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Late Blight takes our Tomatoes

Source: M. Grabowski, UMN Extension Educator

After carefully cultivating tomato plants throughout the summer, many Minnesota gardeners have been shocked to see their plants devastated by blight and fruit rot in a matter of days. The culprit is late blight, a disease of potato and tomato plants. Late blight does not cause significant damage every year in Minnesota but prolonged cool wet weather this year has created ideal conditions for disease.

Late blight is cause by the pathogen Phytophthora infestans, a fungus like organism that thrives in wet conditions. The scientific name of the pathogen translates to ‘plant destroyer’ and the pathogen lives up to its name.  Late blight is the plant disease that ruined the potato crop in the infamous Irish potato famine of the 1840’s and the disease has been responsible for numerous epidemics since then.

How to identify late blight

  • Leaves have large, dark brown blotches with a green gray edge.
  • Stem infections are firm and dark brown.
  • Disease progresses very rapidly in cool wet weather and the entire plant may turn brown and collapse in just a few days.
  • Fruit have firm dark brown blotches. If the tomato is cut open, dry brown rot can be seen extending into the fruit. Fruit only become soft and mushy when bacteria invade after the initial infection.
  • In high humidity, thin powdery white fungal growth appears on infected leaves, fruit, and stems.

Management options for home gardeners
Once late blight has been found in a garden, there is little that can be done to help the plant. The disease simply moves too quickly. Harvest any tomatoes that are not showing symptoms and keep them in a well ventilated area to ripen. It is likely that a few are infected but not showing symptoms at the time of harvest, so check on tomatoes regularly and discard any infected fruit. Potatoes from infected plants should be dug up as soon as possible. Uninfected tubers can be eaten but should be checked regularly for rot during storage. Discard infected tomato fruit and potato tubers along with infected plants.

Infected plants should be removed or destroyed as soon as possible to prevent the thousands of airborne spores forming on the leaves from spreading to neighboring plants.

  • Infected plants can be placed in a plastic bag or under a plastic tarp and left to cook in the sun for several days. Once all of the plant material is killed, the plant can be composted or buried.
  • Plants can also be shallowly buried in soil, as the freezing winter temperatures will kill both the plant and pathogen.
  • Infected plants can be placed in a compost pile that completely heats up and breaks down all plant material. Do not use a compost piles that does not thoroughly heat up or completely break down plant material. These mildly warm piles can sheltered infected plant material from the freezing cold and allow the late blight pathogen to survive from one season to the next. If you are unsure how hot your compost pile gets, heat infected plant material in a plastic bag before placing it on a compost pile.  

Can fungicides help?
Fungicides are not effective once a plant has become infected with late blight. For many gardeners, it is too late to apply fungicides. Fungicides only work to protect healthy plants and prevent new infections. Gardeners that have not yet seen late blight may choose to protect their tomato or potato crops with a fungicide that has copper as the active ingredient. The product MUST list tomato and/or potato on the label and the gardener MUST follow all label instructions to use the pesticide safely. There are a number of copper fungicides registered for use in home gardens. Some are certified for organic use.

Will late blight come back next year?
The late blight pathogen can only survive Minnesota’s winter if sheltered from extreme cold in infected plant material. This typically only occurs in large piles of unmarketable potatoes that are sometimes left on field edges or in infected potatoes that are buried deep enough to survive the winter. Most years the late blight pathogen must be brought into the state on infected potato seed, infected tomato transplants, or as windblown spores from other areas. Once the pathogen arrives, the right weather conditions (cool and wet) must be present for the disease to become established and spread.

There are a few tomato and potato varieties with resistance to late blight. Look for resistant varieties in your seed catalog for next year.

Photo: Late blight on tomato fruit. Mold appears in high humidity. M. Grabowski, UMN Extension


Michelle Grabowski
Extension Educator, Horticulture
(651) 319-2551
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