Sudden Oak Death
Phytophthora ramorum is a water mold pathogen that causes two types of disease. On trunk hosts, it causes the disease known as sudden oak death, a forest disease that has resulted in widespread dieback of several tree species in California and Oregon. On understory plants P. ramorum is a foliar and twig disease which infects but does not kill a wide variety of ornamental plants; on foliar hosts, P. ramorum is referred to as ramorum blight or ramorum dieback.
Since the early 1990s, oaks and tanoaks have been dying in the coastal counties of California. To date, more than 1 million tanoaks have died from the disease known as sudden oak death (SOD). It wasn’t until 2001 that scientists identified the pathogen as a water mold they named Phytophthora ramorum.
P. ramorum has a broad range of hosts, including hardwoods (several oak species), softwoods, like Douglas fir, and many popular ornamental plants like rhododendron, viburnum and camellia. Some may succumb to the disease, whereas others facilitate the spread of the fungus-like pathogen.
Through ongoing surveys, APHIS-PPQ continues to define the extent of the pathogen’s distribution in the U.S. and limit its artificial spread beyond infected areas through quarantine and public education.
P. ramorum thrives in cool, wet climates, but significant infestations can occur in much warmer, drier conditions when nurseries create microclimates that foster the pathogen. It spreads when rain – often driven by strong wind – splashes spores onto new susceptible plants. It also moves long distances in waterways and locally in contaminated irrigation water and soil or gravel substrates. The pathogen can also be moved unknowingly in undiagnosed infected plants, contaminated potting mix, and planting containers.
Sudden oak death is not the only significant “pest” that can spread to new areas by people. Unwanted pests and plant diseases can move into new areas unknowingly on firewood or live plant material. One way to help limit human-assisted spread of these plant pests and pathogens is by burning firewood where you cut it. Learn more about human-assisted spread of plant pests and pathogens from our friends at Don't Move Firewood.
Trunk hosts like oaks and tanoaks are considered terminal hosts.
Phytophthora ramorum infected nursery stock has been found in over half of the states in the U.S. Many nursery and landscape plants are hosts for Phytophthora ramorum. Some foliar and twig hosts include camellia, rhododendron, viburnum, pieris, kalmia, magnolia, leucothoe, and syringa.
To see the most recent list of regulated and associated hosts for P. ramorum visit the USDA Phytophthora ramorum page.
Phytophthora ramorum causes two types of diseases, depending on the host. On members of the oak family (Fagaceae), it causes stem cankers that in time kill the host. The name sudden oak death describes symptoms of this type of P. ramorum. Cankers often show the characteristic “bleeding” symptom. In contrast to the symptoms displayed on trunk hosts, symptoms on other plants include shoot blight, shoot necrosis, and leaf spots or leaf blight. Roots may be infected but appear healthy.
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