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Is late blight a threat to Minnesota tomatoes?

Michelle Grabowski, University of Minnesota Extension Educator

Thumbnail image for 9-09LateBlight 1.JPGMany articles about the late blight epidemic of potatoes and tomatoes in the northeastern United States have been appearing in the news this summer. These reports are making some Minnesotans worried about late blight here in Minnesota. Although the extreme level of disease that is occurring in the northeast is not currently present here in Minnesota, late blight is present in the state and disease is possible. Gardeners should be aware of what late blight infected plants look like, what environmental conditions favor late blight, and what to do if late blight occurs. At this point, however, there is no need to panic.

Photo 1: Late blight on leaves. H.Schwartz, Colorado State University,

About late blight

Late Blight is a disease caused by a fungus-like organism called Phytophthora infestans. Phytophthora literally means 'plant destroyer' in Latin. This pathogen has earned its name by causing several famous epidemics including the Irish potato famine of the 1840's that resulted in the death or emigration of millions of Irish people. Late blight is most severe on potatoes and tomatoes but can also infect related plants like petunias and nightshade. Olive brown spots on leaves or stems are often the first obvious symptoms of disease. These spots grow until the entire leaf is affected, progressing into petioles and stems. Eventually the entire plant is brown and wilted. Under very moist conditions, fine white cobweb like fungal growth may be visible on infected plant parts. Infected tomato fruit have a large greasy brown spot. Infected potato tubers have sunken brown lesions on the surface and reddish granular rot extending into the flesh of the potato. Rot can start in the field and continue in storage.

Thumbnail image for 9-09LateBlight2.JPGPhytophthora infestans thrives in cool wet weather. Temperatures ranging from 60-80 degrees F are ideal. In addition, moisture on leaves and other plant parts is necessary for the pathogen to infect and spread. This moisture can come from rain, irrigation or dew. The late blight pathogen is so devastating because once an infection starts the pathogen reproduces and spreads very rapidly. One lesion can produce 100,000 to 300,000 spores in the right weather conditions. These spores are carried on wind or splashing rain to other plants, devastating an entire field in as little as a week's time.

Photo 2: Late blight causes olive brown lesions on stems. Photo credit: H.Schwartz, Colorado State University,

2009 late blight epidemic in northeastern states

In the northeastern states this summer, several factors combined to create the perfect conditions for a late blight epidemic. First, Phytophthora infestans was brought into several states on infected tomato transplants. These infected plants were then sold at big box stores and distributed throughout the area in home gardens. This early season arrival of the pathogen, combined with unusually cool wet weather, allowed the disease to take hold and spread rapidly from gardens to farm fields.

2009 Minnesota late blight outlook

Until August, most of Minnesota was extremely dry with a few areas in moderate to severe drought conditions (US drought monitor - link to ). These conditions are not favorable to late blight and it is not surprising that the disease was not found in Minnesota until August.

9-09LateBlight 3.JPGWith recent rains, the possibility of late blight on tomatoes and potatoes in Minnesota has increased. Although very few cases have been reported in Minnesota, late blight has been found in North Dakota, South Dakota and Wisconsin. Because the disease spreads so rapidly, there is still time for damage to be done. In large part, the spread and development of late blight in Minnesota will depend on the weather conditions, but spread and disease development also depends on growers and gardeners properly caring for plants that may become infected. Commercial potato growers are already vigilantly looking for new late blight infections and have been spraying fungicides to protect their crop

Photo 3: Late blight on tomato fruit. R.Wick, University of Massachusetts,

Monitoring and controlling late blight

What can you do?
Thumbnail image for 9-09LateBlight 4.JPG
  1. Monitor: First monitor tomatoes and potatoes in your garden at least once a week, and more frequently if rainy weather persists. Examine the lower and inner leaves where humidity is highest and disease is most likely to appear first. Use the UMN extension online diagnostic tool to identify the problem you are seeing on your tomato plant or send a sample to the UMN plant diagnostic clinic (

  2. Remove diseased plants: If late blight appears, immediately remove the infected plant, place it in a tightly sealed plastic bag, and throw it in the trash. This will prevent the pathogen from spreading to nearby gardens and farms.

  3. Apply fungicides: Some gardeners may choose to protect their tomatoes and potatoes with a fungicide spray. Chlorothalonil is the only product available to home gardeners that will provide adequate control of late blight. The vegetable being sprayed MUST be listed on the fungicide label, and all label instructions MUST be read and followed. Organic gardeners can use a copper based fungicide, but should be aware that it will only provide partial protection. Fungicides must be applied before the disease starts in order to control the disease.
Photo 4: Late blight on potato.

S.Bauer, USDA ARS,

 With luck, the season will pass without further disease development. The active management strategies already in place by Minnesota farmers will reduce the amount of fungal spores available to spread the disease. Gardeners can do their part by keeping a watchful eye over their plants and responding quickly if disease occurs.
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