A disease that infects Christmas trees erupted in some Pacific Northwest tree plantations last year, leading to tree loss and triggering a renewed round of research into better understanding the disease.
The disease, web blight, has been a sporadic, but relatively minor problem in Christmas trees since it was first identified in the Northwest in the late 1990s.
“I suspect that one of the reasons it was so severe this past year was because of all of the wet weather that we’ve had,” said Washington State University plant pathologist Gary Chastagner. “That provides an environment that is super conducive for spread of the pathogen.
“You need cool, moist conditions for it to spread from needle to needle,” he said.
The web blight pathogen is a type of Rhizoctonia, Chastagner said, but not the one that causes root rot or damping-off of seedlings. The disease primarily infects Douglas-fir, but this past year also showed up on noble, grand, Nordmann and Turkish fir.
Outside of some research into web blight in forest situations conducted through Oregon State University, little research has been done on the disease since preliminary studies on Christmas trees were conducted at Washington State University in the late 1990s. Chastagner said he now is revisiting that work.
“We are looking at the optimum temperature for the growth of the pathogen and development of the disease and, in collaboration with (OSU Extension Christmas Tree Specialist) Chal Landgren, we are looking at the ability of the pathogen to survive over the summer and cause problems again in the fall,” he said. “We don’t know whether those same trees that are damaged in a planting are likely to be damaged the next year, or whether new infections from the spread of inoculum from other sources, such as nearby forest trees, result in new infections.
“We don’t fully understand the sources of inoculum, what the types of inoculum are and the optimal conditions for the development of the problem,” Chastagner said. “Nor do we fully understand the extent of the susceptibility of some of the species of Christmas trees that we are growing.”
According to the Pacific Northwest Plant Disease Management Handbook, web blight first appears on trees as a browning of outer foliage in roughly circular areas. Chastagner described a typical symptom as volleyball- or basketball-sized patches of brown needles, often connected by a web that can hold infected needles in place.
“What happens is this pathogen is basically growing as a web of mycelia that you can see over the surface of the needle, and it just kind of spreads from one needle to the next,” he said.
According to the handbook, under moist conditions, the fine fungal webbing may be visible. The disease, which can spread to affect as much as half the side of a tree, can be distinguished from Botrytis, or gray mold, in that the latter affects only current-year needles and shoots, and symptoms initially appear on the new growth in the spring.
In cases where trees are marginally infected, new shoots the next year will likely cover up the damage, and trees can still be brought to market. In more severe cases, growers tend to cut out trees.
“This past year there were some Douglas-fir where I would say half the tree didn’t have any needles on it anymore,” Chastagner said. The infection tended to be directional, he added. “In other words, infection was most severe on sides of the tree that were up against a forested area, or the north sides of the trees, where you might expect it to be cooler and moist longer.”
Among questions Chastagner is hoping to answer in his research is how the disease is spreading from tree to tree. “It could be doing so by spores, but that has not been demonstrated. There is a spore stage to this pathogen, but it is not clear what role that stage plays in the development of the disease, and we don’t know when the pathogen would actually be producing those spores. It could be a limited time period, or a long period of time,” he said.
“But there are other ways it can spread,” he said. “If I have trees that have the disease and I was doing culture work on the trees, I could have some of those colonized needles transferred from one tree to the next on, let’s say, a shearing knife. Or, just by walking through a planting, sometimes the needles can fall off and get on your clothes, and the next time you walk by a tree, maybe you transfer the colonized needles to another tree.
“We don’t have a good sense on how it is spreading, but there are a number of ways it could potentially spread, including the wind. If you had a windstorm, it could possibly blow needles from nearby timber into the edges of a Christmas tree planting,” he said.
Researchers also don’t know whether the pathogen survives over the summer on needles on the ground and produces a spore stage that re-enters trees, or whether it is the needles that get hung up in the tree that are able to cause infections the following year.
Chastagner and his team have collected foliage samples from several fields and are looking at the survival of the pathogen on several different types of trees, including Douglas-fir, Turkish fir, Nordmann fir, grand fir and noble fir.
“We want to see whether there is any difference (in survival of the pathogen on different hosts), so we are monitoring those (samples),” he said. “But this is the first year that we’ve done some of those types of studies.”
Among past findings from research Chastagner conducted in the late 1990s, it was shown the pathogen is sensitive to some fungicides, including Bravo, or chlorothalonil. But, Chastagner said, sprays applied in the spring to control a disease such as Swiss needle cast are unlikely to affect web blight, which appears in the fall.
Among cultural control methods identified by Chastagner and the Pacific Northwest Plant Disease Management Handbook are to avoid planting trees in low-lying areas with poor air drainage and avoid planting near native stands of Douglas-fir that appear to have the disease.