MSU Extension in Roosevelt County

This fungus is transmitted by two species of bark beetles or by root grafting.  The American elm, Ulmus americana, is the most seriously affected of all elms.

SMALLER EUROPEAN ELM BARK BEETLES pass the winter as larvae in the bark. When warm weather comes in the spring, larvae complete their growth and transform to pupae and later to adult beetles. The adults begin emerging about the middle of May through holes that they make in the bark.  They continue to emerge during the warm months. The adults feed in the crotches of living elm twigs and, if carrying the Dutch elm disease fungus, may introduce it into healthy elm trees. Later they bore through the bark of dead or dying elm trees or recently cut elm logs. Cold winters kill off most of the smaller European elm bark beetles. Those that survive do so only in protected spots, especially home firewood piles. It is for this reason that elimination of elm firewood is such an important factor in DED control.

Spores of the Dutch elm disease fungus are carried on the bodies of these beetles and deposited in egg galleries and tree wounds. European elm bark beetles transmit the fungus by feeding on small twigs. Native elm bark beetles introduce the Dutch elm disease fungus when making feeding tunnels in the bark. Most emerging beetles feed on healthy elms within 1,000-1,500 feet of where they hatched. However, beetles may rise to altitudes of several hundred feet and be carried by air currents for many miles. Control Measures Effective disease control programs should be considered on a community-wide basis. Dutch elm disease control involves two different but related programs: (1) community-wide sanitation programs designed to reduce the level of elm bark beetles (principal carriers of the Dutch elm disease fungus); and (2) prevention of the spread of the disease through natural root grafts from infected trees to adjacent healthy trees. There is no way to eliminate Dutch elm disease once it begins; control programs have as their object the management of the disease so that losses are spread out over a long period, minimizing the impact of the disease.

 Insecticides:
 As mentioned before, the adult native elm bark beetles bore into the thick bark of large trees in the fall to overwinter. Because most of the elm bark beetles which survive the winter in our climate do so only near the base (lower 4 feet) of large trees, it has been possible to treat such trees with insecticide to reduce beetle populations. Such treatments are feasible both for communities and for individual homeowners, although individual action is of limited value. The insecticide Dursban® (2E and 4E) is registered for control of overwintering native elm bark beetles. Dursban® 4E should be used at the rate of 0.5% active ingredient, which is 1 1/3 fl. oz. (2 2/3 tablespoons) per gallon of water. Dursban® 2E is to be used at the rate of 2 2/3 fl. oz. (5 1/3 tablespoons) per gallon of water. The bottom 9 feet of the trunk should be sprayed to wet the trunk thoroughly but do not spray to runoff. Special care should be taken to clear away grass and weeds from the base of the trunk to get good coverage of the root flare. Sprays can be applied from spring to early fall and are most effective if applied by the end of August. An early spring (late March - early April) treatment may offer some additional protection. Dursban® is a restricted use pesticide and is to be applied by certified pesticide applicators only.

 

Sanitation: 
A year-round community sanitation program is the key to slowing the spread of the disease. Since elm bark beetles breed in dead elm wood with intact bark and in weak or dying elm trees, the first steps toward control of the beetle involve the destruction of all dead or dying elm wood present in the community. This includes Siberian elm as well as American elm. Any dead, dying, or weak elm trees or elm wood with bark firmly attached can serve as a breeding site for beetles. This includes limbs hanging on trees that may have been damaged by storms (hail, wind, etc.) the previous season, trees that are very old or weakened by pests, and fresh elm firewood. Branches less than 4 inches in diameter are generally not a threat because the beetles do not survive in them even if colonized. Firewood is an exception, however, because firewood piles are often in protected locations and partially covered with snow. The chance of root graft spread of Dutch elm disease to adjacent trees increases in direct proportion to the length of time an infected tree stands before removal. Immediate removal (within two weeks) of newly infected trees will substantially reduce the number of trees infected by root grafts. Immediate removal should be a routine procedure where it can be accomplished. Sanitation alone will not stop the spread of the disease, but it will tend to stabilize its spread and prevent epidemic outbreaks. The true value of a good sanitation program is that it allows time for a replacement program so that a community doesn't lose all of its trees at once. Replanting new trees of other species can then proceed on a gradual basis. The value of a good sanitation program is often underestimated because some people believe that, "The elms will die anyway." Although this may be true, the rate of dying can be dramatically affected.

 

Firewood Control:    
In cities with municipal forestry programs where standing dead trees and fallen logs are routinely removed, elm firewood with intact bark may play a major role in overwintering survival of elm bark beetles. Because the interior of a firewood pile offers a protected environment, beetle survival may be higher than in standing trees or fallen logs. It appears likely that elm firewood piles are the only important survival site for the smaller European elm bark beetle in North Dakota cities. Effective reduction of elm firewood cannot be achieved by ordinance alone - it requires the cooperation of an informed public. A few elm logs secreted away by one homeowner who does not understand the importance of the problem can undo all attempts at thorough sanitation and watchful disease surveillance for an area of several city blocks. Some communities have obtained de-barking equipment which permits elm firewood to be utilized. Failure to effectively control presence of bark-on elm firewood will doom any municipal control program, regardless of how well its other aspects are carried out.

 

Root Graft Transmission:
Spread through natural root grafts has accounted for a majority of new cases of Dutch elm disease each year in some Midwestern cities. Elm trees which are growing close together (within 50 feet) over a period of years form root grafts. If one of the trees becomes diseased, the DED fungus will be transmitted along an entire street by moving directly through the root system into adjoining healthy trees. In other words, disease in just one tree in the row could cause infection and death of the remaining trees. Immediate removal of infected trees (see sanitation) reduces the chance of root graft spread by getting rid of the infected tree before the Dutch elm disease fungus reaches the roots. The only way to prevent transmission through the roots is to create a barrier between diseased and healthy trees by severing or killing those roots between the trees. This can be done without harm to the healthy trees either by mechanical trenching or through the use of chemical barriers, which have been found to be quite effective in some situations. Mechanical trenching for disruption of root grafts has the advantage of being quick and effective if the machinery is available and no pipes, underground cables or pavements are encountered. To be entirely certain of disruption, a trench depth of 48 inches is needed, but most roots are much nearer the soil surface and trenching to a depth of 24-30 inches is often adequate. The deeper trenching may require specialized equipment. A 30-inch trench cut immediately is probably better than a 48-inch trench delayed for days or weeks while waiting for the special machine.

A fumigant, sodium N-methydithiocarbamate (SMDC) sold under the trade names of Vapam and VPM, has been found effective a s a chemical barrier. The chemical fumigant is dangerous and should be applied only by trained applicators. Because slogans such as "Save the Elms" have wide popular appeal, chemical tree treatment is sometimes offered by untrained or unscrupulous individuals in competition with reputable trained arborists. Check out the individual offering treatment with your local Chamber of Commerce, Better Business Bureau or local forester before agreeing to any treatments; obtain a written description of all work to be done and get any guarantees in writing. Do not use systemic fungicides with highly alkaline water. If the level of calcium is very high, or if the hardness is over 10 grains per gallon, the fungicide may form a precipitate rendering it ineffective and possibly damaging the tree.

Therapy of Infected Elms Many municipal tree ordinances require removal of Dutch elm disease-infected elms regardless of therapeutic treatment; check your local tree ordinance before arranging therapeutic treatment. Therapeutic tree injection is generally only effective where less than 5 percent of the crown of the tree shows symptoms. The symptomatic part of the tree should be pruned out promptly as it will not recover. Ideally, the infected branch should be removed 10 feet below where streaking of the wood ceases. Therapeutic treatment of trees infected through root grafts has never been successful and cannot be recommended under any circumstances. Community-wide chemical therapy should never be attempted. Such a program cannot be justified either economically or biologically. Protective Treatment of Healthy Elms Preventive injection with currently registered fungicides will require retreatment every one to three years to maintain the protective effect. Trunk or root collar injections injure the tree and the cumulative effect of repeated injections may damage the tree directly or lead to severe wetwood or other infection.

For more information with pictures go to http://www.ext.nodak.edu/extpubs/plantsci/trees/pp324w.htm. Or call the Roosevelt County Extension Office at 787-5312 to receive a copy.