Dutch elm disease is a vascular wilt disease. The earliest external symptoms of infection are often yellowing and wilting (flagging) of leaves on individual branches. These leaves often turn brown and curl up as the branches die, and eventually the leaves may drop off. Although initially only a part of the tree crown may be affected, symptoms may progress rapidly throughout the crown. Highly susceptible trees often die in a single year, but others may linger for several years. Symptoms progress quickly and death may occur rapidly in trees infected in early spring, while trees infected later in the summer may survive longer.
If the bark of infected elm twigs or branches is peeled back, brown discoloration is seen in the outer layer of wood. This discoloration in the xylem actually occurs before the foliar symptoms described above are seen; foliar symptoms result when sap flow ceases in the infected wood. Xylem browning is often discontinuous. In cross section, it appears as a circle of brown dots or a ring. Other wilt diseases of elm, such as Verticillium wilt, also cause sapwood discoloration, so positive diagnosis of Dutch elm disease depends on laboratory culturing and identification of the fungus.
Infections that take place in the spring or early summer involve “springwood” which has very long xylem vessels. In these vessels the fungi can spread rapidly throughout the tree, which then may die quickly. Later in the season, the fungi are restricted to the much shorter vessels of the “summerwood,” and the fungi spread much more slowly in the tree. Localized infections often result, and the tree is likely to survive longer.
In the absence of effective disease management, Dutch elm disease increases exponentially until an affected elm population is greatly depleted. Seedlings and many saplings escape and live long enough to reproduce, so even the most susceptible elm species have never been threatened with extinction by Dutch elm disease. Wild elm populations in the eastern and Midwestern U.S. have increased in recent decades, and this increase has led to renewed prominence of Dutch elm disease in landscapes.
If you think you have trees which are suffering from Dutch Elm Disease or any other tree disease, give Cutting Edge Tree Care Specialists a call. We are a licensed arborist with over 20 year’s experience!