Disease Diagnosis
Dutch elm disease (DED) is one of the most destructive diseases affecting shade trees in the United States today. This disease is caused by the fungus Ophiostoma ulmi, and vectored via either European or native elm bark beetles. It has also been found to spread from plant to plant through root grafts among adjacent trees. Affecting primarily American elms, Ulmus americana, Dutch elm disease results in chlorosis and wilting of leaves, followed by leaf death and defoliation killing individual branches and then eventually the entire tree in just a couple of years. The xylem, water-conducting vessels, of afflicted trees discolors becoming brown which can be seen as a brown ring when bisecting a branch or as dark streaking under the bark.
Since the introduction of Dutch elm disease it has swept through urban and suburban areas, causing the loss of a great number of street and shade trees. It has also altered the role of the American elm in the bottomland ecosystems, although their chances for survival appears higher as a member of natural stands. Many specimens have been surviving to maturity, and are able to produce viable seeds prior to succumbing. Fluctuations in disease incidence may be correlated to rises and falls of the vector population.
Infection Identification
The symptoms of DED are the result of the fungus infecting the xylem of the tree, causing vascular clogging which impairs the ability of the tree to move water from the roots to the crown.
Visual symptoms begin as wilting and yellowing of leaves, eventually leading to browning and branch dieback. The pattern of progression is dependent on the point of infection, if the disease entered through a root graft symptoms may first appear on the lower crown at the side closest to the point of entry affecting the entire crown rapidly. If the infection was transmitted first to the upper crown, symptoms will first appear at the end of the affected branch and spread downward, causing a “flagging” effect.
Vascular symptoms can be seen in cross sections of affected branches and stems, typically dark streaks of discoloration can be seen. In newly infected branches or twigs the sapwood is first affected, as seen in the 1st twig on the right. The ring visible in the middle twig is due to uninfected summer wood overlaying the diseased sapwood. And the 3rd twig is wholly unaffected as of yet.
Spread of DED over land is attributed to and closely linked with the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). Both species of beetles are strongly attracted to weak, stressed or dying elm wood in which to complete their life cycles. The beetles burrow into the wood and lay eggs, when the eggs hatch the larvae feed on the inner bark and sapwood. If the fungus is present in the tree the larvae will eat it or the sticky spores will attach to adult beetles and get transported on them.
DED is spread through root grafts when the roots of trees growing near each other overlap or intertwine and become fused together. Fungus from infected trees can move through the grafted roots. The disease spreads extremely rapidly in this way, as the fungus is transported up to the crown through the tree’s natural sapstream mechanism. Root graft spread is the major cause of DED spread and elm tree death in urban areas where trees are closely planted.
Affliction Administration
Dutch elm disease can be managed by interruption of the disease cycle; the most effective means of breaking the cycle is early and thorough sanitation to limit the population of the insects that transmit the fungus from tree to tree. Systematic pruning, managing beetle populations, disrupting root grafts and fungicides, or a combination thereof, are all viable methods to manage and stem the spread of Dutch elm disease. Learn to identify the signs of stress associated with the disease and look out for them within your community. Contact your town’s tree warden should you suspect an incidence of DED on any town owned trees and consult with your arborist if you are concerned about trees on your property.
