The name is derived from the Greek: Phyto = plant, phthora = destroyer. P. infestans is a member of the oomycetes, a group of organisms sometimes referred to as the "water molds" which are related to brown algae. The mycelium is hyaline and coenocytic (few septa), and the nuclei are diploid. The formal name for this group of organisms is Oomycota which have been assigned to the Kingdom Stramenopila of the eukaryotes.
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.
Potato tubers become infected in the field when sporangia are washed from the foliage into the soil. Infections generally begin in tuber cracks, eyes or lenticels. Infected tuber tissues are copper brown, reddish or purplish in color. Sporulation may occur on the surface of infected tubers in storage or on cull piles. Infected tubers are often invaded by soft rot bacteria which rapidly convert adjoining healthy potatoes into a smelly, rotten mass that must be discarded.
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.
Late blight infections produce dark brown, firm lesions which may enlarge and destroy the entire tomato fruit. Late blight lesions on tomato fruit are often followed by soft rot and disintegration as described for potato tubers.
The asexual life cycle of Phytophthora infestans is characterized by alternating phases of hyphal growth, sporulation, sporangia germination (either through zoospore release or direct germination, i.e. germ tube emergence from the sporangium), and the re-establishment of hyphal growth. There is also a sexual cycle, which occurs when isolates of opposite mating type (A1 and A2) meet. Hormonal communication triggers the formation of the sexual spores, called oospores. The different types of spores play major roles in the dissemination and survival of P. infestans. Sporangia are spread by wind or water and enable the movement of P. infestans between different host plants. The zoospores released from sporangia are biflagellated and chemotactic, allowing further movement of P. infestans on water films found on leaves or soils. Both sporangia and zoospores are short-lived, in contrast to oospores which can persist in a viable form for many years.
Under ideal conditions, the life cycle can be completed on potato or tomato foliage in about five days. Sporangia develop on the leaves, spreading through the crop when temperatures are above 10 °C and humidity is over 75%-80% for 2 days or more. Rain can wash spores into the soil where they infect young tubers, and the spores can also travel long distances on the wind. The early stages of blight are easily missed. Symptoms include the appearance of dark blotches on leaf tips and plant stems. White mould will appear under the leaves in humid conditions and the whole plant may quickly collapse. Infected tubers develop grey or dark patches that are reddish brown beneath the skin, and quickly decay to a foul-smelling mush caused by the infestation of secondary soft bacterial rots. Seemingly healthy tubers may rot later when in store.
P. infestans survives poorly in nature apart from its plant hosts. Under most conditions, the hyphae and asexual sporangia can survive for only brief periods in plant debris or soil, and are generally killed off during frosts or very warm weather. The exceptions involve oospores, and hyphae present within tubers. The persistence of viable pathogen within tubers, such as those that are left in the ground after the previous year's harvest or left in cull piles is a major problem in disease management. In particular, volunteer plants sprouting from infected tubers are thought to be a major source of inoculum at the start of a growing season. This can have devastating effects by destroying entire crops.