Timing: throughout season. Rotate with non-solanaceous crops. Avoid overhead irrigation. Maintain adequate nutrition. These three do the structural work: rotation starves the fungus of the debris it survives on, watering at the base keeps leaves dry so spores cannot establish, and a well-fed crop holds its lower leaves.

Chlorothalonil (Bravo) — chemical

Timing: at first symptoms. Apply fungicide when first symptoms appear on lower leaves. Repeat every 7-14 days. This is one of the few diseases where waiting for first symptoms is the correct trigger rather than a mistake — but it means the lower leaves, checked deliberately, not the ones at eye level. Pre-harvest interval: 14 days. Pesticide registrations vary by country — check local approval before use.

Prevention

• Rotate with non-solanaceous crops; do not follow potatoes with potatoes or tomatoes.
• Water at the base and avoid overhead irrigation so foliage stays dry.
• Keep the crop adequately fed — stressed, hungry plants lose lower leaves to early blight much faster.
• Space plants for airflow so the canopy dries quickly after rain or dew.
• Clear away plant debris and volunteer potatoes, which is where the fungus waits for next season.

Frequently asked questions

Is this the same as late blight? No, and the difference matters. Early blight makes dark brown spots with concentric target rings on the lower leaves and moves slowly. Late blight makes water-soaked lesions with white fuzzy growth on the leaf undersides and collapses plants rapidly.

Can I eat potatoes from an infected crop? Yes. Sound tubers are fine — cut away any lesions. Observe the pre-harvest interval of 14 days after spraying, and do not store lesioned tubers, as they will not keep.

When should I start treating? At the first symptoms on the lower leaves, then repeat every 7-14 days. The cultural measures — rotation, base watering, good nutrition — should already be running from the start of the season.