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Dutch Elm Disease

Dutch elm disease is a vascular wilt disease that causes rapid decline in as few as 6 weeks after infection.  American and European elms are commonly infected in two ways: via elm bark beetles that vector the disease from infected to healthy trees, or via fungi that are transmitted through root grafts. Dutch elm disease is an aggressive disease that is almost always fatal to the host tree once it becomes established.

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Symptoms

Leaves show signs of wilting (flagging), yellowing and browning. Brown or purplish discoloration is found in sapwood under the bark and dieback will occur shortly after symptoms are first discovered.


Photo A taken by Fred Baker, Utah State University, Bugwood.org
Photo B taken by Pest and Diseases Image Library, Bugwood.org
Photo C taken by R. Scott Cameron, Advanced Forest Protection, Inc, Bugwood.org

Treatment

Dutch Elm Disease must be treated proactively before the disease is present in the tree. The disease spreads so quickly that treatment on diseased trees may not be effective. Arborjet recommends a trunk injection of Propizol® Fungicide (14.3% propiconazole) with the TREE I.V., as a proactive treatment, or else at the earliest stages of infection. If treating an infected tree, the following should be performed in addition to the micro-infusion treatment:

  • Prune flagging branches immediately as the vascular wilt moves rapidly in the sapwood.
  • Strip the bark from the branch with flagging symptoms.
  • Inspect the sapwood for streaking (dark staining).
  • Cut the branch 6-10 feet beyond the discolored area to clear wood, ideally to a branch bark collar.
  • Properly dispose of the infected wood or debark it completely.
  • Make sure to disinfect all tools between each cut or drill hole using a bleach solution.
  • Trenching between infected and non-infected trees will slow the spread of the disease by root grafts.

When to Treat

For Dutch Elm Disease, we recommend that Propizol is applied in the spring prior to beetle flight. Best treatment response can be expected when less than 15% of the canopy is affected. In dry conditions, watering the tree will improve uptake of the injection treatment. Generally, the best seasons for injection are fall and spring, as uptake occurs when trees are transpiring. The environmental conditions that favor uptake are adequate soil moisture and relatively high humidity. Soil temperature should be above 40°F for trunk injection. Hot weather or dry soil conditions will result in a reduced rate of uptake, so trees should be watered if applications are made when soil is extremely dry. If treating trees in the summer, inject in the morning for the quickest uptake. Tree health will affect treatment efficacy, so assess tree health prior to treating.  For  example,  a  declining  tree  (>50%  canopy dieback) is a poor candidate for treatment.

What to Expect After Treatment

Tree recovery will be dependent upon the severity of the Dutch Elm Disease infection at the time of trunk injection, as well as upon the extent of eradicative pruning. Propizol is best used as a proactive application before  signs  of  infection  are  present.  Up  to  60%  of infected elm trees have recovered using the pruning and treatment techniques recommended above.

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