As the seasons change and the weather warms, DNR Forest Health staff tend to get more inquiries about tree health issues across the state. Some issues such as root diseases or Douglas-fir bark beetle mortality are relatively common, while others are less common but equally attention-getting. The latter includes red and dead branches, red and newly dead trees, and impacts on trees of various ages and sizes.
A couple of issues in two of our most common tree species have emerged this year, including western hemlock defoliation and mortality, and tip dieback in Douglas-fir. You may have seen some of this damage on trees on your own property or while travelling from here to there.
Western Hemlock Defoliation and Mortality
In 2015, reports of western hemlock trees losing green needles throughout the spring, summer and fall were widespread across western Washington and northwest Oregon.
Western hemlock is considered very susceptible to drought stress and struggles to grow if it doesn’t have enough water. Many tree species shed foliage to reduce transpiration-related water loss in times of drought stress. The foliage loss observed in western hemlock was attributed to drought given the warm and very dry conditions that year.
However, about the same time in 2015, a new foliar disease was observed and identified on western hemlock trees in Oregon. Symptoms of the foliar disease were similar to the foliage loss symptoms observed in western Washington (Figure 1).
The foliar disease affecting western hemlock is a fungus called Rhizoctonia butinii. Professor Jared LeBoldus’s lab at Oregon State University has been investigating this pathogen and disease since 2015. The first detection of the disease was in Washington in 2016 and symptoms have been observed scattered across western Washington since then.
The fungus appears to have a wide range of hosts, including western hemlock, Douglas-fir, Sitka spruce, Pacific yew and true firs. Not all patterns of defoliation among host species look the same as those observed in western hemlock, where the pathogen tends to start in the lower crown causing foliage loss and dead branches and works its way upwards.
The disease appears to have an association with drought conditions. Damage and possible mortality may be seen in western hemlock trees of all sizes and ages that are growing in dry or water-limited soils.
Western hemlock is also susceptible to several root diseases, with the most common being Annosus root disease caused by Heterobasidion occidentale. Damage and mortality to western hemlock may increase when Annosus root disease, Rhizoctonia foliar disease and drought interact at the same time.
Damage may be reduced in some areas by thinning overstocked western hemlock stands to reduce water competition among the trees. In areas where damage and mortality is already observed, it’s best to avoid replanting western hemlock when conducting reforestation and, instead, choose a species that’s known to grow well in your area.
Douglas-fir Tip Dieback
Another tree health issue currently being reported and observed in western Washington is tip dieback in Douglas-fir (Figure 2).
Basically, the branch tips and some tops of young Douglas-fir are red and dead. The dead portions are adjacent to a dark, sunken area on the branch, which is usually indicative of some type of fungus-caused canker.
Samples have been collected and sent to Oregon State University, so we’re likely to have a cause identified soon. There is likely an association with drought conditions.
The DNR Forest Health Program would like to track where these tree health issues are occurring, so if you’ve seen any symptoms of these tree health issues on your property, please report them to me at firstname.lastname@example.org.
by Amy C. Ramsey, DNR Forest Pathologist, email@example.com