DUTCH ELM DISEASE
The Friends has worked to combat Dutch Elm disease in our three parks for four decades. It was one of the early threats that led to a sense of urgency in joining forces to protect these spaces. The Common and Mall were both heavily planted with elms, which were being decimated by the disease. By the 1970s there were blocks of the Mall with only three or four trees remaining on them..
ORIGIN OF THE DISEASE
Dutch elm disease (DED) is caused by a fungus introduced into the U.S. in the early 1930s. Not of Dutch origin, it received its name because of early work done on the disease by Dutch pathologists in the 1920s. Since its entry into the U.S, hundreds of thousands of elms have been killed across the country, over half of the northern U.S. population of elms. New England towns and cities have been profoundly affected by the loss of what was once an essential and defining feature of the landscape.
HOW DED IS TRANSMITTED
The disease is caused through transmission of the fungus by two species of bark beetles, as well as by root graft from nearby infected trees. The beetles carry the fungus from infected trees to healthy trees as they feed on twigs and upper branches. They lay their eggs in the bark and wood of trees. Emerging larvae form tunnels underneath the bark and the fungus grows through the tunnels until it reaches the tree’s water conducting cells, or xylem. Chemicals produced by the tree during its attempt to fight the disease plug up the xylem, causing the tree to wilt. Root graft transmission is particularly prevalent in the kind of conditions found in our urban parks.
The observable symptoms and the progression of the disease differ among trees which are infected through beetle feeding and those infected through root grafts. Trees infected by beetles first show wilting, curling and yellowing of leaves on one or more branches in the upper portion of the tree. Large trees may survive and show progressively more symptoms for one or more years. Trees infected through root grafts wilt and die rapidly; this frequently occurs in the spring soon after the trees have leafed out and progresses from the base of the tree upward. Brown streaks in the wood beneath the bark of affected branches is further evidence, but only laboratory isolation and identification can confirm positively that the tree has DED.
The American elm is extremely susceptible as are some European elms, but the Asiatic elms are highly resistant to the disease, and a number of disease-resistant hybrids have been developed over the years.
CONTROL AND TREATMENT
The Friends of the Public Garden in cooperation with Boston Parks Department are pursuing best management practices for the control of DED. It involves many things, including monitoring, research, sanitation, cultural, and chemical treatments.
All chemical treatments have limitations for the treatment of DED. The number one limitation is the pressure of the disease in any given year and the development of resistance to the chemical. It is always better to protect elm trees before an infection occurs. This is first done by preventing infection from infected elm bark beetles feeding on healthy trees. Insecticides injected into the trunks of healthy trees helps manage the disease indirectly by managing the population of beetles that elm trees are vulnerable to.
Secondly, healthy elms should also be protected from the fungus with injection of systemic fungicides before an infection begins. Elm trees are effectively protected with fungicides internally for up to two years. Fungicides can also be used for trees that have mild infection with success, but should be carefully monitored. Over time, a healthy growing elm tree can compartmentalize the disease with the help of promising systemic fungicides.
The Friends is constantly reviewing and researching fungal/insect resistance and the best treatments for control of DED. When chemicals are no longer effectively controlling insects and disease, they have developed resistance. Changing chemical treatments prevents this problem and preserves the future use of these good chemicals.
It is always important to remove dead, dying, and diseased elm trees as soon as possible. The Friends and the Parks Department are working closely together on this vital step in stopping DED.
Recognizing the symptoms of DED and reporting them to the Friends is important. Documenting the location and the date of the first sign of the symptoms is necessary for our arborists to review records and verify in the field the suspected DED sighting. It is important to remember that other diseases can mimic these symptoms. All sightings are important, but collecting samples and laboratory analysis is the only means of identification of DED.
Another good cultural practice for the preservation of elm trees is plant nutrition. The Friends monitors soil fertility and plant tissue for nutrients that are essential for proper plant growth and development. The fertility program also includes identifying and relieving soil compaction conditions.
ASIAN LONGHORNED BEETLE
The Asian Longhorned Beetle’s (ALB) discovery in Massachusetts was first reported on August 7, 2008 in Worcester by an alert citizen. Since then, nearly 30,000 trees have had to be cut down in the city to stem the outbreak. In July of 2010 six maple trees in Jamaica Plain were found to be infested with the beetle. The trees were immediately cut down and chipped, and no further infected trees have been found in the vicinity. The beetle is unique among invasive forest pests for attacking such a broad array of tree hosts, which is part of its danger. Therefore, they pose a new and potentially serious threat to some of our parks’ most beautiful trees.
Native to parts of Asia, the beetle is believed to have arrived in North America in the wooden packing material used in cargo shipments from China. Until a proven chemical or biological defense against the beetle in North America is found, affected trees must be cut down and the wood destroyed.
HOST TREES SUSCEPTIBLE TO THE ASIAN LONGHORNED BEETLE
- Horse Chestnut
- London Plane
The Friends has hosted a series of ALB surveys in each of our three parks. The best way to protect the tree population of the parks is to train citizens to regularly monitor, detect, and report any signs of the beetle.
WHAT BEETLES DO, AND WHAT TO LOOK FOR
The Asian Longhorned Beetle is up to 1.5” long with antennae that are 1 to 2 times its body length. They are shiny and black with white spots.
- Adult beetles chew their way out of the tree emerging during spring and summer months to mate. The exit holes are about ¾” in diameter often in the larger branches of the crowns of infested trees
- Adult females dig 1/2-inch diameter bowl-shaped pits in the bark to lay their eggs.
- The eggs hatch within 10 to 15 days and then the worm-like white immature larvae tunnel under tree bark through the cambium and bore into healthy hardwood trees.
- This tunneling damages and eventually kills the tree.
- Sometimes sap can be seen oozing from the exit holes with coarse sawdust on the ground or lower branches
TO REPORT POSSIBLE EVIDENCE OF ALB, CONTACT ONE OF THE FOLLOWING:
- Visit the Massachusetts Natural Resources Collaborations website
- Call the Massachusetts ALB Cooperative Program at 1-866-702-9938
- Call the Massachusetts Department of Agricultural Resources Pest Hotline at 617-626-1779