Late Blight

Late Blight Lesions on Potato Foliage
Late Blight Lesions on Potato Foliage

Late Blight Symptoms on Potato Tuber
Late Blight Symptoms on Potato Tuber

Casual Agent

Phytophthora infestans is a fungal-like organism that causes late blight.


Phytophthora affects foliage, stems, and tubers of potatoes. Initially, foliar lesions are greasy-appearing, with a light yellow halo around them. They quickly enlarge and turn black-brown. Infected tubers decay either in the soil or in storage.

Disease Cycle

Phytophthora overwinters in the soil and in decaying potato tubers or cull potato/tomato piles. It produces spores that are blown by wind or splashed by rain onto new host tissue. The disease occurs when temperatures range from 50°F to 78°F and relative humidity is above 90%.


  • Remove cull piles.
  • Allow plants with infected foliage to dry for 2-3 weeks before harvest to ensure Phytophthora has died. It cannot survive on dry dead plant material.
  • Apply a fungicide. Options are: azoxystrobin, chlorothalonil, pyraclostrobin, famoxadone, cymoxanil, dimethomorph, fenamidone, propamocarb hydrochloride or mefenoxam + chlorothalonil.