Phytophthora infestans has been observed infecting roughly 90 plant species . However, the pathogen is most associated with potatoes (Solanum tuberosum) and tomatoes (Solanum lycopersicum).

The infection cycle begins with the development of asexual spore producing structures (sporangia) on the aerial parts of the plant or from oospores, which release zoospores (wind and rain dispersal) or germinate directly to initiate infection. Zoospores discard their flagella and synthesize a cell wall to form a cyst, which germinate within hours and enter into the host via openings such as stomata or form an appressorium-like germ tube to penetrate into the host. After penetration, a primary infection vesicle is formed and it is from which hyphae will emerge to colonize the plant host. The hyphae grow intercellularly, producing haustoria, which project into host cells, initiate an interaction between the pathogen and the host cell membrane and allow the pathogen to siphon nutrients from the host cells. As the infection spreads, P. infestans will produce sporophores, which will lead to the production of further sporangia and cause lethal lesions to be formed on the plant.

If two different mating types come into contact, sexual reproduction can occur and leads to the formation of thick-walled structures called oospores, which allow for survival and dispersal of the pathogen. In the absence of oospores, P. infestans can survive between crops as mycelium in infected tomato fruit or potato tubers.