Introduction Dutch Elm Disease has been a devastating event in the history of tree diseases. It is caused by the fungus Ophiostoma novo-ulmi and vectored by bark beetles. The disease is referred to as "Dutch" Elm Disease because it was first described in Holland in 1921. The pathogen originated in the orient. The disease began its destructive reign in the United States in 1930. Prior to the arrival of this pathogen, many streets and parks were graced by the upright and spreading branch structure of American Elms. The appearance of a mature American Elm can make nature lovers stop and stare in awe. Due to the incredible interest in these trees, plant breeders have been trying to develop resistant American Elms for years. Unfortunately, many of these trees become susceptible to other Elm diseases such as Elm Yellows. Symptoms
Disease Cycle
From the point of inoculation, the fungus moves upward and downward by two modes: in the liquid within xylem vessels and the growth of fungal hyphae between xylem vessels after germination. Ophiostoma novo-ulmi reaches the roots within one season of infection where it continues to grow. The fungus grows in the roots and ascends the trunk in a wave of infection that kills the entire tree or a major part of it. Where elms are planted close together and there is a possibility of root grafting, Ophiostoma novo-ulmi may move from one tree into the next through the roots. The fungus can also survive as a saprophyte in dead plant tissue. Management Strategies Management of DED requires a combination of strategies including sanitation, control of the insect vectors, removal of root grafts, preventative fungicides, and use of resistant varieties. 1. Sanitation: Quick removal of diseased trees and symptomatic branches is necessary in managing this disease. Removal of the damaged parts reduces breeding sites for the elm bark beetle and removes the fungus from the area. Wood from infected trees can be used as firewood but should be used before the elm bark beetles emerge in the spring or should have the bark removed from the pieces prior to stacking. Branches with "flagging" symptoms should be removed making a cut well behind (5-10ft) any visual symptoms. 2. Insecticides to kill bark beetle vectors: Another option for management is to attempt to control the insect vector population by applying an insecticide. Timing of the application may depend on the type of insecticide you have selected. Some insecticides may target the spring feeding sites while others target the overwintering sites and should be applied during late autumn. Use of this management option should be considered carefully. It is difficult to attempt control over this beetle population due to the timing factor, the complete coverage issue, expense and pesticide exposure. 3. Root grafts: The fungus is capable of
moving from tree to tree via root grafts. Trees planted within
25 to 50 feet of each other can easily have grafted root systems. Breaking
these root grafts is an important prevention measure but may be quite
difficult for a homeowner to accomplish. A professional landscaper
may have the necessary equipment, a vibratory plow, needed to perform
this task. Breaking the root grafts prior to removing any diseased
trees is recommended. Transmission of the pathogen may occur
if the diseased tree's roots are pulled away or broken from the healthy
tree's roots. Created, KLS, 1/01; Updated, SLJ, 1/05 This publication contains pesticide recommendations. Changes in pesticide regulations occur constantly, some materials mentioned may no longer be available, and some uses may no longer be legal. All pesticides distributed, sold, and/or applied in New York State must be registered with the New York State Department of Environmental Conservation (DEC). Questions concerning the legality and/or registration status for pesticide use in New York State should be directed to the appropriate Cornell Cooperative Extension Specialist or your regional DEC office. READ THE LABEL BEFORE APPLYING ANY PESTICIDE. __________________________________________________________________________________ The Plant Disease Diagnostic Clinic at Cornell University is located at 334 Plant Science Building, Ithaca, NY, 14853. Phone: 607-255-7850, Fax: 607-255-4471, Email: kls13@cornell.edu or slj2@cornell.edu |
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