American elms represent some of the most culturally and economically significant urban trees. Their contributions to the urban landscape are numerous and include: carbon sequestration, capture of storm water and airborne particulate matter, reduced heating and cooling costs through wind buffering and shade and enhanced aesthetics with their large, sweeping canopies. Prior to the introduction of Dutch Elm Disease, American elms dominated the urban and suburban landscape because of their beauty, rapid growth rates and ability to tolerate difficult growing conditions.
Despite the devastating effects of the disease, millions of American elms still occupy the urban and forest landscape today. But, after decades of regular injection the costs associated with these treatments are adversely impacting tree heath and this issue must be addressed. The UMass Shade Tree Laboratory, now the Plant Diagnostic Laboratory, was founded in 1935 with the sole purpose of combating the DED epidemic. Now, 80 years later the fight against this destructive disease continues in ways that could never be predicted decades ago.
Without a regular fungicide injection, American elms (Ulmus americana) become infected by the Dutch elm disease (DED) pathogen, Ophiostoma novo-ulmi. This lethal disease, caused by a non-native fungal pathogen introduced from Asia, has decimated the American elm population. Fungicide injections into the lower trunk are performed by drilling small holes through the bark and into the water-conducting tissues (xylem), where chemicals are delivered and then transported by the tree to the upper canopy. While the treatments are highly effective, the act of injection creates a wound that can allow wood-decaying fungal pathogens to invade. Once established, wood-decaying fungi are capable of causing severe decay that may lead to structural failure or even death as vital tissues are killed. Additionally, the drilling of the injection site and phytotoxic effects of certain fungicides and insecticides are known to kill living tissue surrounding the injection site. This results in xylem dysfunction, when the water-conducting tissues are damaged and the tree cannot provide sufficient volumes of water to sustain foliage and branches in the canopy. While individual injection site wounds are small, multiple injections on the lower bole are required per treatment and trees must be treated every one to three years. Therefore, over the course of many years the number of wounds becomes substantial.
By using a non-destructive method, sonic and electrical impedance tomography, American elms regularly injected to control DED can be assessed for both the presence of internal decay and xylem dysfunction. Sonic tomography measures sound wave velocities, allowing for estimations of wood density, which decrease when decay is present. Electrical impedance tomography measures the electrical conductivity of wood, which can detect a buildup of moisture in wood that is normally dry (a precursor to internal decay) and distinguish non-functional areas of the outer sapwood (xylem dysfunction). Together, the sonic and electrical impedance data can be used to determine if wood-decaying fungi are present and if significant areas of the xylem have been irreparably injured from regular injection. Tomographic methods are currently the most accurate and minimally invasive techniques to understand internal tree health. They also provide the highest resolution data compared to all other decay detection methods available.
The primary goals and objectives of this study are to non-destructively detect and quantify: (i) the volume of internal decay; and (ii) the extent of xylem dysfunction in American elms regularly injected to control DED. Repeated injection site wounding can facilitate invasion by wood-decaying fungi and result in widespread disruption of surrounding xylem tissue. By accomplishing these two main objectives, we can better understand how repeated injection site wounding influences long-term tree health and formulate more comprehensive management plans to ensure American elms treated for DED remain healthy in the future. The findings could have transformative impacts on how American elms are treated for this destructive disease. An additional objective is to better inform and educate tree care professionals and the public of the threats posed by urban trees harboring high levels of internal decay. Current tree risk assessments often do not attempt to quantify internal decay using non-destructive methods. Because lower trunk decay cannot be readily assessed during a visual inspection, it goes ignored until some other form of disturbance (e.g. a strong windstorm) exposes the resulting structural weakness.