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 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.