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The last 2 months we have looked at what is a major killer of Pines and Oaks in the mid south. I thought it would be fitting to address the Elm species as well. It is important to mention that major pruning cuts on Elms should NOT be done during the spring and summer months as the Elm Bark Beetle is attracted to fresh wounds at this time. Late fall and winter is the best time to do major pruning on Elms in our area to decrease possible disease transmission.
Dutch Elm Disease has been a devastating event in the history of tree diseases. It is caused by the fungi Ophiostoma novo-ulmi and O. ulmi and is vectored (carried) 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.
Figure 1: Characteristic streaking seen beneath
the bark. (provided by Dr. Wayne Sinclair, Cornell University)
Unfortunately, many of these trees become susceptible to other Elm diseases such as Elm Yellows.
Symptoms develop quickly within a 4-5 week period and usually when the leaves have reached full size.
The first visual symptoms are what is referred to as "flagging" within the crown of the tree. Flagging is a branch of a tree that develops symptoms of wilting and/or yellowing of the leaves on a otherwise apparently healthy tree. Prior to this occurring, symptoms have developed internally.
They include the death of xylem cells, the loss of water conducting ability and the browning of the infected sapwood in narrow streaks that follow the wood grain (Fig. 1). The fungus is present in the streaked wood, and isolations taken from this symptomatic tissue are needed to confirm infection by this pathogen. Occlusion of xylem vessels is due to the production of gums and tyloses. In the west where summers are dry, water shortage and heat stress often mask symptoms.
Spores of Ophiostoma novo-ulmi are stored in xylem vessels and reproduce through budding. Dispersal of spores is via the bark beetles that burrow under the bark and lay their eggs in wood galleries (Fig. 2). Ophiostoma novo-ulmi is distributed over long distances in elm logs and in firewood. Elm bark beetles distribute it locally and over distances for several miles. There are two species of beetle vectors known in North America, Hylurgopinus rufipes and Scolytus multistriatus. Insects are attracted to healthy elms by volatile chemicals produced by the trees. Beetles bore into the inner bark and while feeding deposit spores Ophiostoma novo-ulmi. S. multistratus feeds in the crotches of twigs, therefore, most infections occur in twigs. H. rufipes bores in the bark of branches and small trunks causing infections in major branches.
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 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.
4. Preventative fungicide injections: Some fungicides are capable of protecting elm trees from infection, but this method of management should be considered very carefully. Fungicide injection must be performed by someone trained in the technique and may be very expensive. Protection lasts only 1-3 years, and then must be repeated. In addition, some researchers are concerned that repeated wounding of the tree (by drilling holes) for the injection may open trees to decay.
5. Resistant cultivars: Developing or finding a truly resistant American Elm has been quite difficult. Originally the best method for creating a resistant American Elm was to cross it with closely related European or Asian species known to be resistant. Unfortunately, most of the resulting crosses do not carry the upright, spreading growth habit of the true American Elm and, therefore, are not well accepted as an alternative. Still, planting of resistant varieties derived from European or Asiatic elm species is advisable. In addition, after years of breeding and research, a number of clonal cultivars of American Elms with resistance have been made commercially available. These include 'Princeton', 'Independence', 'Valley Forge', 'New Harmony' and 'Jefferson'. Another group of resistant elms are collectively known as 'American Liberty Elm'. However, all of these elms are susceptible to Elm Yellows and should not be used where that disease occurs on native elm.
Please contact us if you are concerned about your elm trees. Email firstname.lastname@example.org or call us at (901) 309-6779.