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Texas Invasive Species Institute

Texas Invasive Species Institute

Late Blight of Potatoes and Tomatoes

Phytophthora infestans

Class: Oomycota
Order: Peronosporales
Family: Pythiaceae

Phytophthora infestans

Photographer: G.D. Franc Affiliation: Colorado State University Source: www.bugwood.org Copyright: CC BY 3.0

Phytophthora infestans

Photogtrapher: Marco Thines Source: http://www.ncbi.nlm.nih.gov Copyright: Public Domain


Late Blight, Phytophthora infestans, is an eukaryotic, water mold (Oomycete) which caused the 1845 Irish and 1846 Highland potato famines. Unlike the other 60 Phytophthora species that produce soil-borne, root-rotting diseases, Late blight infects foliage, stems, potato tubers and tomato fruits. P. infestans infects the foliage of the plants. Late blight lesions can occur on both leaves and stems, and usually occurs after periods of wet weather. Black lesions appear within 3-7 days of infection of leaves. When it is humid, whitish growth (pathogen spores) is produced at the edge of the lesion, particularly on the underside of the leaf. Lesions turn brown when they dry up and the white spore masses will not be visible. Symptoms on tomato leaves and stems are similar to those on potato.

Infected potatoes have shallow, brownish or purplish lesions on the surface of the tuber. On tomato fruit, Phytophthora infestans causes a firm, dark, greasy looking lesion where, under humid conditions, the spore producing structures emerge. For potatoes, these lesions can serve as pathways for other tuber diseases like bacterial soft rot to enter, so late blight symptoms can sometimes be obscured by symptoms of other diseases.

Host Plant: Mainly potatoes (Solanum spp.) but also tomatoes and other members of Solanaceae family; such as petunias, nightshades and tomatillos.

Ecological Threat

Since the oomycete 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 entire potato and tomato commercial fields very quickly under wet conditions; and farmers who grow potatoes or tomatoes are at serious risk of losing their entire income from these crops.


The outbreaks in the early 1990s were caused by 3 genotypes of Phytophthora infestans (US-6 in 1992; US-7 in 1993-1994 and US-8 in 1998). Since then, there have been almost 20 more different genotypes. Each version is unique in its susceptibility to chemical controls (such as mefenoxam), or if it infects both potatoes and tomatoes, if it is A1(prevalent) or A2(rare) mating type, if sexual reproduction occurs, temperature restraints and quick versus long latent periods. The presence of the second mating type (A2) along with the first mating type, allows sexual reproduction and quicker gene recombination, which, leads to more rapid development of resistance to pesticides. In order to treat the infestations it is necessary to know what genotype and strain variation you are dealing with.

The general life cycle of P. infestans starts with sporangia (pictured right) being blown into an area by prevailing winds and germinating (at temperatures between 44 and 55°F) when free water is present on leaves and form 8 to 12 motile zoospores per sporangium. The zoospores swim freely in water films, attach to the leaf surface (encyst) and infect the plant. Encysted zoospores infect leaves by penetrating the leaf surface with a germ tube, either through stomata or by direct penetration. When temperatures are a little higher (55-70oF) sporangia germinate through a single germ tube. Under conditions of high humidity the asexual cycle of the pathogen can occur and only takes 5 to 7 days to complete.

Tubers may become infected if sporangia produced on the foliage are washed down into the soil by water. Then water-borne spores appear to follow stems and stolons into the soil where they reach tubers and cause infection. P. infestans can survive only in living potato tissue. It usually survives from year to year in infected tubers placed in storage, in piles of cull potatoes or in infected tubers missed during harvest that remain unfrozen over the winter (volunteer potatoes). In the spring, the pathogen can be transmitted from infected tubers in cull piles or volunteers to potato foliage by airborne spores, and the cycle continues.


One hundred fifty years after the first appearance of late blight in the United States and Europe, Late blight resurfaces in the early 1990s. It possibly came from imported tomatoes from Mexico that were pre-symptomatic that became symptomatic once in the United States  Even though those tomatoes would have been discarded P. infestans reproduces profusely on infected tomatoes. An aerial dispersal could allow P. infestans to transfer from the infected from those tomatoes to potato or tomato fields.

Native Origin

Native Origin: Thought to be native to Central America, particularly Mexico

Current Location

U.S. Habitat: Anywhere potatoes and tomatoes can be grown; and preferably where there is also wet weather with moderate temperatures (60-80oF), high humidity and frequent rainfall.

U.S. Present: CT, FL, KY, LA, MA, MD, ME, NH, NJ, NY, OH, OR, RI, PA, TN, VA, VT, WI, WV


The best method of preventing an outbreak of P. infestans is close management of farming operations and, scouting for signs should as soon as green tissue emerges. Searching for signs should be intensified in areas where soil moisture is the highest. First, it is important to eliminate sources of inoculum. The initial sources of inoculum can be infected potatoes in cull piles, infected volunteer potato plants that have survived the winter and infected seed tubers. It is important to keep a clean operation by destroying all cull and volunteer potatoes. Cull piles should be kept as small as possible because piles of about 500 hundredweight do not freeze throughout; and P. infestans can then survive the winter. It is also very important to make sure that you plant only certified seed. Using seed saved from local crops may increase the risk of late blight. Seed sources should be selected carefully to avoid bringing in late blight on seed, especially new strains of the pathogen.

No potato cultivars are immune to late blight only resistant. Planting resistant varieties will slow down (but not prevent) the development of late blight. Currently, “Defender” and “Elba” are the most resistant varieties available. Potato varieties with moderate levels of resistance include: Kennebec, Sebago, and Allegany.

Some chemicals are able to help manage late blight; but as previously mentioned some strains of P. infestans are more resistant than others. In some states, like Michigan, both Headline and Amistar have provided very effective late blight control, but these products should be used in strict adherence with anti-resistance development strategies, i.e., always mix with a protectant fungicide (e.g., EBDC or chlorothalonil-based products) and never apply consecutive treatments of the same product. Consult your local advisor for appropriate rates and additional combinations.

There is computer software out there that can help forecast the probability of a late blight infestation and when weather conditions are favorable for P. Infestans. Blightcast is one of the programs that are available to track weather conditions and help predict or forecast when the disease may occur. Disease forecasting programs predict when environmental conditions are most favorable for disease and recommend when fungicide applications should be applied. Unless you know that the forecast information is appropriate for your fields, the forecasts should be used only as a general indication of how favorable weather has been for late blight.




Anderson, P. K., Cunningham, A. A., Patel, N. G., Morales, F. J., Epstein, P. R., & Daszak, P. 2004. Emerging infectious diseases of plants: pathogen pollution, climate change and agrotechnology drivers. Trends in Ecology & Evolution, 19(10):535-544.

Fry, W.E. and S.B. Goodwin. 1997. Re-emergence of potato and tomato late blight in the United States. Plant Disease 81(12):1349-1357.

Goodwin, S. B., Spielman, L. J., Matuszak, J. M., Bergeron, S. N., and Fry, W. E. 1992. Clonal diversity and genetic differentiation of Phytophthora infestans populations in northern and central Mexico. Phytopathology 82:955-961.

Lambert, D. H., and Currier, A. I. 1997. Differences in tuber rot development for North American clones of Phytophthora infestans. Am. Potato J. 74:39-43.


Internet Sources






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