Sudden Oak Death: An International Epidemic?

California's oaks are dying by the thousands from a disease known as Sudden Oak Death. Now, the pathogen that causes the disease has been linked to infestations of rhododendron shrubs in Europe and in a California nursery, suggesting that international trade is spreading the destructive fungus.

A newly discovered fungus threatens critical oak habitat in California. Until recently, the pathogen, a member of the genus Phytophthora, the same group that caused Ireland's great potato blight of 1845-1850, was known only to infect California's coast live oaks, black oaks, and tanoaks from Sonoma County to Big Sur.

map of oak death in california
The Phytophthora fungus wreaking havoc on California's coastal oaks prefers moist and cool conditions.

sap bleeding from discolored bark sap bleeding from discolored bark
The characteristic symptom of Sudden Oak Death is sap bleeding from discolored bark.

Now scientists say it matches a fungus isolated from rhododendron shrubs in Europe, and an infected rhododendron has recently been discovered in a Santa Cruz County nursery. However, it's unknown if the fungus originated in Europe or another part of the world, or whether it is a California native, only recently risen to infamy because of some environmental trigger.

Even more disturbing is recent evidence that the fungus may spread through the air. UC Davis plant pathologist David Rizzo isolated Phytophthora 60 feet up a tanoak that did not show the usual symptoms on its trunk.

Scientists are now examining natural rhododendron populations and other trees in an attempt to understand the spread of this potentially devastating fungus.

different stages of dieback of oak forest
Tanoak forest showing different stages of dieback in Mill Valley