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Plantwise Factsheets for Farmers

Potato Late Blight

Phytophthora infestans

Recognize the problem

This fungal disease affects leaves, stems, roots and tubers of potato. It is an important disease in Afghanistan as it causes high economic losses to farmers. The disease is identified by black or brown spots on the underside of leaves and stems. They are small in the beginning and appear wet. The disease spreads quickly, as the spots become larger and destroy the leaves. The whole plant can be destroyed within a few days. Diseased tubers are often copper brown, reddish or purplish in colour and feel soft to the touch.


During winter, the fungus generally survives underground in the tubers and in other host plants. The disease can be spread by wind so it can affect neighbouring fields very quickly. Cool and moist conditions also help to spread the disease. The fungus is washed off from the leaves, enters the soil and infects the potato tubers. The fungus remains in the tubers and can cause disease tp affect potato in the next season.


  • Use disease-free, healthy seeds
  • Use resistant varieties
  • Ensure plant to plant distance is 25cm in a row
  • Plant in recommended time for sowing
  • Keep plants healthy by using balanced fertilizers
  • Collect all dry plants and burn
  • Harvest only when plants are dead
  • Avoid harvesting in wet conditions
  • Store potatoes in good storage conditions with an air ventilation facility
  • Spray with a fungicide like mancozeb (e.g. Dithane or manzate), metalaxl-M (e.g. Ridomil gold) and meneb with recommended doses during all the season
  • Treat the seed before sowing with fungicide like mancozeb 80% w/w @ 4 g/litre

When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.

The recommendations in this factsheet are relevant to: Afghanistan

Authors: Shir Ahmad Omid
Aga Khan Foundation
tel: (+93)772681094 email:
©CAB International. Published under a CC-BY-SA 4.0 licence.

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