What is Late Blight?

Information about late blight of potatoes and tomatoes with links to photographs of symptoms

Late blight is a plant disease that attacks both potatoes and tomatoes. Late blight caused the Irish potato famine in the 1850's, during which millions of people in Ireland starved or were forced to emigrate. Entire potato crops rotted in the field or in storage because of late blight infection. Late blight is caused by a fungal pathogen that survives from one season to the next in infected potato tubers.

This fungal pathogen is well known for its ability to produce millions of spores from infected plants under the wet weather conditions that favor the disease. Early in the season, the disease can be introduced into a field or garden on infected seed potatoes, from volunteer plants growing from diseased potatoes that were not harvested last season, from infected potatoes in cull piles (rejected potatoes), compost piles, or infected tomato transplants. Later in the season, spores from infected potatoes and tomatoes in nearby fields or gardens can travel through the air, and if the weather is sufficiently wet, cause new infections. Spores can also be washed through the soil to infect potato tubers, which may rot before harvest, or later in storage.

Because the fungus that causes late blight produces so many spores, and the spores can travel long distances through the air, it is very important that everyone who grows potatoes or tomatoes is able to identify late blight and know how to control it, to avoid being a source of spores that infect potatoes and tomatoes in neighboring gardens and commercial fields. This disease is capable of wiping out not only your entire potato and tomato crop but also commercial fields very quickly under wet conditions, and farmers who grow potatoes or tomatoes are at serious risk of losing their entire income for the season.

Some History

Late blight used to be a frequent problem for potato farmers in the U.S., but a new fungicide introduced in the early 1970's was extremely effective against late blight, and for many years it was found only rarely. Starting in the late 1980's, new strains of the late blight fungus were found in Mexico that were resistant to this fungicide. These new strains made their way into the U.S., and have caused serious late blight epidemics in nearly every major potato growing area of the country in the last few years. A very intensive effort will be needed to keep these new strains of the fungus under control. You can play an important role in this effort by controlling late blight in your garden. This will prevent fungal spores from being produced that could cause infection in nearby gardens and commercial fields. To control late blight, gardeners must learn to recognize the disease symptoms and use a variety of practices for disease management. To minimize risk until we learn more about the new strains of the fungus, potato and tomato growers need to use regular fungicide applications.

Preventing Late Blight

Avoid sources of inoculum: The most effective management strategy for late blight is to avoid sources of early season inoculum (spores). Late blight can only survive on living tissue, so potato tubers or tomatoes (transplants or imported fruit) are the only source of early season inoculum. One important way to avoid introducing late blight on potatoes is to plant certified seed potatoes. Many states where potato seed is produced have seed certification programs to ensure that the seed meets certain standards for disease levels. Certified seed is not a guarantee that late blight will not be present, however. Examine your seed carefully before you plant, and plant only sound, blemish-free tubers. Destroy any rejected tubers that you don't plant. Other ways to avoid early season inoculum: make sure any potatoes put into compost piles are completely decomposed, and pull up and destroy any potato plants that come up from tubers that were left in the garden last season.

Resistant Varieties: Planting resistant varieties will slow down (but not prevent) the development of late blight. A variety called "Elba" is currently the most resistant variety available. Varieties with moderate levels of resistance include: Kennebec, Sebago, and Allegany.

Fungicides: If the growing season is wet, late blight spores will be present in many areas, and fungicides will be necessary to protect your plants from infection. Theonly fungicides that are effective against late blight are protectant materials, which means that they must be on the foliage before spores land on leaves that are wet from rain, irrigation, fog, or dew. Therefore, continuous fungicide coverage is necessary to protect plants from infection. Tomatoes and potatoes are susceptible to late blight at any time during the growing season. Choose a fungicide that has maneb, mancozeb, chlorothalonil, or fixed copper as an active ingredient AND has tomato and potato late blight on the label. Contact your Cornell Cooperative Extension office for more information about controlling late blight. And remember, all pesticides should be used in accordance with instructions on the label.

Scouting: During the growing season, check your potatoes and tomatoes for symptoms of late blight twice each week. Check more often during periods of wet weather. If you find any late blight in your garden, intensify your fungicide applications (by increasing application frequency or rates) within the guidelines listed on the label. If late blight becomes severe, destroy diseased plants by thoroughly tilling them under, or by cutting them off and immediately burying or bagging them to avoid producing large numbers of spores that could put nearby farmers and gardeners at risk.

To avoid tuber infection: Hill up soil around the base of potato plants to provide a barrier to spores that can wash through the soil. Vines should be dead 2-3 weeks before digging potatoes for storage. If you want to harvest before the vines have died naturally, cut stems just above the surface of the soil 2-3 weeks before harvesting.

Late Blight Symptoms

On potato tubers : Infected potatoes have shallow, brownish or purplish lesions on the surface of the tuber. If you cut across the surface of these infected areas, you'll see a reddish-brown, dry, granular rot that extends up to half an inch into the flesh. Late blight lesions can serve as pathways for other tuber diseases to enter, so late blight symptoms can sometimes be obscured by symptoms of other diseases. Plant only sound, blemish free tubers to avoid late blight and other tuber-borne diseases. Don't allow any discarded tubers to sprout and possibly produce late blight spores.

On potato plants :Late blight lesions can occur on both leaves and stems. The first appearance of lesions commonly occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. Under humid conditions, delicate, whitish fungal spore producing structures are produced at the edge of the lesion, particularly on the underside of the leaf. Lesions turn brown when they dry up,and are often surrounded by a halo of gray-green tissue. Once lesions dry up, the white spore masses will not be visible. To help identify late blight if outdoor conditions are not humid enough for spores to be produced, you can place suspect leaves or stems in a closed container with a damp paper towel. Check the leaves after about 12 hours to see if the delicate, white fungal material is growing from the tissue at the edge of the lesion. On stems, late blight causes brown, greasy looking lesions that frequently appear first at the junction between the stem and leaf, or at the cluster of leaves at the top of the stem.

On tomatoes : Symptoms on tomato leaves and stems are similar to those on potato. On tomato fruit, late blight causes a firm, dark, greasy looking lesion from which the fungal spore producing structures emerge under humid conditions.

Need More Information?

The following publications are available from: Resource Center-GP, Cornell University, 7 Business and Technology Park, Ithaca, NY 14850.

Late Blight of Potatoes and Tomatoes (102VCFS726.20) Fact sheet including photographs of foliage and tuber infection. $3.00

Detection of Potato Tuber Diseases and Defects(153IB205) Fact sheet with photos of assorted tuber diseases and defects including late blight. $2.50

Contact your county Cooperative Extension Office or your local garden center for recommendations on fungicides for late blight control.

Authors: Abby Seaman, Extension Area Educator, Vegetable IPM, Integrated Pest Management Program, NYSAES, Cornell University.

Rosemary Loria, Dept. of Plant Pathology, Cornell University.

William Fry, Dept. of Plant Pathology, Cornell University.

Photographs by: William Fry, Rosemary Loria, Tom Zitter, Vaughn James, Abby Seaman, Mike Orfanedes