Late Blight


Late blight is caused by the water mold fungus Phytophthora infestans.


P. infestans has a wide host range, including tomato, potato, pepper, and eggplant. It is most notably known for its destructive role in the 1840's Irish potato famine.
Late blight occurs commonly in coastal environments and sporadically elsewhere. The fungus spores can originate from seed tubers, cull piles, volunteers, closely related weed hosts such as nightshade, and adjacent plantings of potatoes or tomatoes that already are affected. Late blight mainly attacks the foliage, fruits and tubers of potatoes and tomatoes. However, it can sometimes be found on other crops such as tomatillos, weeds and ornamentals in the botanical family Solanaceae (nightshades and petunias).

The unaffected parts probably are safe to eat. Parts with symptoms likely do not pose a health risk when consumed either, but they do not look appetizing and will have an off flavor. This pathogen is not toxic, but no published scientific studies on this specific issue were found to confirm this conclusion, therefore consumers need to make their own decision on food safety.


The fungus may appear differently depending on the host and which part of the host has been infected. The following describes the appearance of different infected plants and their parts.

  • Potato tubers: lesions appear as a shallow, coppery-brown, dry rot, which spreads irregularly from the surface through the outer 1/8-1/2 inch or more of tissue. Infected tissues immediately beneath the skin appear granular and tan to copper-brown. If stored in cool, dry conditions, lesion development slows and over time lesions may dry out. Late-blight lesions allow for secondary bacteria and fungi to enter, resulting in a slimy, foul-smelling breakdown of the tubers.
  • Tomato: develops large, firm, brown, leathery-appearing lesions usually concentrated on the sides or upper fruit surfaces. Fluffy, white fungal growth often develops under humid conditions. A secondary bacteria often follows resulting in a slimy wet-rotted fruit.
  • Potato and Tomato Leaves: pale green, water-soaked spots appear at the tips or edges of the leaves. Leaf surfaces will have circular or irregular lesions, often surrounded by a pale yellowish-green border that merges with healthy tissue. Lesions enlarge rapidly and turn dark brown to purplish-black. In wet weather conditions a cottony, white mold growth appears at the edges of lesions. This indicates that the fungus is producing spores. In dry weather, infected leaf tissues quickly dry up and the white mold growth disappears.
  • Stems: appear brown to black and when moist weather persists white mold growth can be seen.

Life Cycle

*The fungal pathogen that causes late blight requires a living host (usually potato tubers) to survive overwinter. The tubers might be in storage or in soil but the pathogen cannot survive in soil alone or on dead plant debris. Tubers that have been discarded at any stage of crop production or handling (harvest, storage, shipping, spring cleanup, or planting) are called "culls." Culls may survive if they are not destroyed (frozen, crushed, composted, or buried at least 2 feet beneath the soil surface) Late blight epidemics can also occur when the fungus becomes reintroduced on seed potatoes, tomato transplants, when live spores blow in with rainstorms, or when spores wash down through the soil and contact the fleshy organs of tubers.

  • Lesions enlarge as the pathogen grows through the tissues, and the pathogen can produce spores from older lesions when the environment is favorable (leaf wetness for more than 10 to 12 hours at moderate temperatures).
  • Spore production may occur on lesions that are only four to six days old. Under dry conditions spore production does not occur and the lesion has a brown dead center, surrounded by host tissue that has collapsed and appears either water soaked, gray-green, or yellowed.
  • Infected stems and stem lesions are capable of producing sporangia for a longer time than the lesions on leaves.
  • Individual lesions can produce 100,000 to 300,000 sporangia per day. Each sporangium is capable of initiating a new infection that will become visible within three to four days and produce sporangia within another day or two under optimal conditions.
  • Disease development (growth of the fungus) favors moderate temperatures of 60-80 degrees F and humid to very wet weather conditions. This fungus has a high reproductive potential making epidemics all the more devastating.

Natural Enemies

Not applicable


During the growing season, monitor your garden for symptoms of late blight regularly- at least weekly. Check more often during periods of wet weather.

Action Thresholds

Late blight can develop and spread rapidly if spores are present and conditions are conducive, so the removal of infected material as soon as possible is recommended.

Cultural and Physical Controls

The most effective strategy for managing late blight is to avoid sources of spores. The following lists non-toxic ways to avoid spreading spores before, during and after growing season.

  • Plant only blemish free and symptom free tubers. Examine tubers for late blight symptoms by cutting them open with a knife. Immediately sterilize or thoroughly wash the knife if symptoms are found.
  • Destroy all rejected, unplanted seed tubers by burying them deep in an area away from the garden, burning them, or discard them in a plastic bag in the trash.
  • Make sure any tubers put into compost piles are completely decomposed.
  • Pull up and destroy potato plants that come up from tubers left in the garden last season.
  • Harvest all potato tubers in the garden. If late blight occurs when the tubers have already 'sized up', harvest the crop as soon as possible to avoid post-harvest tuber rot
  • Plant healthy-looking tomato transplants. Fortunately, the late blight fungus is not known to infect tomato seed.
  • If late blight becomes severe, remove diseased plants by digging them up. Destroy these plants immediately by: burying them deeply in an area away from the garden, burning them, or by bagging them in a plastic bag and discarding the bag.
  • Avoid spraying the plants with water, which wets the leaves and instead soak the ground around the plants. Not possible, water mid-day so that foliage can dry quickly.
  • Avoid harvesting in wet conditions.
  • Eliminate weeds around the garden thus maximizing air circulation around the plants.
  • For potatoes, hill up the soil around the plants to provide a more substantial barrier against tuber infection from spores washing down through the soil.
Resistant Varieties
  • Plant potato varieties that show resistance to late blight. Elba is currently the most resistant potato variety available. Potato varieties with moderate levels of resistance include: Kennebec, Sebago, Allegany, and Rosa. (These varieties will slow down, but do not prevent, the development of the disease).
  • Unfortunately no late blight resistant tomato varieties are available.

Biological Controls


Chemical Controls

Bordeaux mixtures may be used as a protectant. Although there are many formulas for preparing Bordeaux mixture, generally a ratio of 10-10-100 works well for many disease-causing pathogens. The three hyphenated numbers represent the amount of each material to add.

The first number refers to pounds of copper sulfate, the second to pounds of dry hydrated lime, and the third to the total gallons of water. For the home gardener a 1-gallon mixture of 10-10-100 Bordeaux, which would contain 1/10th of a pound of each of the dry ingredients, which would be 3-1/3 tablespoons of copper sulfate and 10 tablespoons of dry hydrated lime in 1 gallon of water. You can purchase copper sulfate and hydrated lime at most garden centers.


Fry, William E. VEGETABLE CROPS: Late Blight. Fact Sheet. Department of Plant Pathology, Cornell University, July 1998. Web. 4 May 2010. <>.

Late Blight Food Safety Statement." Rev. of Late Blight by Meg McGrath. Web log post. Cornell Cooperative Extension Community Horticulture. Cornell University, 8 Sept. 2009. Web. 7 May 2010. <>.

Rowe, Randall C., Sally A. Miller, and Richard M. Riedel. Ohio State University Extension Fact Sheet: Late Blight of Potato and Tomato. Fact Sheet. Ohio State University. Web. 5 May 2010. <>.

The Plant Disease Diagnostic Clinic. Late Blight, Phytophthora Infestans, Fact Sheet. Publication no. Fact Sheet. Cornell University, Mar. 2010. Web. 5 May 2010. <>.

"Tomato Late Blight." UC Davis IPM. January 2008. Web. 5 May 2010. <>.

USA. United States Department of Agriculture. Beltsville Agriculture Research Center. Late Blight Phytophthora Infestans. By Kenneth Deahl, Kathleen Haynes, Richard Jones, and Leslie A. Wanner. USDA. Web. 7 May 2010. <>.

  • No labels