Late Blight of Potato


	Image link BBC - The Irish Famine

Late Blight of Potato

One of the most famous diseases of all time, late blight of potato caused the Irish potato famine of the 1840's. The late blight epidemic occurred not only in Ireland but throughout much of Europe and in North America. In Ireland alone 1 million people died and another 1.5 million immigrated to other countries, primarily the United States. What kind of beast wreaked all this havoc? The fungal pathogen Phytophthora infestans (Mont.) deBary is the culprit. P. infestans continues to plague potato grower today. Late blight also causes severe injury on tomato. Other members of the Solanaceae including eggplant, pepper and nightshade are susceptible hosts but infections by P. infestans are not as severe. The fungus is quite variable, consisting of several mating types. Development of late blight is favored by cool, wet temperatures in late summer and early fall. Optimum fungal growth occurs between 15-25^oC and 100% relative humidity, although a high temperature race exists that functions well at higher temperatures. The fungus overwinters as mycelium in infected tubers. It may be present in tubers buried in the ground, in cull piles, and stored for seed piece production. The fungus can also survive as oospores outside the host but this does not occur in North America. Sprouts arising from infected tubers are invaded by the fungus shortly after emergence. Sporangia are produced on the stems and leaves and are wind blown or splashed to other plants. Sporangia germinate directly and infect the leaf tissue or produce zoospores that infect the leaf. Infection most commonly occurs directly through the cuticle but can also occur through host stomata. Once an infection occurs, the fungus can reproduce in as little as four days resulting in numerous secondary infections. Tubers become infected late in the season during periods of wet weather. Sporangia are washed from the aerial plant parts by rain or irrigation water onto the ground. Zoospores produced from sporangia infect exposed tuber tissue or are washed through cracks in the soil. Infection occurs through the lenticels or wounds. The fungus thus becomes established in the tuber and remains until next growing season.

Symptoms

Initial leaf symptoms appear as water-soaked spots of margins of lower leaves. The spots enlarge turning brown to purplish black. A pale yellow margin may be present around the spot. A white downy growth of sporangiophores and sporangia forms at the edge of the spot, most commonly on the lower leaf surface, when moist conditions occur. Entire leaves may become infected and the pathogen may spread to petioles and stems, killing the entire plant. When the disease progresses rapidly, the decaying plant material gives off a foul odor. Tuber infections are characterized by irregular, slightly depresses areas that are variable in size. The areas are brown to purplish in color with an indistinct border between healthy and infected tissue. Tuber infection occurs in the field but symptoms may continue to develop in storage.

Plant Health Management

Plant resistant varieties

Two types of resistance are available, monogenic or specific and polygenic or nonspecific. In general, nonspecific resistance is more desirable since the pathogen is so variable.

Use blight-free seed

Apply protectant fungicides

These types of fungicides must be applies before infection occurs to be affective. It is, therefore; necessary to combine scouting with a predictive system such as a pestcaster ( set of instruments set up in the field which monitors environmental conditions and predicts when an infection is likely to occur. In potato growing states, the extension service routinely issues information as to when infections may occur. Many fungicides are labeled for control of late blight. The list includes products containing chlorothalonil, copper sulfate, mancozeb, maneb and metalaxyl (1997)

Destroy cull piles

Keep plants adequately hilled to avoid tuber infection

Remove infected tubers before storage

References

Agrios, G.N. 1997. Plant pathology, 4^th ed. Academic Press. Dan Diego, CA. 635 pp.

Hooker, W.J., ed. 1983. Compendium of potato diseases. American Phytopathological Society. St. Paul, MN.125 pp.

Large, E.C. 1962. The advance of the fungi. Dover Publications, Inc. New York, NY. 488 pp.

Stevenson, W.R. 1993. Management of early blight and late blight. Pages 141-147 in Rowe, R.C., ed. Potato health management. APS Press. St. Paul, MN. pp.

Useful Links

Idaho State University

Ohio State University

Oregon State University

Highlighted Area

The Advance of the Fungi

Interesting links to Irish Potato Famine

University of Wisconsin

American University

University of Massachusetts

Trinity College

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

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