Published by Utah State University Extension and Utah Plant Pest Diagnostic Laboratory
PLP-030-23PR
September 2023
Sudden Oak Death and Ramorum Blight Marion Murray, Extension IPM Specialist and Ann Mull, Research Assistant
Quick Facts • Phytophthora ramorum is a pathogen that causes sudden oak death (SOD) on susceptible oaks and ramorum blight on foliage of approximately 120 additional hosts. It has not been found in Utah. • Phytophthora species are fungal-like organisms called oomycetes and are more closely related to diatoms and brown algae than fungi. • SOD results in large cankers (death of tissue) on the tree trunk that kills the tree. • In California and Oregon, SOD has killed more than 1 million tanoak and oak trees and has greatly impacted coastal ecosystems. • Once established, this pathogen is extremely difficult, if not impossible, to eradicate or control. • Long-distance spread by humans can occur from moving infested nursery plants and soil. Fig. 1. Coast live oak killed by sudden oak death (Phytophthora ramorum).1
INTRODUCTION
Currently, P. ramorum is established in 18 California
udden oak death (SOD) and ramorum blight are
counties. The pathogen has also been detected and
S
caused by Phytophthora ramorum, a non-native
water mold (oomycete) that originates from parts of Asia. Sudden oak death was given its name due to the appearance that entire trees were dying in just 2 to 4 weeks (Fig 1). In reality, the disease progresses over an extended period, estimated at more than 2 years after initial infection. Ramorum blight is nonlethal, typically
counties as well as in Oregon’s Curry and Lincoln contained in other U.S. nurseries. Surveys for P. ramorum are ongoing, including in Utah, and state and federal quarantines exist for proven hosts and associated plants to limit human-assisted spread.
IMPACT
causing foliar browning and wilting.
Tanoak and oak (Quercus spp.) are dominant tree
Phytophthora ramorum is currently only established
hardwood forests. Death of these trees caused by
in forests of California and Oregon, where it has killed tanoak and other oak species. In 1995, the pathogen was first detected in the U.S. in California’s San Francisco Bay Area, where it caused widespread tanoak mortality.
species in California’s redwood and mixed conifer/ SOD impacts forest ecosystems by altering species composition, decomposition rates, and nutrient cycling. To date, SOD has killed 63% of tanoaks in parts of California’s Monterey County, and 90% to 100% in some areas of Point Reyes National Seashore. Although the resulting dead trees can increase fuel for fires, wildfires Page 1