What is Late Blight?

Potato late blight is arguably one of the most infamous diseases in agriculture.  Historically, it is best known as the cause of the Irish potato famine of the 1840s, which resulted in the death or emigration of over 2 million people from Ireland (Martin et al. 2013).  In addition, the disease spurred the emergence of the field of plant pathology.  During studies to understand the cause of the disease, botanist Anton deBary demonstrated that the fungal-like growth observed on blighted plants was the cause of late blight, rather than the result of the disease’s presence.  His work led other scientists to investigate other fungi and bacteria associated with plant disease and, with the development of Koch’s postulates for establishing pathogenicity, resulted in the identification of the causal pathogens of many plant diseases (Schumann 1991).

Late blight is caused by the oomycete pathogen Phytophthora infestans.  It is primarily a pathogen of potatoes and tomatoes, but has been known to infect other members of the Solanaceae plant family as well.  It is also notable for having both an asexual and sexual life cycle (Fig. 1).  P. infestans reproduces asexually predominantly, but in certain locations of the world (e.g. Mexico), the sexual cycle is dominant.  In the US, P. infestans reproduces primarily asexually, but there have been reports of occasional sexual reproduction taking place (e.g. Danies et al. 2014).  As a result, outbreaks of P. infestans in the US can be identified as part of a clonal lineage, which can then be characterized for certain attributes, such as aggressiveness and fungicide sensitivity.
Phytophthora infestans life cycleFig. 1  The asexual and sexual stages of the P. infestans life cycle.  Modified from Schuman and D’Arcy (2000).

Late blight has been referred to as a ‘community disease’, due to its ability to spread rapidly from field to field under the right weather conditions.  Asexual spores travel easily on the wind when the weather is cool and moist, and can rapidly infect neighboring fields.  As such, understanding the symptoms of the disease and what to do when it is detected are essential to preventing an outbreak from rapidly turning into an epidemic.

In 2009, an epidemic of P. infestans occurred in the northeast, due in part to a lack of public understanding of the symptoms and impact of the pathogen (Fry et al. 2013).  As part of the effort to increase communication between growers, home gardeners, extension agents, and the research community, USABlight.org was formed as a means for tracking outbreaks of P. infestans.  It is hoped that through this website, we can provide both a tool for potato and tomato growers for managing late blight, and a way to better understand the movements and impact of this disease. 


Danies, G., Myers, K., Mideros, M. F., Restrepo, S., Martin, F. N., Cooke, D. E. L., Smart, C. D., Ristaino, J. B., Seaman, A. J., Gugino, B. K., Grünwald, N. J., and Fry, W. E.  2014.  An  ephemeral sexual population of Phytophthora infestans in the northeastern United States and Canada.  Plos ONE.  Online journal.  DOI:10.1371/journal.pone.0116354.

Fry, W. E., Myers, K., Roberts, P., McGrath, M. T., Everts, K., Secor, G., Seaman, A., Gevens, A. J., Seebold, Jr., K., Zitter, T. A., Gugino, B. K., Snover-Clift, K., McLeod, A., Johnson, S. B., Wyenandt, A., Danies, G., Judelson, H., Grünwald, N. J., Small, I. M., Ristaino, J., and Smart, C. D. 2013. The 2009 late blight pandemic in the eastern United States – causes and results.  Plant Disease 97:296–306.

Martin, M. G., Cappellini, E., Samaniego, J. A., Zepeda, M. L., Campos, P. F., Seguin-Orlando, A., Wales, N., Orlando, L., Simon, Y. W. H., Dietrich, F. S., Mieczkowski, P. A., Heitman, J., Willerslev, E., Krogh, A., Ristaino, J. B., and Gilbert, M. T. P.  2013.  Reconstructing genome evolution in historic samples of the Irish potato famine pathogen.  Nature Communications.  Online journal.  http://dx.doi.org/10.1038/ncomms3172.

Schumann, G. L.  1991.  The Irish potato famine and the birth of plant pathology.  In: Plant Diseases:  Their Biology and Social Impact.  APS Press, American Phytopathological Society.

Schumann, G.L. and D’Arcy, C. J. 2000. Late blight of potato and tomato. The Plant Health Instructor. DOI: 10.1094/PHI-I-2000-0724-01