Phytophthora is a genus of Oomycota (oomycetes or water molds), a phylogenetic group of fungus-like eukaryotes in the phylum Heterokonta, or stramenopiles. Many heterokonts are unicellular flagellates, and some, including Phytophthora, are multicellular with a flagellated single-celled stage in the life cycle called a zoospore. The name heterokont refers to the characteristic form of these cells, with differentiated flagella, one whiplash and one tinsel. The whiplash straminipilous flagellum is covered with tripartite (with three regions each) mastigonemes (lateral bristles).
Much like fungi, oomycetes are filamentous, microscopic organisms that reproduce both sexually and asexually and absorb nutrients from their substrate either saprophytically or pathogenically. Oomycetes differ from fungi in that their cell walls are made of cellulose and beta glucans rather than chitin, their hyphae lack cross-walls (septations), and their life cycle is primarily diploid rather than haploid. Oomycetes include some of the most serious plant pathogens; almost all known Phytophthora species can cause disease symptoms in plants.
Phytophthora has become one of the most studied genera of plant pathogens. The genus includes many species reported to cause root, crown, and collar rots, wilts, leaf and stem blights, fruit and tuber rots, cankers, bleeding cankers, and stem dieback, resulting in economic impacts to crops, ornamentals, and forest ecosystems around the world. Some species are particularly devastating plant pathogens that have a significant impact in agriculture or natural ecosystems; among them are P. austrocedri, P. capsici, P. cinnamomi, P. infestans, P. kernoviae, P. quercina, and P. ramorum.
Chlamydospores or oospores germinate in wet conditions to form sporangia. Sporangia release tiny, single-celled swimming spores called zoospores. Zoospores can swim through water on leaf surfaces or through water-logged soil, but they are susceptible to drying. Zoospores are attracted to plant roots as they swim through the soil, and when they find one, they form a cyst. Some Phytophthora species may encyst and infect leaves as well as roots. Cysts germinate to form microscopic thread-like structures called hyphae, and the pathogen grows into the plant tissues to obtain nutrients, infecting the plant. The Phytophthora then produces more chlamydospores (asexually) or oospores (sexually), then sporangia, and the life cycle continues.
Please visit the morphological identification section to learn more about how Phytophthora morphology can be used to identify species within the genus.