Farming Magazine - May, 2010


Be Prepared to Prevent Late Blight

By Vern Grubinger
These rows of tomatoes were destroyed by late blight in 2009 before any marketable fruit could be harvested.
Photos by Vern Grubinger.

Many tomato growers in the Northeast, mid-Atlantic and beyond will long remember 2009 as the year of the late blight epidemic. Prolonged periods of cool, wet weather and the early-season appearance of late blight on tomato transplants combined to promote the spread of the disease, resulting in substantial crop loss and/or frequent fungicide applications in an effort to protect plants long enough to get marketable fruit. Potatoes were also affected, but in many locations, not as severely as tomatoes because the strain of late blight was more virulent on the latter.

Let’s hope that the weather is less conducive to late blight in 2010, and that homeowners and commercial growers are better prepared to prevent the disease, or manage it, if it does show up. To help with that preparation, the New York State IPM Program has developed the following information, used with permission.

What is late blight?

Late blight is a fungal-like plant disease caused by the organism known as Phytophthora infestans. It attacks potatoes and tomatoes, although it can sometimes be found on other crops, weeds and ornamentals in the same botanical family. Other plants that late blight may infect include petunia, nightshades and tomatillos.

Where does late blight come from?

Late blight survives from one season to the next in infected potato tubers. 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 brought into the area.

This organism is well-known for its ability to produce millions of spores from infected plants under the wet weather conditions. Spores produced on infected potatoes and tomatoes can travel through the air, land on healthy plants, 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.

Knowledge is key to management. Because the pathogen that causes late blight produces so many spores, and the spores can travel long distances through the air, it is 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 entire potato and tomato crops quickly under wet conditions, and farmers who grow potatoes or tomatoes are at serious risk of losing their entire income from these crops.

History of late blight

Late blight was a factor in the Irish potato famine in the 1850s, 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 just getting started in this tomato crop.

Late blight used to be a frequent problem for potato farmers in the U.S., but a fungicide introduced in the early 1970s was extremely effective against it, and for many years it was found only rarely. Starting in the late 1980s, new strains of the pathogen were found in Mexico that were resistant to this fungicide. These new strains made their way into the U.S., and in some years cause serious late blight epidemics in tomatoes, potatoes or both.

Preventing late blight

In years when late blight is a problem, farmers and gardeners can play an important role in minimizing its spread by controlling the disease or destroying infected crops. This will prevent spores from being produced that could cause infection in nearby gardens and commercial fields. To control late blight, one must learn to recognize the disease symptoms and use an assortment of practices for disease management. Fungicide applications may be needed in some years to prevent loss of tomatoes and potatoes, and if the disease becomes severe it may be necessary to destroy infected plants to prevent it from spreading.

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 healthy 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 (not just putting them into cull piles) any rejected tubers that you don’t plant.

Make sure any potatoes that were put into compost piles are completely decomposed (rotted), and promptly destroy any potato plants that come up from tubers left in the ground last season.

Plant resistant varieties

Planting resistant varieties will slow down (but not prevent) the development of late blight. Currently, Defender and Elba are the most resistant potato varieties available. Potato varieties with moderate levels of resistance include: Kennebec, Sebago and Allegany. Resistant tomato varieties will be available soon. The first resistant tomato variety will be a cherry tomato called Mountain Magic.

Fungicide use

If the growing season is wet, and late blight is present, fungicides will be necessary to protect plants from infection. Garden fungicides that are effective against late blight are protectant materials, which means that they must be on the foliage before spores land on leaves and initiate infection. Infection only occurs when the leaves are wet. 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. Homeowners should choose a fungicide that has maneb, mancozeb, chlorothalonil or fixed copper as an active ingredient, and has tomato and potato late blight listed on the product’s label. Of these fungicides, only some of the fixed copper products are approved for organic production. Remember that any pesticide must be used in accordance with instructions on the label.

Commercial growers should contact Cooperative Extension for the latest information on fungicides for controlling late blight.

If you choose not to use fungicides, it’s especially important to keep an eye on your potatoes and tomatoes and remove and destroy infected plants to avoid spreading the disease to nearby gardens and farms. If possible, destroy infected plants on a dry, sunny day when dislodged spores will die quickly. If the weather is continuously wet, it is better to destroy plants sooner rather than waiting for a dry day.

Scout your crops

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, but stay 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.

Avoid tuber infection. Hilling up soil around the base of potato plants provides a barrier to spores that can wash through the soil. Potato vines should be dead for two to three weeks before you start digging potatoes for storage. If you want to harvest before the vines have died naturally, cut the stems just above the surface of the soil, then wait two to three weeks before harvesting.

Recognizing late blight 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 including bacterial soft rot to enter, so late blight symptoms can sometimes be obscured by symptoms of other diseases.

Recognizing late blight 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 three to seven days of infection of leaves. Under humid conditions, delicate, whitish growth (pathogen spores) are produced at the edge of the lesion, particularly on the underside of the leaf. Lesions turn brown when they dry up. Active lesions 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 pathogen sporulation is visible on 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.

Recognizing late blight 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 pathogen spore producing structures emerge under humid conditions.

For more information

Detailed descriptions of the biology and management of late blight, along with excellent photos of symptoms on both potato and tomato can be found at Vegetable MD Online, Many photos of late blight on tomato and potato are posted at Long Island Horticultural Research & Extension Center’s pages on late blight at

The author is vegetable and berry specialist with University of Vermont Extension based at the Brattleboro office. He can be reached at . Comment or question? Visit and join in the discussions.