Pruning is an extremely effective way to treat a tree infected with DED.
Timely removal of diseased wood checks the spread of the disease. It is however less effective for well-established infections or for root-graft infections.
When you prune is extremely important. Pruning opens fresh wounds in the tree that take time to heal. The elm bark beetles are attracted to these wounds.
Pruning should not take place while the beetles are active (from early April to late July). Some municipalities have by-laws defining when you are legally permitted to prune elm trees.
On an infected tree, locate the wilted branch and remove the bark until sapwood can be found with no discoloration.
Cut the branch approximately 10 feet below the discolored area. The farther below the discolored area the limb is cut, the greater the chance of freeing the elm of the disease.
Properly dispose of dead wood.
Disinfect tools using a 10% household bleach solution.
The loss of a major limb is regrettable but it can prevent the loss of the tree. One study reported that 60% of the trees that had been pruned at the early signs of the disease survived.
If the symptoms are allowed to continue, however, the success rate plummets. Studies have shown that if less than 5% of a tree is infected, pruning successful stops the disease 65% of the time. If 20% of
the tree is infected, the success rate drops to 0%. 1
Treating elm trees with systemic fungicides is a popular way to combat the effects of Dutch elm disease. Chemicals are injected into the tree either through holes drilled in the base of the tree or in the root-flares. Some
chemicals that are currently used include Arbotect 20-s, Alamo and Lignasan.
Chemical fungicides can be effective for trees in the early stages of the disease. If, however, a tree has advanced DED or has contracted the disease through a root graft, fungicides have little or no effect.
Fungicides are not a substitute for pruning and other measures.
Due to the difficulty of the procedure, fungicide injections are best left to professional arborists. A preventative injection into a healthy tree can adversely affect its health making it more susceptible to a DED
1 (Himlick and Ceplecha, 1976)