A field of tomatoes wilts. A stand of tanoak trees dies. A forest of bay laurels and manzanita withers. An orchard of citrus yellows and decays. A wildland restoration project crumbles into dust. Potatoes turn rancid and spongy. These are the calling cards of Phytophthora, the destroyers of plants.

Phytophthora are oomycetes, water molds, a distinct phylogenetic lineage of microorganisms. Once mistaken for fungi because of their long, filamentous branches (hyphae), water molds are actually Heterokonts, close relatives of the brown rack and kelp that hug the California coasts. There are hundreds of species of oomycetes. They live everywhere, streams, ponds, oceans and soil. Some even live in arid, desert climates. Regardless of the environment, oomycetes feed by absorption, decomposing dead material and taking in nutrients or by parasitizing other species.

Oomycete parasites infect fish, amphibians, plants and people. Saprolengnia parasitica causes lethal infections in bass, trout, catfish and flounder. Pythium insidosum causes chronic skin lesions in mammals. Saprolengnia and Leptolengina kill frogs. Some infect passing microscopic organisms with spore harpoons. The most infamous and notorious Oomycete parasites infect plants. The genus Phytophthora (Greek phyton “plant” pthora “ destroyer) is made up entirely of plant pathogens.

Some Phytophthora are famous. Phytophthora infestans was responsible for the blight during the Great Irish Famine, causing a diaspora of ~1 million Irish people and the deaths of approximately 174,000. Some, like Phytophthora fragariae, which infects strawberries and raspberries, are more obscure. the vast majority are entirely unknown, living undetected in their native habitats.