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.
Phytophthora are dangerous to individual plants in their native environments but devastating to ecosystems when introduced to new habitat. In California there are several invasive Phytophthora species. Phytophthora cinnamoni can infect over 1000 species including avocado, bay laurels, manzanita and madrones. P. ramorum causes Sudden Oak Death and can infect hundreds of different species. P. cambivora infects beech, chestnuts, almonds, manzanitas and oaks. P. cactorum kills native shrubs including coffeeberry. Douglass fir are felled by P. crytogea. P. tentaculata rots the roots of many species and has recently been detected at Alameda and Peninsula restoration sites. Some of the invasive phytopthora haven’t been conclusively identified. They grow in both wet and dry areas, colonizing dryland and chaparral just as easily. If left unchecked they can denude entire forests or shrublands, leaving the area dead, vulnerable to fire and inhospitable to native animal life.
So we know they’re dangerous, but how do phytophthora infections work? They start as free-swimming, microscopic spores called “zoospores”. They swim toward nutrients and chemical signals released by the fine root hairs of plants. Zoospores don’t need open or flowing water to travel this way but flowing water can accelerate their spread. They move through the thin layer of water trapped within soil particles. Once they find a root hair (or get splashed onto a leaf) the zoospores latch and produce hyphae. These are thin, hair-like roots that penetrate between plant cells. The hyphae produce haustoria, tiny, needle-like roots that penetrate plant cells, secrete digestive enzymes consuming the plant from inside out.
Plants will try to wall off the infection with suicidal, hypersensitive cells and kill it with antimicrobial peptides,or oxidative bursts. These tactics are ineffective against introduced phytophthora species which do not share the same susceptibility to antimicrobial peptides as their native cousins. Plants may even have a difficult time detecting the infection because invasive phytophthora appear “different” to plant senses, evading detection and actively suppress the expression of anti-oomycete genes.
After the hyphae have absorbed enough nutrients they reproduce asexually. The hyphae burrow back out through the plant and produce sporangia, sacs full of new zoospores. These release new spores into the environment. The hyphae also grow upward into the shoots and leaves. Under optimal environmental conditions this cycle can perpetuate the rapid spread of the disease between neighboring plants.
Under dry conditions the hyphae can differentiate into chlamydospores. These tough, hardened sacs that can withstand heat and drought and can re-introduce zoospores to the environment following rain or directly infect a plant if it should happen to bump into it. Oospores are made after sexual reproduction between hyphae. This increases genetic diversity in the phytophthora population, making it more difficult for plants and plant pathologists to control. The oospore itself is also resistant to harsh environments. Oospores may germinate on contact with a susceptible plant or release zoospores under moist conditions.
It’s not all doom and gloom. We can help contain invasive infections even if eradication in the wild isn’t feasible. If you hike, mountain bike, run trails or drive off-road you can contain the spread of phytophthora by thoroughly cleaning your boots, shoes and tires before and after your day of fun. This prevents chlamydospore or oospore-bearing soil particles from travelling between different parks, trails or forests. If you notice dead, standing, bay laurels, manzanita, madrone or oaks clustered next to each other or spreading downhill this can be a sign of infestation. Try to avoid areas like that, and notify parks officials (There's an app for Apple. Seriously. Use it). If you're buying plants, shop at places that are clean, with good drainage and use controlled irrigation from municipal water sources. The best way to control phytophthora is by quarantine and removal of the afflicted plants. The earlier we catch these infections the better.
As our world gets more connected, as travel becomes easier and more humans move into previously sparsely populated areas, other invasive species will arrive and cause problems for local wildlife. We have a duty and responsibility as stewards of our environment and our urban forest to do what we can to mitigate future outbreaks and contain current ones. It's difficult. We won't stop all of it. We need to do what we can so that our native plants have a chance to develop resistances to introduced parasites and pathogens. To do otherwise is to cede responsibility for a problem we had a hand in causing and devalues our wild spaces.