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Potato Diseases - Late Blight

Figure 8.21New late blight lesions may have a yellow halo

Figure 8.22Late blight lesions quickly enlarge and turn black-brown.

Figure 8.23Tubers infected with late blight decay either in the soil or in storage


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.


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 weeks 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.


Image 1, William M. Brown Jr.,

Image 2, Howard F. Schwartz, Colorado State University,

Image 3, Sandra Jensen, Cornell University,