HPIPM:Potato Late Blight

Taxonomy

Domain	Eukarya
Kingdom	Chromista
Phylum	Oomycota
Class	Oomycetes
Order	Peronosporales
Family	Pythiaceae
Genus	Phytophthora

Scientific Name

Phytophthora infestans

Common Name

late blight

Author: Howard F. Schwartz, David H. Gent, and Gary D. Franc

Identification and Life Cycle

Late blight, caused by the fungus-like organism Phytophthora infestans, is a potentially devastating disease of potato, and may be the most important disease of potato worldwide. The pathogen can infect several Solanaceous plants, including tomato, eggplant, petunia, and hairy nightshade. P. infestans can infect both potato foliage and tubers. Infection begins when sporangia or zoospores form germ tubes and infect foliage when temperatures are cool to moderate (46 to 75ºF) and free moisture is available. Lesions can develop in as little as three days after infection, producing additional sporangia that are responsible for secondary spread of the disease in a field. Tuber infections are thought to occur when sporangia are washed from leaves and contact buried tubers through cracks in the soil. Tuber infections may also occur during harvest and handling. In the High Plains, the pathogen requires a living host to survive. Therefore, cull potatoes, volunteers, and contaminated seed are the primary means of pathogen survival and dissemination. Survival in soil for months or possibly years occurs by oospores in certain areas of the world where P. infestans sexual mating occurs.

Plant Response and Damage

Late blight symptoms first appear on foliage as small, irregularly-shaped necrotic spots. As lesions develop, they become more circular with a necrotic center surrounded by a yellow or pale-green margin. Under humid conditions, white downy mycelium is visible surrounding necrotic lesions. Petioles and stems also become infected and produce water-soaked, dark green to black lesions.

Tuber infections appear initially as reddish brown, dry lesions that can extend an inch or more into tubers. The boundary between healthy and infected tissue is discontinous.

Yield losses of 100% in the field and continuing into storage are not uncommon with susceptible varieties if not regularly treated with fungicides. The disease reduces yield, seed quality, marketability, and processing value.

Biological Control

No biological control practices have been developed for potato late blight.

Cultural Control

Plant high quality seed free from the late blight pathogen. Varieties less susceptible to late blight can slow epidemics and reduce the need for fungicides, and should be planted if available. Promoting air movement and leaf wetness in the crop canopy can slow secondary disease spread. Since the pathogen must have a living host to survive in the High Plains, destruction of volunteers, alternate hosts, and culls will reduce initial inoculum sources.

Chemical Control

Due to constantly changing labels, laws and regulations, the sponsors of this web site assume no liability for the suggested use of any chemicals noted in the management section. Contact your local pest management personnel for current information on labeled pesticides that can be used for the intended plant disease, insect pest or weed.