Integrated Pest Management Manual
Dutch Elm Disease
IDENTIFICATION AND BIOLOGY OF DUTCH ELM DISEASE
Is it Dutch Elm Disease?
An IPM program for elms is much more than managing a disease (Dutch elm disease) and its insect vectors (european elm bark beetle and native elm bark beetle). Not only are there several alternatives available for the management of these pests, there are also several other pest problems of elms which can cause symptoms similar to Dutch elm disease; it is essential that the tree manager be familiar with these as well. Insect and disease problems which cause symptoms similar to Dutch elm disease are summarized in Table 1.
Table 1. Diagnosis of elm disorders.
|Symptom||Time of appearance||Possible causes||Diagnosis (see references for more information)|
|Failure of new growth to develop in spring.||March-April||DED, scale insects, limb injury||DED--examine wood for streaking, culture|
|New growth develops poorly in the spring (slow leaf expansion and shoot elongation).||March-April-May||DED, scale insects, nutrient deficiency, elm yellows.||DED, scale--see above. Have soil and tissue tests performed for
nutrient levels. Determine fertilization history of site. Phloem
necrosis--sudden necrosis and death of undeveloped leaves. Streaking
in phloem tissue accompanied by wintergreen odor.
|Sudden yellowing and necrosis of leaves on a branch or branches.||Anytime during growing season||DED, elm yellows.||See above|
|Uneven browning of leaf margins on a section of the tree||July-end of growing season||Biotic leaf scorch, branch or root system injury, chemical phytotoxicity.||Biotic--positive culture for bacterial leaf scorch. Trace injury
or phytotoxicity to chemical application.
|Uniform browning of leaf margins||May-end of growing season.||Abiotic leaf scorch, root system injury, chemical phytotoxicity.||Abiotic--negative culture for bacterial leaf scorch. Hot, dry,
weather prior to development of symptoms. Trace injury or phytotoxicity
to chemical application.
|Skeletonization or small holes in leaves||May, July||Elm leaf beetle||Small holes in leaves from feeding by adults, skeletonization
from larvae. Both are 1/4" to 3/8" in length and yellow
with black stripes.
|Brown spots along leaf veins.||late April-late May||Anthracnose||Results after a cool, wet spring.
Dutch Elm Disease and Its Insect Vectors
Dutch elm disease is caused by the fungus Ophiostoma ulmi (Buism.) Nannf. (=Ceratocystis ulmi (Buism.) C. Moreau). (For a complete description of the life cycle of the pathogen, see Sinclair et al. 1987.) O. ulmi is an introduced pathogen that arrived in North America in elm logs from Europe. The pathogen overwinters in the bark of infected trees or logs cut from infected trees. It is carried from infected to uninfected trees by two insect vectors, the European elm bark beetle (Scolytus multistriatus) and the native elm bark beetle (Hylurgopinus rufipes). Althogh the European elm bark beetle is the major vector of the disease, temperatures below -6
F kill the larvae; thus the native elm bark beetle is the primary vector
in parts of the northern United States, New England, and all of Canada.
Both species of beetles bore into the bark of infected trees and excavate
egg galleries. The galleries of S. multistriatus run parallel
to the grain of the wood, while those of H. rufipes are at a
45 degree angle to the grain. Larvae hatch from the eggs in approximately
one week; the white, legless grubs tunnel perpendicularly to the maternal
gallery, feeding on elm phloem cells for 4 to 5 weeks. A 1 to 2 week
long pupal stage follows. The 1/8" adult beetles tunnel to the
bark surface to emerge and fly to new trees. As they move to the surface,
fungal spores that have germinated and spread throughout the feeding
galleries attach to the beetles' bodies. These spores are then carried
to new trees as the beetles move there to feed. The beetles can fly
for several miles, allowing the disease to spread over a wide area.
The pathogen also moves between trees via root grafts. The fungal spores
move passively within the tree in xylem vessels (both up and down the
tree from the point of infection). The fungus also moves actively between
xylem vessels as fungal hyphae. The pathogen kills the tree by blocking
solute movement in the xylem as well and by producing a toxin. Acute
and chronic forms of the disease are recognized. The acute form is thought
to cause wilt and branch death while the chronic form is thought to
lead to more gradual chlorosis and leaf drop.
After emergence from the brood tree, adult beetles fly to other elms to feed and to breed. Bark beetles can also feed on logs cut for lumber and fuel. Such colonized wood may become a reservoir for beetles and hence Dutch elm disease even if the wood is of a resistant elm species. This is why debarking of elm wood that is being stored for use as fuel is stressed as part of the management strategy for control of Dutch elm disease.
S. multistriatus commonly produces two generations per year; the first overwinters as larvae, pupates in early spring, and emerges as elms reach full leaf. The second generation flies in late summer, producing the broods that overwinter. H. rufipes usually overwinters as adults in the bark at the base of healthy elms, and may infect the tree with Dutch elm disease during fall feeding in lower boles or spring feeding in branches. H. rufipes may produce one to one and one-half generations per year, overwintering as either larvae or adults.
Pheromone traps for elm bark beetles are available and can be used to monitor beetle populations. Information on flight activity is useful if insecticides are to be used to control adult beetles.
The Dutch elm disease pathogen, O. ulmi, grows and sporulates in elm tissues throughout the growing season. The sporulation of the fungus is temperature-related. Asexual spores are most commonly produced during the warm months. When sexual reproduction takes place (which is rare in nature) the production of fruiting bodies increases, occurring most commonly between November and February.
Diagnosis of Dutch Elm Disease by Testing for the Pathogen
Pathologists now recognize that there are several strains of Dutch elm disease, and that some are more aggressive than others. These strains kill elms more rapidly (Richards and Takai 1984). The production of toxins by the pathogen was first recognized in 1947 (Diamond 1947), and aggressive and non-aggressive strains were first recognized in 1984 (Sinclair et al. 1987). More recently, an antibody specific for some of the Dutch elm disease toxins was produced (Benhamou et al. 1985). This will enable the development of a fast, accurate test in which fluids from the suspect tree are matched against Dutch elm disease toxins. The nature of the reaction would confirm or deny the presence of the disease. Current tests for the pathogen involve culture of the disease organism from diseased wood, a procedure which can take several days. This test is usually recommended only in areas where the disease has not been previously reported.
NON-CHEMICAL CONTROL OF DUTCH ELM DISEASE
Prior to the introduction of Dutch elm disease into the United States in 1930, the American Elm (U. americana L.) was one of the most popular street trees throughout the Northeast, Middle Atlantic, and Midwest; about 77 million elms were growing in the United States. By 1976, about 43 million of those trees had been lost to Dutch elm disease (USFS 1977). The impressive shape, size, fall color, and shade quality of the American elm led to the institution of a near monoculture of this species in many urban areas, which has served to enhance the spread of Dutch elm disease. Maintenance costs for Dutch elm disease management vary considerably depending on the community and the nature of its elm plantings; they were estimated to range between $26,000 and $152,000 (in 1982 dollars) annually in several communities in a four-year demonstration project (Hanisch et al. 1983).
A management program for Dutch elm disease encompasses many different strategies. Factors to consider in deciding which are most appropriate for a situation include the time of year, the resources at your disposal, and the number and location of trees affected.
Resources available for Dutch elm disease management are often limited, so the tree manager must often balance the value of the trees in the landscape and the degree of infection in making a decision about managing Dutch elm disease. From an aesthetic point of view, trees in natural areas are generally considered least important when funding for maintenance is limited. The drawback to this approach is that these trees may be close enough to valuable plantings to form root grafts or to serve as a breeding ground for beetles which can then fly to those plantings and infect them with the disease. In a study on the use of pheromone traps for mass trapping, beetles were caught as far as five miles from the nearest elm tree (Birch et al. 1981).
The total amount of tree biomass affected must also be considered. Several authorities on Dutch elm disease feel that it is most realistic to expect to control a small infection on a large tree (Lanier 1988). Source of infection is also important; as presented in Table 2, trees with root graft infections were not successfully treated by any available method; trees with current season's infections were treated with a higher success rate than those with residual infections. It was also suggested that fungicide injections directly into branches with localized infections rather than into the bole of the tree (the current practice) would greatly increase the success rate of therapeutic injections (Lanier 1988).Use of Resistant Varieties
The severity of Dutch elm disease and the desirable aesthetic qualities of the elm have led to the development of several elm varieties with resistance to Dutch elm disease as well as re- planting with tree species that have some of the ornamental characteristics of the elm. American, English, red, and winged elm are among the most susceptible, while Chinese, Japanese, and Siberian elms are among the most resistant. Resistance is not the same as immunity, however. The presence of Dutch elm disease was recently confirmed in several Chinese elms planted adjacent to the National Mall in Washington, D.C. Several resistant cultivars have been developed from crosses of European and Asian elms. While many of these trees are promising, none seems to be as attractive as the American elm. They also have not been planted long enough for all their possible insect, disease, and cultural problems to have been recognized. For example, the Siberian elm is highly susceptible to the elm leaf beetle, as is the Japanese zelkova. (See Sinclair et al. 1987 for more information.) Other disorders, such as bacterial leaf scorch and elm yellows must also be considered. The mechanism of resistance seems to be related to the ability of the tree to quickly heal the wounded area and thus prevent the movement of the pathogen to other parts of the tree.
This is the most important element of a Dutch elm disease management program for existing elms because it removes the elm bark beetle's breeding habitat from the system. No Dutch elm disease management program will be successful without good sanitation. It consists of the immediate removal of any dead or wounded branches, and the debarking of branches stored for use as firewood. Flagging branches on which streaking has been observed are also removed. Ideally, branches should be cut back 10' from the last point where streaking is evident. This is determined by making small cuts in the bark to look for streaking. The final pruning cut for removal of the branch should be made approximately 10' behind the point at which healthy wood is first observed (Lanier 1988).
Sanitation should be viewed as a community-wide management tactic. Considering the distance that elm bark beetles can travel, removal of branches from a single tree will have little impact in the infection status of that tree if there are other infected trees in the area. Sanitation, while a key component of a Dutch elm disease management program, is most effective when combined with the judicious use of fungicides, as outlined by Lanier (Lanier 1988). This is discussed in more detail in the section on chemical control.
Wounding trees by pruning will attract the bark beetle vectors of Dutch elm disease (Byers et al., 1980). Ideally, routine pruning should be done in the dormant season. If this is not possible, pruning of healthy elms should be restricted to periods of beetle inactivity. This can be determined by the use of pheromone traps to monitor beetle flight periods.
Mass-trapping of Elm Bark Beetles
Mass-trapping of beetles using pheromone traps has also been investigated as a way to control the European elm bark beetle and thus reduce the spread of Dutch elm disease. A study in California estimated that only 1%-20% of marked beetles released in three study areas were recaptured, indicating that traps alone are not sufficient to reduce beetle populations (Birch et al., 1981).
CHEMICAL CONTROL OF DUTCH ELM DISEASE
Lanier (1988) suggests that pruning combined with fungicide gives better disease management than pruning or fungicides alone when dealing with a residual infection. Fungicides were most effective when injected directly into large limbs where an infection had been found, as well as into the bole. For current year infections, pruning alone was as effective as pruning with the use of a fungicide. These results are summarized in Table 2.
Table 2. Effectiveness of Dutch elm disease management strategies based on history of the infection (based on Lanier 1988).
|INFECTION HISTORY||MANAGEMENT STRATEGY||EFFECTIVENESS|
Infection history can be determined by noting when and where on the
tree the symptoms develop. Symptoms that appear during the first eight
weeks after leaf development are most likely the result of a residual
infection or a root graft, while those that appear later than this result
from a current season's infection. Symptoms appearing on several branches
at the same time suggest that either a root graft or multiple beetle
infections. If there is a multiple-branch infection on a limited section
of the tree, suspect disease transmission via a root graft.
As shown in Table 2, the effectiveness of fungicide injections varies considerably depending on the nature of the Dutch elm disease infection source. Recommended timing of sprays to control elm bark beetle varies as well; both winter (Hanisch et al., 1983) and early spring (Davidson 1991) treatments are recommended. Only the latter corresponds to a point in the beetle's life cycle when effective control can be obtained. See Birch et al.(1981); Johnson and Lyon (1988); Pajares and Lanier (1989) for more information on choice and timing of insecticides for beetle control.
Trap-tree Strategy for Dutch Elm Disease Management
The use of trap trees is an alternative type of sanitation for Dutch elm disease management. In this approach, most recently reviewed by Lanier (1989), trees infected with Dutch elm disease that cannot immediately be removed are injected with the herbicide cacodylic acid which causes the tree to die quickly. The goals of this approach are to remove infected trees from the system when the labor or money to take them down is not available and to attract beetles away from healthier trees to the dying tree. Beetles oviposit in the dying tree but the larvae do not survive because the tree is dead when the eggs hatch. Cacodylic acid has been used successfully in parks where site factors made tree removal impossible. This strategy is affected by the following conditions (Lanier 1989; Lanier and Jones 1985; O'Callaghan et al. 1980).
1) The amount of beetle mortality is affected by the degree of dieback in the crown at the time of cacodylic acid application. There will be much greater mortality if less than 50% of the crown is dead than if more than 90% is dead.
2) If possible, time cacodylic acid applications to periods just before adult flight begins.
3) Herbicide will move through root grafts, so trees to be treated must also be trenched.
4) For maximum beetle mortality, bait the tree with a pheromone trap and treat the lower bole with an insecticide. Lanier and Jones (1985) found that addition of an insecticide to this system increased beetle mortality. For every beetle which landed on and bored into a tree treated with cacodylic acid, 40 landed and did not bore into the tree. Thus the use of an insecticide greatly enhanced beetle mortality.
MANAGEMENT OF OTHER ELM PESTS
IDENTIFICATION AND BIOLOGY
Elm yellows (also known as elm phloem necrosis) is caused by an organism called a "mycoplasma-like organism" (MLO) which biologists classify between a virus and a bacterium. It is carried between elm trees by leafhoppers and possibly spittlebugs (Sinclair and Johnson 1987). Roots are infected first, where root tips and root hairs are killed. Foliar symptoms appear in mid-summer of the second season of infection. Leaves droop and curl, turn yellow and then brown. The disease is reported to be fatal in elm species native to North America but not in elms of European or Asiatic descent. Death is sudden, occurring within a few weeks after the onset of foliar symptoms.
MLOs cannot be cultured in the laboratory, so diagnosis is performed in the field on living but infected trees. When the bark is peeled back it is be yellow or tan, sometimes speckled with brown (the normal bark color is creamy white). Infected bark also emits a strong wintergreen odor. If this is not noticed at first, warming a bark sample in a pocket will enhance the odor (Holmes 1987).
CONTROL OF ELM YELLOWS
The management strategy for this disease is simple. Dead trees must be removed immediately, and control measures must be applied for leafhoppers. Timing of leafhopper control coincides with the two flushes of elm shoot growth (Holmes 1987). While the disease has been reported throughout the eastern United States and southern Ontario (Sinclair et al., 1987), its occurrence is sporadic; thus control of the vectors is recommended only when the disease is known to be in the area.
IDENTIFICATION AND BIOLOGY
The European elm scale (Gossyparia spuria) as well as several species of leucanium scales produce symptoms similar to Dutch elm disease on elm. For more information on the biology and identification of scale insects that feed on elm, consult Johnson and Lyon (1988). The important point is that dieback and poor growth, which are symptoms of Dutch elm disease could also be caused by scale organisms, so this must be ruled out (or the presence of Dutch elm disease confirmed) before a tree is treated for Dutch elm disease.
CONTROL OF SCALE INSECTS
Management of scale insects is straight-forward. Once the presence of scale has been determined, remove several scale covers to confirm that the scales are viable. If this is so, apply horticultural oil either during the dormant season or during the growing season when scale crawlers are active. The timing of the latter spray will depend on the specific scale insect and geographic location of the trees. A dormant oil application in late March would be the most effective treatment since there is no leaf surface to intercept the spray. An application in March will have some impact on the elm bark beetles that will have started to feed.
ELM LEAF BEETLE
IDENTIFICATION AND BIOLOGY
The elm leaf beetle (Pyrrhalta luteola) is a common defoliator of all elm species as well as the Japanese zelkova, a frequent elm replacement. It overwinters as an adult in buildings or protected outdoor sites. It lays eggs on the expanding leaves in the spring. Females feed on elm leaves before ovipositing. The larvae feed by skeletonizing the leaves, which causes the leaves to appear yellow or brown from a distance; thus their injury might be mistaken for Dutch elm disease at first. The beetle larvae move to the base of the tree to pupate; adults emerge one to two weeks later. The pupae are bright yellow and may be seen in masses on the soil at the base of a tree. There are one to three or more generations per year depending on the latitude.
Leaf beetle larval activity can be monitored by the use of tree bands. Place a band coated with sticky material around the tree; as the beetles move up or down they will be caught. A large increase in larval catch indicates they are moving to the tree base to pupate. The larvae are about 3/8" long, and are yellow with black stripes running the length of their bodies.
CONTROL OF ELM LEAF BEETLE
The beetle can be controlled with a strain of Bacillus thuringiensis and several conventional pesticides, some of which may be applied to the bark of the tree to kill migrating larvae. In the case of a small infestation on a few trees, the sticky bands described above for monitoring may serve as sufficient controls. For more information on the biology and host preference of this beetle, see Johnson and Lyon (1988) and Hall (1986).
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2. Birch, M.C., T.D. Paine, and J.C. Miller. 1981. Effectiveness of pheromone mass-trapping of the smaller European elm bark beetle. California Agriculture (Jan/Feb).
3. Byers, J.A., P. Svihra, and C.S. Koehler. 1980. Attraction of elm bark beetles to cut limbs on elm. Journal of Arboric. 6: 245- 246.
4. Davidson, J. 1991. Recommendations for Insect Monitoring and Control: Trees and Shrubs. Cooperative Extension Service, University of Maryland System Bulletin 258.
5. Diamond, A.E. 1947. Symptoms of Dutch elm disease reproduced by toxins of Graphium ulmi in cultures. Phytopathology 37 (7).
6. Johnson, W.T. and H.H. Lyon. 1988. Insects that feed on trees and shrubs. Cornell University Press, Ithaca, NY.
7. Hall, R.W. 1986. Preference for and suitability of elms for adult leaf beetle (Xanthogaleruca luteola) (Coleptera: Chrysomelidae). Environ. Entomol. 15: 143-146.
8. Hammerschlag, R., J. Sherald, and S. Kostka. 1986. Shade tree leaf scorch. Journal of Arboric. 12(2): 38-43.
9. Holmes, F.W. 1987. Elm yellows (=Elm phloem necrosis). Journal of Arboric. 13(7): 188.
10. Lanier, G.N. 1988. Therapy for Dutch elm disease. Journal of Arboric. 14(9): 229-232.
11. Lanier, G.N. 1989. Trap trees for control of Dutch elm disease. Journal of Arboric. 15(5): 105-111.
12. Lanier, G.N. and A.H. Jones. 1985. Trap trees for elm bark beetles. Journal of Chem. Ecol. 11(1): 11-20.
13. Hanisch, M.A., H.D. Brown, E.A. Brown (eds). 1983. Dutch elm disease management guide. USDA Forest Service.
14. O'Callaghan, D.P., E.M. Gallagher, and G.N. Lanier. 1980. Field evaluation of pheromone-baited trap trees to control elm bark beetles, vectors of Dutch elm disease. Environ. Entomol. 9(2): 181- 185.
15. Pajares, J.A. and G.N. Lanier. 1989. Pyrethroid insecticides for control of European elm bark beetle (Coleptera: Scolytidae). J. Econ. Entomol. 82(3): 873-8.
16. Richards, W.C. and S. Takai. 1984. Characterization of the toxicity of cerato-ulmin, the toxin of Dutch elm disease. Canadian Jour. of Plant Pathol. 6:291-8.
17. Sinclair, W.A., H.H. Lyon, and W.T. Johnson. 1987. Diseases of Trees and Shrubs. Cornell Univ. Press, Ithaca, NY.
18. Stipes, R.J. and R.J. Campana. 1981. Compendium of Elm Diseases. American Phytopathological Society, St. Paul, MN.