DISEASE: Late blight of potato and tomato PATHOGEN: Phytophthora infestans Late blight is the disease that triggered the Irish potato famine of the 1840s. It also 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. P. infestans is a member of the oomycetes, a group of organisms sometimes referred to as the "water molds" Potato stems and leaves Late blight of potato is identified by black/brown lesions on leaves and stems that may be small at first and appear water-soaked or have chlorotic borders, but soon expand rapidly and become necrotic. In humid conditions, Phytophthora infestans produces sporangia and sporangiophores on the surface of infected tissue. This sporulation results in a visible white growth at the leading edge of lesions on abaxial (lower) surfaces of leaves . As many lesions accumulate, the entire plant can be destroyed in only a few days after the first lesions are observed Tomato stems and leaves Tomato plants are also susceptible to late blight, and the foliar symptoms are similar. Like potato, infected tomato plants may be rapidly infected and destroyed by P. infestans. White sporulation (sporangia and sporangiophores) may be visible in humid weather.