Impact
Late blight is caused by the fungus Phytophthora infestans. Late blight is one of the most devastating diseases of potato worldwide. This disease is most serious where the weather is consistently cool and humid. In recent years, epidemics of late blight have developed in crops under center-pivot irrigation in arid production areas. Late blight has occurred twice over the last 20 years in the San Luis Valley as a result of imported seed. Imported seed or commercial potatoes pose a rapidly increasing risk because of the recent North American late blight epidemic.

Late blight foliar symptoms (G. A. Secor, North Dakota State University).

Descriptions and Symptoms
	Symptoms of late blight appear on leaves as pale-green, water-soaked spots, usually beginning at the leaf tips or edges. The circular- or irregularly-shaped lesions are often surrounded by a pale, yellowish-green border. The lesions enlarge rapidly and turn brown or purplish black.
	During periods of high relative humidity and leaf wetness, lesions may be bordered by a cotton-like white mold growth on the underside of the leaf.
	In dry weather, infected leaf tissues turn brown and quickly dry up. Infected strems and petioles turn brown to black, and entire vines may be killed.
	Under cool, moist conditions, the fungus can form spores on the foliage of infected plants. These spores are easily dislodged and can be moved by the wind into neighboring potato fields and cause new infections.
	On the tuber surface, lesions appear brown and sunken while tissues immediately beneath appear granular and tan to copper-brown.
	The fungus survives between crops in infected tubers. Tubers removed during grading and dumped in cull piles or unharvested tubers in teh fields or rock piles are sources of the pathogen.
	Newer strains of the fungus are more aggressive, resulting is a higher incidence of tuber infection. Infection can occur on other Solanaceous species such as pepper, tomatoes and various nightshade species.
Integrated Management
	Do not import seed or commercial potatoes.
	Scout fields weekly after emergence until harvest.
	Apply fungicides to all fields after confirmation of late blight in the San Luis Valley. Continue applications at recommended intervals for the remainder of the season. Complete coverage of all foliage with a fungicide is essential. Ground application is most effective. Fungicide applications should continue until vines are dead.
	Harvest only after vines are completely dead. Handle tubers gently to avoid mechanical damage and to reduce the risk of infection.
	Dispose of cull piles promptly.

This material is based on work supported by the San Luis Valley Water Quality Demonstration Project Best Management Practices Advisory Committee, Colorado State University Cooperative Extension, Farm Service Agency, Natural Resources Conservation Service, and the U.S. Department of Agriculture under authorization as 1991 U.S. Department of Agriculture Water Quality Demonstration Project.