LIFE

Douglas-fir trees the hardest hit in tree die-back

Glenn Ahrens and Mary Stewart
Special to the Statesman Journal
OSU Extension Forester Glenn Ahrens examines conifer branch for signs of stress.

Landscape and forest trees are experiencing widespread dieback this spring according to Glenn Ahrens, Oregon State University Extension Forester for Marion County.

"Browning or dieback is usually caused by weather-related stress, sometimes in combination with pests and diseases," he said.

Douglas-fir trees are the most common victims, but stress due to the weather is affecting many tree species, and a variety of different problems are showing up.

The discoloring on this Douglas-fir is evidence of flare out.

Flare out

Symptoms of flare out on young Douglas-fir trees include branches, tops and sometimes whole trees turning red or brown and dying. Older trees typically have milder symptoms.

"This sudden mortality or 'flaring out' of branches and tops is a classic symptom of drought in conifers," Ahrens said.

Possible stressors include last year's long, dry summer ending with a hot period, followed by an early freeze in November and then a relatively warm winter.

Drought-related injuries to the stem and leader are not always apparent when they occur but often show up the following spring as the weather warms up and it's time for trees to grow. That seems to have begun with the warm weather of February and March, with symptoms becoming obvious in April.

Similar drought damage events have recurred over the past 15 years, most recently in 2013.

Heat and drought can kill trees outright or just put the trees under severe moisture stress. Subsequent problems can ensue when secondary insects or diseases take advantage of a tree's weakened condition.

A Douglas-fir, suffering from stem canker fungus, shows the transition from live to dead tissue. This tree suffered from drought stress.

Douglas-fir is most commonly affected, but similar problems occur with other conifers including grand fir, noble fir, western redcedar and western hemlock. Grand fir around the Willamette Valley is notorious for health problems due to drought followed by secondary agents such as bark beetles and fungi.

Ahrens has observed that drought-stressed Douglas-fir trees often are afflicted with Dermea or Phomopsis stem canker fungi — normally weak pathogens that become damaging in trees under stress. The cankers can coalesce to girdle branches or stems and also can become sites of attack by bark beetles.

Insect pests that take advantage of drought-stressed trees include the Douglas-fir engraver (Scolytus unispinosus) and the pole beetle (Pseudohylesinus nebulosus). Grand fir and noble fir are vulnerable to engraver beetles (Scolytus ventralis), which attack true firs of all sizes.


Severe Swiss needle cast on a Douglas-fir causes thin, yellowing foliage.

Swiss needle cast disease of Douglas-fir

Douglas-fir trees in some foothills around the Willamette Valley are afflicted with Swiss needle cast. The disease produces a pale overall appearance and sparse crown as individual needles turn yellow and drop.

"Swiss needle cast disease has been a problem in coastal Douglas-fir since the 1990s," Ahrens said. "But last year, we had increased reports of the disease in the valley, and we are seeing it again this year."

Weather also is a contributing factor, and the disease is most severe in years with a combination of a warm winter and abundant spring moisture.

Ahrens advised tree owners and managers to watch for progressive yellowing and shedding (casting) of needles, beginning with the older needles. A healthy tree may carry four to five years' worth of needles, while heavily infected trees may carry only one or two years' of needles. Although the disease is not generally fatal to the tree, it often has a significant impact on growth.

What you can do

Tree damage usually is due to a combination of factors. Multiple actions or changes may be needed to relieve stress and increase tree vigor. Possible strategies include:

  • Prevent soil compaction caused by vehicle or animal traffic near trees. Compaction can damage surface soils and roots, especially in clay soils.
  • Avoid direct damage to trees and roots by animals or machinery.
  • Reduce competing vegetation.
  • Irrigate landscape trees during dry spells. Apply water slowly over many hours, and avoid frequent shallow watering. Apply mulch to maintain soil moisture.
  • Do not alter drainage near established trees (ditches, ponds, fill or removal of soil).
  • Plant trees that are well suited for the site. Where Douglas-fir mortality is occurring, consider planting Willamette Valley Ponderosa pine or hardwoods.
  • If insects or branch/stem cankers are evident, prune and destroy affected branches to reduce spread.
  • Do not fertilize during drought conditions; fertilization can increase a tree's water requirements.

See the Forest Health Notes on the Oregon Department of Forestry website at oregon.gov/odf/privateforests/pages/fhpests.aspx. Or for more information on forest and tree health, contact the Oregon State University Extension Service in Marion County at (503) 588-5301.