Plant pathologists at Washington State University are studying a surge of fungal diseases striking down young Christmas trees already weakened by climate change.
Two different culprits, Armillaria root disease and Phytophthora root rot, killed firs in rows at separate farms in Washington and Oregon in 2022 and 2023. The scale of the outbreaks surprised WSU scientists, who are sampling dead trees to identify the specific pathogen species responsible.
“We’re seeing these diseases show up on a wider scale than we’ve typically seen before,” said Gary Chastagner, a WSU professor known among growers as “Dr. Christmas Tree” for his 46-year career working with the ornamental conifers.
The Pacific Northwest is a prolific producer of Christmas trees, which are trucked to sellers throughout the West. Oregon is the number one Christmas tree-producing state; Washington follows at number six. Chastagner’s team works with growers to manage diseases that stunt and kill the valuable crop.
The more prevalent disease, Phytophthora root rot, is spread by water molds, which thrive and spread in wet conditions and can shrink harvests of susceptible trees by as much as 75%. Once in the soil, it’s virtually impossible to remove and is especially problematic for noble and Fraser firs grown in poorly draining soils.
Less frequently encountered, Armillaria root disease is caused by 11 different species of North American fungi , several of which are ancient inhabitants of Pacific Northwest forests.
“Armillaria pathogens always been present in natural forests,” said Ned Klopfenstein, research plant pathologist with the USDA Forest Service’s Rocky Mountain Research Station and a WSU adjunct scientist. “A single Armillaria organism has been reported that is miles across and has been alive for thousands of years.”
In the forest, Armillaria kills maladapted trees, making room for the next generation. During an Armillaria infection, black, stringy strands of fungus called rhizomorphs can spread 10 feet or more from diseased roots or debris to colonize a healthy neighboring tree. Armillaria root disease can kill trees by destroying the roots or girdling the trunk.
On one farm, WSU scientists noticed an Armillaria species that’s different from those that typically dwell in Northwest forests.
“We may be seeing the emergence of a species we haven’t seen before,” Chastagner said.
Another surprise is disease’s appearance on farms that are removed from natural forests and have been growing Christmas trees for decades.
“Growers are in their third or fourth rotation of trees,” Chastagner said. “They’ve never seen anything like this before.”
Climate change may play a part in exacerbating tree diseases. Wetter conditions promote the water-loving Phytophthora, while hot summers weaken trees’ defenses.
“The droughts and heat dome of the past few years created moisture stress that can make trees more vulnerable to Armillaria,” said Marianne Elliott, a WSU research associate who is collecting samples to confirm which Armillaria species is linked to the recent tree deaths.
Funded by the Real Christmas Tree Board, Chastagner’s team will survey Northwest farms in 2024 for presence of these pathogens. Future research will help answer more questions about the diseases, including whether stump removal minimizes infection and if increasingly planted Eurasian conifers can stand up to the rots.
While the fungi can never be totally purged from the soil, this knowledge would buy growers enough time to nurture their trees along until harvest.
“For Christmas trees, delaying disease for several years should be enough time to allow them to be harvested successfully,” Klopfenstein said.