Phytophthora ramorum and other Phytophthora species are members of the Oomycota, or water molds. Based on DNA analysis, the Oomycota are placed in the kingdom Stramenopila, which also includes diatoms and kelp. Most Phytophthora species attack living plants, and many are serious plant pathogens affecting forests, agricultural crops, and horticultural plants worldwide.
The Oomycota were previously grouped with the fungi based on their overall form and life habits. Like the true fungi, Phytophthora and related organisms are made up of a network of filaments (referred to as hyphae, or collectively as mycelium). As P. ramorum hyphae penetrate and grow into a host plant, they secrete chemicals that kill and degrade plant tissues. The hyphae absorb nutrients released from dead and dying host cells to fuel their continued growth.
P. ramorum is disseminated via several spore types that are produced by the hyphae. These spores can only be observed with the aid of a microscope. During periods of rainy weather, especially when temperatures are relatively warm (see section 1.3.1 Environmental conditions), sporangia (fig. 1-2) are produced on the surfaces of infected leaves and twigs of some hosts. The sporangia detach readily and can be dispersed via splashing water and wind-blown droplets. Relatively few hosts support production of sporangia (see Sidebar 1-3—Host Susceptibility Versus Spore Production).
Sporangia can germinate directly, producing hyphae that can penetrate the host. However, when they are in water, including water films on plant surfaces, sporangia can instead release zoospores. About 30 motile zoospores can be produced by each sporangium (Widmer 2009). Zoospores can swim up to 7 cm in still water and smaller distances through saturated soil (Erwin and Ribeiro 1996), using their threadlike flagella, but can be carried large distances (10 to 100 meters or more) by flowing water.
Zoospores actively swim toward certain chemicals released by host plants, which helps them aggregate on the host. Upon reaching the host, each zoospore transforms into a nonmobile cyst that germinates by producing a single hypha that can penetrate and infect the plant. If zoospores aggregate before they encyst, host tissue may be attacked by many hyphae in a small area. This mass attack increases zoospores’ ability to overwhelm host defenses.
Zoospores in free water are short lived, typically swimming for a few hours to perhaps a day. Zoospore cysts persist a short while longer, but do not tolerate drying. Sporangia can persist for days, but are relatively thin-walled and do not survive dry conditions. P. ramorum also produces thick-walled, long-lived spores (chlamydospores) within and on various infected plant tissues. Chlamydospores are relatively resistant to environmental degradation and help P. ramorum persist though periods unfavorable for growth.
Sporangia, zoospores, and chlamydospores are asexual spores. When these spores germinate, they give rise to new clones (genetically identical copies of the individual that produced the spores). Phytophthora species also can reproduce sexually by forming oospores. Oospores give rise to individuals that are genetically distinct from the parental strains. Oospores are thick-walled and can persist in the environment for extended periods. P. ramorum can only form oospores when strains of opposite mating types come in contact with each other (Werres and others 2001). Only a single mating type of the pathogen is currently widespread in California, and sexual reproduction of P. ramorum is not yet known to occur in California forests (see Sidebar 1-4—Strains of Phytophthora ramorum).