Potato Late Blight

Late blight (Phytophthora infestans) fungus is in the same genus as the fungus causing pink rot (P. erythroseptica) (see "Tuber Rots"). Late blight was responsible for the Irish potato famine in the mid-nineteenth century (Daly, 1996). In the late twentieth century, there have been major re-occurrences and concern around the world over this pathogen and its disease due to recent mutations (Fry and Goodwin, 1997). These mutations, most notably strain US-8, have made the pathogen resistant to control by metalaxyl, the stand-by fungicide for many years. Some of these new pathogenic strains are also more aggressive and virulent than the old (US-1) strain that affected the world in the 19th century.

There is also concern over the appearance of A2 mating types of the pathogen. The late blight pathogen (US-1) that has appeared in the past is an A1 type. But, there have been reports in North America of the appearance of strains that are A2 type (Goodwin and Drenth, 1997). The concerns about the A2 mating type are that sexual reproduction can now occur between the two types when they are in the same field. This would result in a much greater genetic diversity. The A2 mating type can result in oospores which do not need a living host. Soil survival from season to season may be possible. A2 may multiply faster than the A1 mating type.


Late blight will first appear as water-soaked spots, usually at the tips or edges of lower leaves where water or dew tends to collect. Under moist, cool conditions, water-soaked spots rapidly enlarge and a broad yellow halo may be seen surrounding the lesion (Mohan et al., 1996). On the leaf underside, a spore-producing zone of white moldy growth approximately 0.1 - 0.2 inches wide may appear at the border of the lesion. Under continuously wet conditions, the disease progresses rapidly and warm, dry weather will slow or stop disease development. As conditions become moist and cool, disease development resumes. See pictures of late blight on stem and leaves.

Tuber lesions first appear as irregular, dark blotches. When cut open, affected tissue is water-soaked, reddish brown and extends with an irregular margin into the tuber flesh. Lesions may start as a superficial decay that continues to develop after tubers are harvested and placed into storage. Older lesions may become firm and sunken due to water loss and tubers will appear shrivelled. Infected tubers are commonly invaded by secondary decay organisms such as soft-rot bacteria and, therefore, are quite likely to decay during storage. Refer to the "Compendium of Potato Diseases."

Potato late blight in canopy Potato late blight on leaf
Potato late blight on canopy. Late blight on leaf.
Late blight on leaf underside Late blight on stem
Late blight on underside of leaf. Late blight on stem.

Disease Cycle

Late Blight is caused by the fungus Phytophthora infestans. The late blight fungus is especially adapted for growth under conditions where water is present and cool temperatures persist. Temperatures ranging from approximately 50 to mid 90s oF will enable disease progression in the field (Franc et al., 1996).

Disease is initiated by fungus spores or "inoculum" produced on the surface of living, infected plant tissue. Infected seed tubers, tomato transplants, potato cull piles and/or other volunteer potatoes are extremely important sources of inoculum. Spores are readily disseminated among plants and fields by splashing rain, overhead irrigation and wind. Repeating cycles of spore production, dissemination, followed by additional spore production, give late blight its explosive disease potential.

Late blight has to be controlled before it gets out of hand which may take only a few days. What makes late blight such an aggressive disease is its polycyclic nature (see diagrams on Movement and Life Cycle); it goes through many disease cycles in a year.

Significant crop loss can result from tuber infection. During rains, late blight spores may wash down stem and 'funnel' into the ground around the base, going through the soil to the tubers. Spores washed from foliage and stems can infect tubers in the hill before harvest and spores present on vines readily infect tubers during harvest. Although tuber to tuber spread may occur in storage, its significance is not known. Once late blight is in the bin, it cannot be graded out; it will show up as a defect on the other end.

Late blight movement
Testing for Late Blight

Cultural tests can be used to determine the presence of the late blight pathogen on a plant. If late blight is sporulating on the leaf, it can be identified right away. To get the fungus to sporulate put the leaf in a zip-lock bag with wet paper towel. One can also put a leaf in a styrofoam cup or container with an ice cube and paper towel, cover it with saran wrap, and by morning, it should sporulate. It just requires humidity and looking for the spores using a microscope.


  • "Compendium of Potato Diseases" 2nd Edition. 2001. Eds. Stevenson, W.R., Loria, R., Franc, G.D., and Weingartner, D.P., Publ. Amer. Phypathological Soc. Press, St. Paul, MN.
  • Daly, D.C. 1996. The leaf that launched a thousand ships. Natural History issue 1, p. 24-32.
  • Fry, W.E. and Goodwin, S.B. 1997. Re-emergence of potato and tomato late blight in the United States. Plant Disease 81:1349-1357.
  • Franc, G.D., Brown, W.M. Jr., and Kerr, E.D. 1996. Potato Late Blight. Univ. Wyoming Coop. Ext. Serv. B-1032.
  • Goodwin, S.B. and Drenth, A. 1997. Origin of the A2 mating tupe of Phytophthora infestans outside Mexico. Pytopathology 87:992-999.
  • Mohan, S.K., Thornton, M.K., Nolte, P., and Bijman, V.P. 1996. Late Blight of Potato and Tomato. Univ. Idaho Coop. Ext. Sys. CIS 1051.