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Late blight of potato

Late blight of potato:

Causative AgentPhytophthora infestans (Fungus)

Host Crop: Potato and other members of the potato family (the Solanaceae) including the tomato.

Symptoms: The disease is first seen as damp areas on the lower leaves of plants that may fuse to form a large brown area of dead or damaged tissue. The pathogen grows out from the leaves on the lower surfaces of the leaves where it is seen as a white downy mass. The disease is at most aggressive under damp conditions and will rapidly kill all the aerial parts of a plant. A cycle of infection to sporulation can take as little as four days. If there is a dry period, the disease pauses but will resume when the weather turns damp again.Infected potato tubers show surface damage only, but the damage may allow other microorganisms to enter the tuber and destroy it. The rot can be so severe that entire fields may smell of rotting vegetation.

Control: The first stage in control of the disease is prevention by good field husbandry. Disease-free seed potatoes should be used for planting and potato waste should be burned or treated with herbicides as should volunteer plants. Disease-resistant varieties should be used when possible and farmers should keep abreast of news of outbreaks to select varieties and treatment. The pathogen is at its most virulent in areas with cool, damp climates or where the soil has become overwatered or over-irrigated so good management of soil water content becomes important when an outbreak is reported.

The infection can be treated by repeated spraying with fungicides including:

  • Chlorothalonil
  • Copper preparations such as Bordeaux mixture
  • Mancozeb
  • Mancozeb-metalaxyl mixtures
  • Maneb
  • Metalaxyl
  • Ridomyl/TATA Master

Repeated spraying may be necessary and even resistant varieties of plant may need more than one application. Ground spraying is more effective and economical, but aerial spraying may be required for some some cultural practices.The disease can spread widely and rapidly, effective communication and monitoring of outbreaks through local or national authorities is essential for control.

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