Late Blight Disease: Causes, Symptoms and Agricultural Impact

Introduction

Late blight is a destructive plant disease primarily affecting potatoes and tomatoes. It is caused by Phytophthora infestans, an oomycete, which is fungus-like but is not a true fungus.

The disease is historically significant because it contributed to the Irish Potato Famine of the 1840s.

Causative organism

  • Scientific name: Phytophthora infestans.

  • Biological group: Oomycetes.

  • Major hosts: Potato and tomato.

  • Favourable conditions: Cool, humid weather and prolonged leaf wetness.

Symptoms

On leaves

  • Water-soaked lesions.

  • Rapidly expanding brown or black patches.

  • Whitish growth may develop around lesion margins under humid conditions.

On stems

- Dark lesions can develop and cause stem damage.

On potato tubers

  • Brown to reddish-brown internal lesions may occur.

  • Infected tissue can become vulnerable to secondary decay.

Disease spread and management

The pathogen can spread through infected planting material, windborne sporangia, and water under suitable conditions.

Control measures include:

  • Use of healthy seed potatoes and disease-free planting material.

  • Field monitoring and early diagnosis.

  • Resistant or less susceptible varieties where available.

  • Crop sanitation and appropriate management of infected material.

  • Fungicides applied according to approved recommendations and resistance-management practices.

UPSC Prelims Key Pointers

  • Causative organism: Phytophthora infestans.

  • Late blight is associated particularly with potatoes and tomatoes.

  • The pathogen is an oomycete, not a true fungus.

  • The Irish Potato Famine is historically associated with potato late blight.

  • High humidity and leaf wetness favour disease development.

  • Late blight is different from early blight, which is commonly associated with Alternaria solani.