Scientific research of Phytophthora infestans, or late blight, has been an issue of concern ever since the plant disease triggered the Irish potato famine of the 1840s. Plant pathologists say it was the first plant disease for which a microorganism was proved to be the causal agent, leading to the birth of plant pathology as a science.
The fungal-like organism that causes late blight affects both tomatoes and potatoes. Unlike the other 60 Phytophthora species that produce soil-borne, root-rotting diseases, late blight infects foliage, stems, potato tubers and tomato fruits. Lesions can occur on both leaves and stems, and usually occur after periods of wet weather.
Outbreaks in Texas and other parts of the U.S. in the early 1990s were caused by three genotypes of Phytophthora infestans. Since then, there have been almost 20 more different genotypes discovered, each version unique in its susceptibility to chemical controls and/or its ability to infect potatoes or tomatoes, or in some cases both.