‘Sudden Oak Death’ – Not so sudden… Not infecting oaks

May 2022 Posted in Gardening

By Melissa Wagoner

The term Sudden Oak Death (SOD) is really a bit of a misnomer,  according to Sarah Navarro, the Regional Sudden Oak Death Pathologist for the US Department of Agriculture (USDA).

“Tanoak is the tree species most affected because it’s readily killed,” Navarro said. Noting that unlike the white oak – a true oak – tanoaks are a part of the broadleaf family.

“And in terms of the lifespan of a tree it’s not sudden,” she added. Because the time it takes from the first sign of infection to canopy death is one and a half to two years.

Especially contagious in moist environments, often through wind and rain events, the pathogen that causes SOD is phytophthora ramorum – a fungus-like organism in the category of water mold.

“The Irish potato famine was caused by a phytophthora,” Navarro said.

In this case, the spores form in the trees’ canopy, awaiting wind, rain or even fog that will carry them to the understory, which is where the infection spreads.

“The spores go from the canopy and then girdle the tree,” Navarro described. “It forms these red, bleeding cankers.”

And it’s not just tanoaks affected, over 100 other types of plants can be infected as well including evergreen huckleberries, camellias and especially rhododendrons – but not white oaks, at least not without some help.

“Here in Oregon, we’ve never seen a faction in Oregon white oak except when the pathogen has been placed under the bark in a laboratory setting,” Navarro pointed out. Adding, “But a lot of our Oregon native flora is susceptible. And where it’s not readily killed you can find leaf spots or twig dieback.”

Fortunately, although the disease is becoming a serious problem in California – where it was first discovered in the late 90s and where it has spread to 16 counties – here in Oregon, it is still relatively rare, primarily seen in Curry County.

“But the potential is there for it to spread to other counties,” Navarro admitted. Noting that, when found, the infected tanoaks, as well as a perimeter trees surrounding them, are immediately harvested and burned.

And as for the rhododendrons, “We haven’t seen natural spread from rhody to rhody in natural landscapes in Oregon. Though they did have an instance in Washington in a botanical garden.”

What the USDA has seen is spread from infected nursery plants.

“The three introductions we’ve had thus far – and possibly a fourth – have all been from infected nursery plants,” Navarro said. Urging, “Buy nursery plants from reputable sources. The OSU Extension Service has great guides for best management practices for buying reputable plants.”

And all landowners should be on the lookout for infected bushes and trees.

“The symptoms are easier to see on rhododendrons,” she pointed out. Describing, “We look for leaf spots on rhodies, typically on the petiole or mid-rib. It’s good to be aware and knowledgeable.”

If an infection is suspected, Navarro suggests landowners contact a local OSU Extension agent, the Oregon Department of Agriculture or – if the plants are growing in a forestry setting – the Oregon Department of Forestry.

While Navarro takes the threat of SOD in Oregon very seriously, she is optimistic about the situation currently.

“We know from research done by OSU that the eradication treatments we’ve done on the ground have slowed the spread in Oregon,” she said.

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