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.
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