As the seasons change and the weather warms, we tend to get more inquiries about tree health related issues across the state. Some issues — root disease or Douglas-fir bark beetle caused mortality — are relatively common, while others are less common but equally attention getting. These latter issues include red and dead branches, red and newly dead trees, and impacts on trees of various ages and sizes. A couple of issues in some 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 the region, including in western Washington and northwest Oregon. The foliage loss was initially attributed to the very dry, warm weather conditions that occurred across the state. Western hemlock is considered very susceptible to drought stress, which basically means that it doesn’t grow very well when it doesn’t have enough water. Many tree species shed foliage in an effort to reduce transpiration-related water loss in times of drought stress. The foliage loss observed in western hemlock was attributed to this cause. 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 of the patterns of defoliation among host species look the same as the disease does in western hemlock where the pathogen tends to start in the lower crown causing foliage loss and dead branches and work its way upwards.
The disease appears to have an association with drought conditions, so damage and possible mortality may be seen in western hemlock trees of all sizes and ages that are growing in water-limited, or dry 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 all 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.
Western hemlock defoliation and mortality caused by Rhizoctonia butinii and drought. Notice the different sizes of trees affected, as well as those trees that have recently died (left and center photo). The photo on the right shows characteristic defoliation symptoms starting in the lower crown.
Douglas-fir Tip Dieback
Another tree health issue being reported and observed right now 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 droughty and dry soil 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