Tree and Shrub Disease

Dutch Elm Disease is spread through the native elm bark beetle and the smaller European elm beetle. As these beetles feed on the elm tree, they introduce a fungus which spreads through the plant causing branch dieback (it is also important to note that roots may also spread this disease from infected trees to healthy trees via root grafts). The symptoms differ between the two beetles:

Native Elm Beetle
The native elm beetle prefers to feed on branches 2" - 4 " in diameter. This is important to know because the infected branch will be many feet below the top of the tree and that is where the first visual symptoms will occur. Entire branches will begin to show wilting of leaves along with browning of the branch. As this disease travels through the tree, other branches will start to take on symptoms. The thicker branches will also show streaking on the wood. The Native elm beetle will overwinter either as larvae or adults in the bark (near the base of the tree) of healthy elm trees. When they emerge around May, they will often feed on the healthy tree that housed them over the winter. As summer approaches, the native elm beetle, much like the smaller European elm beetle, will search for weakened or dead elm trees to breed in. They bore galleries which are horizontal to the grain of the wood, the females lay eggs within the galleries and the cycle continues. Usually there is one generation per year, though if the weather is mild two generations may exist.

Smaller European Elm Beetle
The smaller European elm beetle feeds on smaller branches or twigs. The first visual signs of Dutch Elm Disease by the smaller European elm beetle will start near the top of the tree where branches are considerably smaller. The branches will have yellowing and wiling leaves with symptoms progressing downward in the tree. The streaking on the wood will also be visible, though smaller twigs will be effected as well. The smaller European elm beetle overwinters in the bark of a weakened or dead tree as larvae after hatching from eggs. After pupation, the adult beetle will emerge (first appearing around May) to feed on healthy elms, they will then begin to bore galleries through dead or dying elm trees parallel with the grain of the wood for breeding. The females will lay eggs within the niches of these galleries and the cycle continues again. Multiple generations can occur per year.

The beetles themselves to little damage to the trees, though the fungus they may carry and spread is deadly to elms. In an attempt to stop the disease, the tree forms a gum on its own water conducting vessels which causes the tree to wilt and die. Dutch elm disease spreads rapidly and should be treated promptly.

Disease Development Favored by: Pruning cuts and wounds on the tree (attract the beetle) Weak trees Corrective Measures: Remove weakened or dead branches Destroy dead limbs and stumps of elms Use elm firewood immediately or strip bark off and tarp the logs Fungicide injections can help to stop the disease if less then 5% of the tree is infected Insecticide injections in late summer will target adult beetles Pruning of susceptible elms should be done in late fall and wounds dressed Use resistant elm trees when planting new elms for landscape Sever root grafts between healthy and infected elm trees

Dutch Elm Disease dying branch

Dutch Elm Disease, severe infection