Landscape and forest trees are experiencing widespread dieback, according to Glenn Ahrens, OSU Extension Forester for Marion County. “Browning or dieback is usually caused by weather-related stress, sometimes in combination with pests and diseases,” he says.
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.
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 explains.
Possible stressors include long, dry summers ending with a hot period, followed by an early freeze 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.
Similar drought damage events have recurred over the last 15 years, including 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.
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 are often 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.
Swiss needle cast disease
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,” says Ahrens.
Weather is also a contributing factor and the disease is most severe in years with a combination of a warm winter and abundant spring moisture.
Ahrens advises 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.
Treatment strategies — what you can do
Tree damage is usually 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; 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.