Late blight can quickly cause demise of tomato, potato garden

Published Jul 15, 2018 at 9:00am

Late blight is a potentially destructive disease of potato and tomato plants. It was responsible for the devastating epidemic in the northeastern United States in the early 1840s and it caused the Irish potato famine in the late 1840s.

This disease can infect and destroy the leaves, stems, fruits, and tubers of potato and tomato plants. Late blight spreads quickly in fields and in gardens and can result in total crop failure. 

Host and pathogen 

Late blight is caused by the fungus-like organism Phytophthora infestans. Three conditions are needed for late blight to be manifested.

The pathogen (Phytophthora infestans) must be present in combination with a susceptible host (tomato, potato) and the environmental conditions (cool and wet) must be present for the pathogen to develop and cause disease. Late blight spreads most in cool (60°F to 70°F), damp weather. Prolonged hot dry days can halt pathogen spread.

Signs and symptoms

  • Leaves have large, dark brown blotches with a green gray edge 
  • Infections progress through leaflets, and petioles, producing dry brown foliage
  • Stem infections are firm and dark brown 
  • Firm dark brown circular spots are on fruits Spots may become mushy as secondary bacteria invade 
  • In cool, wet weather entire fields turn brown and wilted as if hit by frost 
  • In high humidity, thin powdery white fungal growth show up on infected leaves, stems, and fruits 

Disease managment

Even resistant plant varieties will show some late blight symptoms when conditions are highly favorable for disease so it is important to remain vigiliant.

Home gardeners should examine their plants daily. You might be able to save plants when only a few plant symptoms are seen, the weather conditions are hot and dry, and late blight spores are not nearby.

Create less favorable environment for the late blight pathogen by avoiding wetting the plant leaves with overhead irrigation. Instead water midmorning so the leaves can dry quickly. It is also helpful to eliminate weeds from the garden to improve air circulation around the plants.

Home gardeners can try to slow further late blight development by regularly removing affected plant tissue. Cut off affected plant tissue and bag it daily. Try to do this on a dry sunny day.

Application of fungicides containing the active ingredient chlorothalonil can be used. These fungicides are only effective if used before the disease appears and should be reapplied according to label directions. Fungicides cannot cure a spot or lesion that has already developed.

If your plants are severely infected, be ready to destroy them. Remove the entire plant and discard it in a garbage bag. The plants can also be burned. Once the plant tissue is dead, the pathogen can no longer survive.

Control tactics for late blight are modified as new information becomes available. Consult our website for the best recent specific recommendations.