Late Blight

Casual Agent

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

Symptoms

Phytophthora affects potato foliage, stems, and tubers. 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%.

Managment

• 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.