- Dutch elm disease
- How does Dutch elm disease survive and spread?
- Dutch elm disease
- Dutch Elm Disease
- The most common bugs
- The most common diseases/ailments
- Other Pest Resources
- Elm Tree Diseases: Tips On Treating Diseases Of Elm Trees
- Diseases on Elm Trees
- Treating Diseases of Elm Trees
- Problems of Elm
- How Dutch Elm Disease Spreads
- Dutch Elm Disease Signs and Symptoms
- Managing the Disease
- Need Help with Dutch Elm Disease?
- The Middlebury Landscape
Dutch elm disease
How does Dutch elm disease survive and spread?
Galleries of the smaller European bark beetle in elm wood
Dutch elm disease is caused by two closely related fungi, Ophiostoma ulmi and Ophiostoma novo-ulmi. Ophiostoma novo-ulmi is the more aggressive species and is the most common pathogen associated with DED today.
The fungus that causes Dutch elm disease is an invasive species and was first introduced to Minnesota in 1961.The devastating history of Dutch elm disease in Minnesota was recorded by plant pathologist David W. French. Today, the disease can be found in every county in Minnesota yet it is estimated that 1 million elms still remain within communities.
How does Dutch elm disease spread?
Carried by bark beetles
The native elm bark beetle (Hylurgopinus rufipes), the smaller European elm bark beetle (Scolytus multistriatus) and the banded elm bark beetle (S. schevyrewi) can all carry spores of the DED fungus from one tree to another.
Adult females of all three species of elm bark beetle lay eggs under the bark of recently dead or dying trees, or in firewood or logs with firmly attached bark.
Larvae feed on the inner bark and sapwood of the tree creating galleries and tunnels as they feed.
If the tree is infected with or was killed by DED, the DED fungus will be present in the wood. Sticky spores of the DED fungus will be produced within tunnels and galleries created by the bark beetles.
When the new beetles emerge as adults from infected elms, they carry spores of the fungus on and in their bodies.
Newly emerged beetles fly to healthy trees to feed.
As the beetles chew through the bark, spores on the beetle’s body are knocked off in the process. These spores start new DED infections.
Smaller European elm bark beetles and banded elm bark beetles feed in twig crotches of healthy trees. Therefore new infections are seen at small twigs.
Native elm bark beetles feed on larger branches that are 2-10 inches in diameter. Theses infections occur on main or secondary branches.
Several beetles may feed in a single tree resulting in multiple infections scattered throughout the canopy.
Occasionally native elm bark beetles introduce DED into the lower branches of the tree when burrowing to create an overwintering site.
Movement within the tree
Once in the trees’ vascular system, the fungal spores are carried up the tree with the flow of water.
The tree produces plug-like structures called tyloses in the water transporting cells of the tree’s vascular system in an attempt to stop fungal movement through the tree.
Unfortunately susceptible trees do not produce tyloses quickly enough to block the fungus. Instead this poorly timed defense response can cause wilt and decline within the canopy.
Movement through root grafts
The DED fungus produces a thread-like growth called mycelium that grows downward towards the root system.
In susceptible trees, the fungus is often capable of reaching the root system within the first season in which it is infected.
Neighboring elm trees will form root grafts, that allow for water and nutrients to flow from one tree to the other.
Root grafts commonly occur between neighboring trees of the same species.
Root grafts occasionally occur between neighboring trees from different species.
The DED fungus can move through root grafts to infect neighboring trees.
Infection that begins through a root graft often moves very quickly through the tree.
Dutch elm disease
This disease is not of Dutch origin, but because early work on the disease was done by Dutch pathologists in the 1920s, the disease has been called Dutch elm disease (DED). In all probability the disease is of Asiatic origin.
DED is caused by a fungus called Ophiostoma ulmi (formerly Ceratocystis ulmi) that was introduced to the U.S. in the early 1930s. The American elm, Ulmus americana, is extremely susceptible and the disease has killed hundreds of thousands of elms across the U.S. All native elms are susceptible, as are European elms, but the Asiatic elms, U. parvifolia (Lace bark elm) and U. pumila (Siberian elm) are highly resistant to the disease. The disease is still a threat today, but fortunately, several resistant American elm and hybrid elm selections are available or being developed.
There are two insect vectors responsible for transmitting DED: the native elm bark beetle (Hylurgopinus rufipes) and the European elm bark beetle (Scolytus multistriatus). These beetles lay their eggs in infected trees. When the adult beetles emerge, they carry the fungus with them when they travel to healthy trees to feed on twigs and upper branches. From the feeding sites, the spores travel to the tree’s water-conducting cells, or xylem. Chemicals produced by the tree during its attempt to fight the disease contribute to the plugging of the xylem, causing the tree to wilt. The beetles typically have two generations per year in the Midwest. DED can also be transmitted through root grafts. Root grafts between trees are especially prevalent in cramped urban and suburban parkways. The disease usually does not spread in this manner beneath roads because the road foundation prevents root grafts between trees on opposite sides. Driveways and sidewalks are usually not effective barriers to root growth.
The disease is most easily detected during early summer when the leaves on an upper branch curl and turn gray-green or yellow and finally brown. This condition is known as “flagging,” but a flag alone is not absolute assurance that the tree has DED. Brown streaks in the wood beneath the bark of affected branches is further evidence, but only laboratory isolation and identification can confirm positively that the tree has DED.
Samples should be sent to a diagnostic laboratory as soon as DED is suspected. Secure branch samples at least 1/2 inch in diameter and at least 8 inches long from a branch that shows active wilting (but is not completely dead). Wrap the sample in plastic wrap or place in a plastic bag to prevent the sample from drying out (do not add water or damp paper towels to provide moisture). The University of Illinois Plant Clinic will confirm DED for a nominal charge. The address: Plant Clinic, University of Illinois, 1102 South Goodwin, Urbana, IL 61801. Phone: 217-333-0519.
Both the beetles and the fungus need to be considered for control of DED. Control is possible through prevention, early detection of the disease, and replanting with resistant elms. Valuable trees should be inspected frequently, e.g. weekly, from early May through July, and monthly through September. An infected tree may be saved by pruning out the diseased branch promptly after seeing the first “flag.” A final pruning cut 7-10 feet below the lowest evidence of discolored (streaked) wood is necessary, but the saw blade should be wiped (sterilized) with 10% bleach (1 part bleach: 9 parts water) or denatured alcohol before the final cut is made. Injecting trees with systemic fungicides (see below) may be done at this time. If a tree shows many flags or completely wilts and dies, it must be removed quickly so that beetles and root grafts do not transmit the disease further. Root grafts should be severed before removal of a diseased tree whenever possible.
The bark beetles breed in standing dead or dying elm trees and piles of elm wood with the bark attached. Therefore, trees that completely wilt and die are suitable for beetle reproduction and should be felled. Destroy the infected wood and bark by chipping and composting (chips must attain temperatures of at least 120 degrees F), or at a minimum, remove the bark from cut logs and let the logs dry out. Cut logs from diseased trees should not be kept for firewood unless all of the bark has been removed and there is no evidence of bark beetles. Transporting diseased elm firewood may spread DED to otherwise disease- free areas. Covering and sealing cut logs and chips in clear plastic during the summer will allow the sun to heat up the wood and is another way to kill the beetles and fungus. Prolonged sunny weather and high temperatures are necessary, however, for this method of sanitation, called “solarization,” to be effective.
Chemical Protection and Therapy
At present, treatments of affected trees with injected fungicides show promise and should only be applied by licensed, certified arborists. If properly applied, American elms may be protected for 3 years. Be aware that repeated injections with a systemic fungicide may damage the bark and water-conducting tissues.
Dutch Elm Disease
Scientific name: Ophiostoma ulmi and O. novo-ulmi
Native range: Europe
Regulatory Status: Locally Regulated
Some municipalities require control of elm trees infected with Dutch elm disease in order to prevent its spread to other elm trees in the municipality. Because it is widespread, there are not state or federal regulations related to Dutch elm disease in Minnesota.
Dutch elm disease was first found in Minnesota during 1961 in St. Paul and can now be found throughout Minnesota. The spread of Dutch elm disease in Minnesota was documented by the University of Minnesota.
Dutch elm disease is caused by the fungi Ophiostoma novo-ulmi and O. ulmi. These fungi are often vectored by elm bark beetles of which there are a few species found in Minnesota. When bark beetles feed on twigs and branches, the fungus is introduced into the vascular system and spreads to other parts of the tree, including the roots. The tree tries to stop the spread of the fungus by producing plug-like structures which actually block the flow of water and contribute to its wilt. Very susceptible trees may die the same season they were infected; others may take several years. The fungus can spread to adjacent elm trees through root grafts.
The first symptom in trees infected with Dutch elm disease is usually a small area of yellow or brown wilting foliage called “flagging,” often beginning with a branch on the edge of the crown. The area expands and progresses toward the trunk. Wilted branches may have brown streaking in the sapwood which can be seen if the bark is removed. The University of Minnesota plant disease clinic can test elm samples for Dutch elm disease.
Elm trees are still a significant part of many forests and urban landscapes. Elm trees planted in communities today are usually of cultivars considered resistant to Dutch elm disease.
What Can I Do?
Check and follow local regulations regarding removal and disposal of Dutch elm-infected trees as well as storage of elm firewood. One of the primary means of minimizing the incidence of Dutch elm disease in urban areas is good sanitation of infected material. Injections with fungicides can help protect elms against the disease when it is spread by insect feeding.
Contact to report sightings
Black Leaf Spot (fungus – Gnomonia ulmea): Small, yellow spots appear first on upper surface of leaves, then gradually develop a shiny black appearance. Heavy spotting causes leaf yellowing and early defoliation in wet seasons. Usually defoliation does not occur much before normal leaf fall so control is not warranted. If trees have been affected seriously in previous seasons, fungicidal sprays applied when leaves are unfolding, when they reach full size, and again two weeks later will help prevent serious defoliation. Raking and burning fallen leaves will reduce inoculum for future infection.
Other Leaf Spots (fungi – Gloeosporium sp., Cercospora sp., Phyllosticta sp., and others): Dark, elongated spots develop on midribs, veins and margins of leaves, or spots of various shapes and colors may develop on any portion of leaf surface. Destroy fallen leaves and control as for black spot.
Wet wood or Slime Flux (bacterium – Erwinia nimipressuralis): Chronic bleeding of sap from crotches, wound or other weakened areas of trunk, with unsightly discoloration of bark in affected area. Sap frequently is sour smelling. Bleeding or fluxing is most pronounced during spring months or during wet weather. The problem results from fermentation processes of the causal bacteria creating pressures up to 60 pounds per square inch within the tree. Tapping directly into the trunk just below the affected area to provide an outlet for abnormal sap and gasses will relieve internal pressure and may aid in recovery. Drill a small hole (one-half inch diameter or less) directly below the bleeding site and slightly upward into the center of the trunk. Install a tight fitting drainpipe in the drilled hole making sure the end of pipe extends far enough outward so that sap does not fall on the tree.
Dutch Elm Disease (fungus – Ceratocystis ulmi): Symptoms may appear on one or more branches on any part of the tree in contrast to phloem necrosis where tops of infected trees show first abnormalities. Leaves on individual branches wilt and turn yellow; in some instances leaves wilt very rapidly, dry out, then fall while still green. Twig terminals of affected branches sometimes become curved to resemble a shepherd’s crook. As a further diagnostic aid, twigs when cut across, show discoloration or browning of water-conducting tissues in the sapwood. Tree defoliation may occur rapidly or take place over an entire season. Likewise infected trees may die in a single season or live for several years. The disease is spread by elm bark beetles infested with the causal fungus. Development of this disease has been limited in Texas.
Elm Leaf Scorch – (bacterium) – A rickettsialike bacterium has been associated with this condition. Vascular bundles are plugged to the point where water movement in the tissues is impaired. No control is known.
Powdery Mildew (fungi – Phyllactinia guttata, Uncinula macrospora, Microsphaera alni): Powdery whitish to gray growth on both sides of leaves. Affected leaves may be cupped, stunted, and show yellowing. The disease usually occurs so late in the growing season that chemical control is not necessary.
Mistletoe (parasitic plant – Phoradendron flavescens): (See Mistletoe in Plants that Grow on Other Plants)
Cotton Root Rot (fungus – Phymatotrichum omnivorum): (Chinese Elm is highly susceptible) (See section on Cotton Root Rot)
Mushroom Root Rot: (See section on Mushroom Root Rot)
Verticillium Wilt: (See section on Wilt)
Wood Rot: (See section on Wood Rot)
Root Knot Nematodes: (See section on Root Knot Nematodes)
The best defense against tree problems and disorders is a well-maintained and healthy tree. However, should your tree develop problems, these tips may help.
Here are the basic concepts of “integrated pest management,” an effective and environmentally sensitive approach to pest management:
- Put the right tree in the right place. Don’t force something to grow under conditions it can’t handle. FUF helps you with that decision when you plant through us.
- Improve the “cultural conditions” for the tree (water, light, pollution, wind and soil).
- Know your pest. Is it a bug, disease, or something else? Find out as much as possible so your efforts aren’t wasted.
- Choose the least toxic method of pest control. If improving cultural conditions isn’t sufficient, then try a non-chemical method of control. Chemicals are a last resort.
- Know your chemical, if you must use one. What specifically does it kill? What are the risks to you or the environment around you?
Here are some common problems and possible solutions:
|Low vigor, wilting or leaf drop, leaf dieback (leaves die from the tip back) or very little new growth.||Water stress||See our watering page for info.|
|Yellowish leaves. However, the soil may be swampy or even smell bad.||Overwatering/poor soil drainage||Choose an appropriate species; amend soil with organic matter to increase drainiage; raise tree if possible.|
|Leaves to appear yellowish, or chlorotic.||Nitrogen Deficiency||Adding organic nitrogen fertilizer on a regular basis can help – but don’t overdo it.|
|Leaves will yellow, but the veins remain green. This is known as iron chlorosis.
|Iron deficiency||Wind is drying, so try increased watering if the soil drains well. Windburn is usually not fatal if the tree is generally tolerant of conditions. This is usually more noticed on young trees.|
|In rare cases of frost, some plants will partially or fully die back.||Frost Dieback||Don’t prune off the frost-burned branches until you are sure no further frosts are expected. The outer foliage is keeping the inner foliage warm. To prevent frost burn, keep plants well watered. Cover with an old sheet or towel at night if frost is expected, trying to leave an air gap between the sheet and foliage. Uncover during the day. Outdoor rated string lights (“Christmas lights”) in the tree at night can also help, especially if it is covered. Jacaranda (Jacaranda mimosifolia) and New Zealand Christmas trees (Metrosiderus excelsus) are some of the more common San Francisco street trees subject to frost damage.|
|The leaves generally wilt and brown within 1-3 days and the tree does not recover.||Chemicals dumped into tree basin||This could include but is not limited to: painting chemicals, motor oil, cleaning solvents, etc. Painting chemicals seem to cause the most sudden and dramatic tree death when dumped into tree basins.
Be sure that any contractors you hire understand that they are not to dump any toxins in the tree basins (including the neighbor’s!) and that they are responsible for the cost of removing and replacing any trees that die if they do. If you suspect chemical dump, gather some soil with a trowel and smell it. You may detect a chemical odor.
The most common bugs
Here we describe pests and their effects, and list some least-toxic methods of control. (Some text adapted from the Department of the Environment website; see links below)
Aphids, Ants and Sooty Mold
Although sooty mold is not a bug, it is generally associated with aphids.
Aphids are among the most common garden pests, especially in the spring and summer. It’s not cause for panic, but you may want to control their population.
Winged adult cotton aphid next to several green peach aphids (photo by Jack Kelly Clark).
Aphids are small (less than 1/4 inch long), soft-bodied insects that suck sap from leaves, twigs, or roots. They may be green, yellow, brown, red, or black depending on the species and the plants they feed on. They are pear-shaped insects with long legs and antennae. Adult aphids can be winged or wingless. They are often clustered on new growth.
Although aphids are most common in spring and summer, some species mate and produce eggs in fall or winter, which provides them a more hardy stage to survive harsh weather. Under ideal temperatures, many aphid species can complete their life cycle in less than 2 weeks, and because of their prolific reproductive capacity, enormous populations of aphids can build up in a short time.
Although aphids seldom kill a plant, the damage and unsightly mold growth they cause sometimes warrant control. Aphids cause curling, yellowing, and distortion of leaves and stunting of shoots. Moreover, aphids serve as carriers of viral diseases on certain vegetable and ornamental plants. In urban environments, aphids can produce copious amounts of “honeydew” excretion, which often turns black with the growth of a sooty mold fungus. Sooty mold is usually just unsightly, coating leaves with a black residue, but can kill a small tree that is already under stress.
Check plants frequently for aphids, including the undersides of leaves. Look for curled green leaves and/or wilted buds. Many species of aphids cause the greatest damage when temperatures are 65-80 F. The presence of ants often indicate aphids, because ants act as “farmers,” protecting the aphids, in order to harvest their honeydew excretions.
First, is it a big deal? If there is little damage, some aphids are OK. Even in our urban environment, aphids have natural predators that will keep the population under control. We actually want some aphids in order to sustain the predators so they can reproduce to eat more aphids! The key here is not to panic if a few aphids are feeding on a tree.
The easiest thing to do is to wash off the aphids with a strong jet of water one morning per week (morning is best, allowing the leaves to dry during the day). Doing it more than once a week helps keep the population down. You can wash leaves when you’re watering your young tree, washing your car, or watering plants.
If aphids are causing sufficient damage to warrant further treatment, or if washing with plain water has not worked, we suggest one of two insecticides. Always follow manufacturer’s application instructions:
Safer Insecticidal Soap – A contact insecticide that is fully biodegradable. It works by smothering rather than poisoning.
Neem Oil – Works quickly by suffocating the aphids. Also serves as a repellant and a fungicide. Local nurseries, such as Sloat Garden Center, carry neem-based insecticides.
Scale insects are usually slow moving or don’t move at all. They may look like little bumps clustered on the plant. They are also often accompanied by ants. Their honeydew excretions can also cause sooty mold. There are many kinds of scale.
Traps, such as Tanglefoot®, can be smeared on a stiff band of paper taped around the trunk and stakes, and can catch scale during its young “crawler” stage. Follow the manufacturer’s application instructions. Also, place it where pets or people will not brush up against it. Don’t leave anything on the tree long enough to strangle it as it grows!
Horticultural oils or soaps can also work if timed correctly. See more information and photos here.
The most common diseases/ailments
Fire blight is a bacterial disease affecting plants in the rose family such as pear, cherry and apple. It’s characterized by branches looking black and wilted as if they have been burned by fire. See information and photos here.
Powdery mildew on crape myrtle shoot (photo by Jack Kelly Clark).
Powdery mildew appears as a fine white dusting on leaves, causing leaves to look whitish and a bit rumpled. It’s caused by cool, damp, foggy weather, of which San Francisco has plenty. Rain and direct sunlight inhibit powdery mildew. Often seen on London Plane/Sycamore trees in the city (Platanus acerifolia), it is generally cosmetic and not fatal. The ‘Yarwood’ variety of Sycamore is more resistant. See more information here.
Take control measures on highly susceptible species. Prune out infected tissue and dispose of it away from plants. Avoid excessive fertilization or irrigation – these promote susceptible new growth. Overhead sprinkling may reduce powdery mildew infection because spores cannot germinate, and some are killed, when plants are wet. Sprinkle plants in the mid-afternoon when most spores are formed; this allows plants to dry before nightfall, reducing the likelihood that sprinkling will promote other diseases. Fungicides are only effective in early stages, not once there is a lot of mildew. Fungicides should be specifically for powdery mildew, and only used as a last resort.
Chinese Elm Anthracnose
Most often seen on Chinese Elms (Ulmus parvifolia), this fungus causes branches to die back from the tips, causing a “frizzy hair” look to the tree (see photo). The ‘Drake’ variety of Chinese Elm is resistant to anthracnose, and is the most common variety. To control, prune out and dispose of infected twigs during the fall and winter. Fungicides have not been found to be effective.
Chinese Elms other than the ‘Drake’ variety may also get anthracnose canker, which looks like large wounds in the bark (see photo). Consult an arborist for control methods and consider replacing severely infected trees.
See more info about anthracnose here.
Shot Hole Fungus
Shot Hole Fungus affects Prunus species such as plum, almond and apricot. In San Francisco, it’s most often seen on Purple-Leaf Plum trees. It looks like little holes in the leaves. Our (relatively) warm, wet winters and wet springs encourage shot hole, but it is cosmetic and usually not fatal.
Sooty mold on leaves (photo by Jack Kelly Clark).
Sooty mold is dark fungi that grow on plant surfaces that have been covered with insect honeydew. Leaves will look black and “dirty.” See aphids and scale info, above. Sooty mold in itself is generally harmless and can be ignored, except when it covers leaves so much they can’t get enough light. Fungicides are not effective; instead, wash leaves with a forceful stream of water and control the insects (usually aphids) that are producing the honeydew.
Other Pest Resources
Free Sick Plant Clinics
At Berkeley Botanical Garden, on the first Saturday of each month, 9 a.m. to noon, UC Berkeley plant pathologist Dr. Robert Raabe, UC Berkeley entomologist Dr. Nick Mills, and their team of experts will diagnose what ails your plants (a photo or bagged sample may help). The sick plant clinic is free with no reservations required. Get directions and info here or call 510-643-2755.
The San Francisco Botanical Garden sometimes has plant clinics. For more information, call 415-661-1316 x354.
Pests of Landscape Trees and Shrubs, publication # 3359, published by University of California, has very good pictures and helpful tables matching symptoms to causes. It’s available online or by phone at 800-994-8849. You also might find it at the San Francisco Botanical Garden Bookstore.
Write for advice
Email Dr. Hort at [email protected] or send a stamped, self-addressed envelope to Dr. Hort, c/o San Francisco Chronicle, 901 Mission St, San Francisco CA 94103.
For more information about other pests and their least-toxic control methods, visit these excellent sites:
San Francisco Department of the Environment
UC Davis Integrated Pest Management
UC Davis Integrated Pest Management En Español
Elm Tree Diseases: Tips On Treating Diseases Of Elm Trees
Stately elms once lined the streets of Midwestern and Eastern towns. In the 1930s, Dutch elm disease nearly wiped out these lovely trees, but they are making a strong comeback, thanks in part to the development of resistant varieties. Elm tree diseases still play a major role in the life of the trees and complicate their care. Anyone with an elm in their landscape should know the symptoms of disease so they can address problems promptly.
Diseases on Elm Trees
There are several elm tree leaf diseases that cause spotting, discoloration and defoliation. By the time the leaves fall from the tree, the spots have often grown together and other discolorations have developed, making it hard to differentiate between the diseases without a lab test.
Most elm tree diseases that attack the leaves are caused by fungi, but elm leaf scorch, caused by a bacterium, is a little different. With this disease, the bundles of veins in the leaves become clogged so that water can’t move within the leaf. This causes the leaf to look scorched. There is no known treatment for elm tree leaf scorch.
The most devastating elm tree diseases are Dutch elm disease and elm phloem necrosis. Dutch elm disease is caused by a fungus spread by elm bark beetles. The microscopic organism that causes elm phloem disease is spread by white-banded leafhoppers.
The diseases look similar, with all of the leaves browning on affected branches, but you may be able to tell the difference by the location of the damage. Dutch elm disease usually starts on lower branches, and may appear random, affecting only part of the tree and leaving another part unscathed. Elm phloem necrosis affects the entire crown at once. Agricultural extension services in most areas ask that you report incidences of these diseases.
Treating Diseases of Elm Trees
Once elm tree leaf diseases take hold, there is no effective treatment. Rake and burn leaves to help prevent the spread of the diseases. If you have problems with leaf diseases, try using an anti-fungal spray early in the season the following year. This may help prevent disease. Powdery mildew is another leaf disease that sometimes effect elms, but it occurs so late in the season that treatment is unnecessary.
There is no cure for Dutch elm or elm phloem disease. Trees infected with Dutch elm disease sometimes respond to pruning. This is a treatment that extends the life of the tree for several years if caught early and done properly, but it is not a cure. It’s best to hire a certified arborist for the job. Trees with elm phloem necrosis should be taken down as soon as possible.
Since there is no easy cure, it is important to learn how to protect elm trees from disease. Here are some tips:
- Watch for the insects that cause elm tree diseases, and start a control program as soon as you see them.
- Rake and destroy elm tree leaves promptly.
- Use an antifungal spray if you had problems with elm leaves the previous year.
Problems of Elm
The European elm bark beetle and the native bark beetle are the major elm pests, partly because they burrow under the bark as larvae and spread the Dutch elm disease.
Elm Leaf Beetle – Soon after leaves unfurl in spring, the brownish yellow, ¼-inch long beetles chew out rectangular areas in the leaves. After laying eggs on leaf undersides, their 1/2-inch long, yellow-and-black larvae skeletonize the foliage, and drop to the ground at the base of the tree to pupate. In fall, they may migrate indoors, but they normally over winter in tree bark and outbuildings.
Holes In Bark, Bud and Twig Damage
Smaller European Bark Beetle – This beetle, a principal transmitter of Dutch elm disease, frequently attacks weakened trees. Adults are reddish black, 1/10 inch long, and deposit eggs along galleries they excavate in the sapwood surface. Hatching larvae tunnel out at right angles to the main gallery. Adults emerge through the bark, leaving tiny holes, and feed on buds and bark of twigs during the summer.
Leaves Wilt, Branches Die
Dutch Elm Disease – This lethal fungus is spread from tree to tree by Elm bark beetles, as well as by natural inter-grafting of tree roots. It also enters through wounds and pruning cuts. The most obvious symptom is the wilting and yellowing of one or more branches. In cross-section, infected wood shows long brown streaks following the grain of the wood and spots or flecks near the bark. Older American Elms are susceptible, recent releases of American Elm and Lacebark Elm are highly resistant. Promptly remove diseased or dying elms before May, regardless of the cause of their problem. Isolate infected trees by trenching to sever any possible root connections with neighboring trees. (Root grafts can interconnect large elms 30 to 50 feet apart.) Trees can be injected with benomyl, a systemic fungicide, by a professional arborist. Trees with more than 5 percent of their crowns infected are probably beyond help and should be removed. Keep surviving elms in vigorous growing condition by careful feeding and watering.
Bleeding Holes In Trunks, Branches Girdled
Borers – The elm borer, dogwood twig borer, flatheaded apple tree borer and leopard moth borer all bore into weakened elm trees. Sap bleeds from borer holes, often accompanied by fine sawdust. Their feeding sometimes girdles branches or even main trunks. Borers are white, pinkish or yellow grubs, 3/4 to 1 1/4 inch long. Keep trees well watered, watch for signs of injury and dig borers out with a wire or inject holes with a BT (Bacillus thuringiensis) solution. Prune off and destroy infested branches.
Branches Encrusted With Small Bumps
Scale Insects – At least a dozen scale species attack elms. They all form groups of small bumps or blister-like outgrowths on tree stems and leaves, from 1/10 to 2/5 of an inch across. These white, yellow, brown or black bumps are waxy shells that protect the insect beneath. When the scale infestation is heavy, many branches may turn yellowish brown, foliage turns gray green and wilts.
Swollen Lesions on Stems and Trunk
Cankers – At least eight species of fungi cause cankers and dieback of twigs and branches of Elms. Many cankers can be removed by pruning well beyond the affected area to ensure complete removal of infected tissue. Be sure to burn the pruned limbs. Mildly infected trees have been known to recover without any special treatments, but once a tree is heavily infected, it should be removed.
Leaves Curl and Wither
Elm Phloem Necrosis – This disease is thought to be caused by a mycoplasma-like organism (MLO), similar to a virus. Affected trees show drooping and curling leaves, which become yellow, then brown, then fall off. Usually all the branches on the tree show symptoms at once, but sometimes the symptoms develop branch by branch. The phloem tissue of the inner bark develops a butterscotch color and a distinctive wintergreen scent. Highly susceptible trees are killed, while survivors become stunted and sometimes develop witches brooms. The disease is spread from tree to tree by natural root grafts and by leafhoppers. The organism also over winters in infected tree roots. Spray with rotenone or pyrethrum to control the insects. Injections of tetracycline into trunks may stop the disease, but this method is too expensive except where valuable specimens are concerned.
Leaves Spotted, Turn Brown, Drop Prematurely
Leaf Spot – A number of leaf spot fungi attack elms. Often, small white or yellow flecks appear on upper leaf surfaces. These enlarge and turn black in the center. In heavy infections, leaves may drop prematurely. Usually the disease predominates in late fall about the time the leaves drop normally, so little damage to the tree occurs. Gather and destroy the fallen leaves in the fall. A copper fungicide applied 3 times at 10 to 12 day intervals, starting when the leaves are half grown, gives control.
Popular for their beauty and ability to grow fast and thrive in a variety of conditions, the American elm tree used to line many main streets in our country. But that all changed as Dutch elm disease began its devastating domination in the United States around 1930. Considered one of the most destructive shade tree diseases in North American, once it affects one elm tree, others nearby are soon to follow.
How Dutch Elm Disease Spreads
Dutch elm disease is caused by two related species of fungi—Ophiostoma ulmi and the more aggressive of the two, Ophiostoma novo-ulmi, which is responsible for most of the devastation. This fungus attacks the tree’s vascular system, preventing the proper flow of water and nutrients. The fungus is spread in one of two ways:
Transported by the elm bark beetle. Adult beetles tunnel into the bark to lay eggs, creating galleries in the sapwood below the bark. Once the eggs hatch, the larvae begin to feed on the sapwood. When the larvae mature and leave the fungus-infested tree, the adults carry the fungus, introducing the disease to neighboring trees.
Spread through the connected root systems of nearby trees. The Dutch elm disease fungus can move from an infected tree to neighboring trees through their interconnected roots (or root grafts).
Dutch Elm Disease Signs and Symptoms
Mostly in late spring (but also occurring any time during the growing season), symptoms spread first to individual branches, then throughout the entire tree, and eventually kill it. This process can happen all in one season or take a couple of years. What you will see:
- Leaves wilt, turn yellow, and ultimately turn brown.
- Premature leaf drop.
- Flagging or branch death.
- Brown streaking in sapwood—the newly formed, softer outer layer of wood underneath the bark. This can be revealed by cutting a cross section of the dying branch and looking for discoloration in the sapwood.
Managing the Disease
Plant trees that are more resistant to the disease. While the American elm tree is highly susceptible to the disease, other species, like many European and Asiatic elms, may be more tolerant. No native elms are immune, but some hybrids have been selectively bred for better survival.
Pruning. This method only works on trees that have newly infected crowns, but the disease must be caught early enough and cannot have already spread via the root system. Wood pruned from infected elm trees must be destroyed. Pruning is more effective when paired with the use of a fungicide. Of course, if the damage is serious enough, the entire tree should be removed before it infects others.
Breaking the connection. If larger trees are within 25 to 50 feet of each other, it is likely that some of their roots are connected. All it takes is one tree with the disease to infect the others. Breaking of the root grafts should take place before the infected tree is removed. This is done with professional machinery.
Insecticides. One way to stop the disease cycle is to kill the elm bark beetles responsible for spreading the disease. Certain insecticides are often used, but are not always the most effective method.
Proper protection. There are fungicides that can prevent the elm bark beetle from spreading the infection. An expensive process that needs to be repeated seasonally, this treatment is injected into the tree by a professional.
If you want to learn more about identifying and controlling Dutch elm disease or other tree diseases, contact Elite Tree Care at 610-935-2279.
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The Middlebury Landscape
Dutch elm disease, Ophiostoma novo-ulmi, is a fungus that targets certain members of the Elm (Ulmus) genus, including American elm, Scotch, and Red Elm. The fungus attacks the xylem within tree, causing an immune response. The tree reacts by plugging the xylem with gums, shutting water transport down, which in turn prevents vital nutrients from flowing as well. The leaves then wilt from lack of water, and this appears in the tree as flagging, which is the yellowing of foliage on the upper branches in the crown. Generally this is seen in late May or early June. The disease quickly moves through the rest of the tree, often killing even a large mature tree in less than a year.
The fungus is primarily transmitted in two ways. The first is by bark beetles, either the native Elm Bark Beetle, Hylurgopinus rufipes and the European Elm Bark Beetle Colytus multistriatus . Both insects over winter as adults under bark of unhealthy infected elms, and emerge in the spring covered by fungal spores of DED. The adults feed on tender bark and twigs of healthy trees, spreading the fungus, and then lay eggs in weak and dying bark of elms, starting the cycle again. Insecticides to slow the progress of the beetle are minimally effective against the native beetle, and not at all against the European beetle, so sanitation (removal of dead wood) is the recommended course of action.
The second method of DED transmission is root grafting. American Elm trees within 50-60’ or closer to each other can graft their roots together, effectively linking the trees. The fungal spores are carried in water throughout the xylem, and the shared roots can quickly infect an otherwise healthy tree. While a beetle sourced infection may take a year or two to kill the host tree, root grafting can spread the disease and cause mortality in a couple of months. Root grafting has turned out to be the major cause of DED transmission in cities, as street after street lost elms like dominos falling.
Combatting and Preventing Dutch Elm Disease
Middlebury College uses a multi-step approach to saving the American Elm on campus. The first trick is to maintain vigor in the older trees. This is done by prescriptive fertilizing every year, to make sure the tree is not lacking in any macro or micro nutrients. We also apply compost from our composting operation, and mulch leaves in the drip zone of the canopy to increase organic matter.
The most effective way to prevent the spread of Dutch elm has been in sanitation, removing any dead and dying elm trees. Several larger urban areas have had good luck with this method. By removing diseased wood, the source of the infection, as well as the overwintering sites of the bark beetles, the source of infection is greatly reduced. A single piece of elm wood firewood sized can produce over 1800 bark beetles. Middlebury prunes the heritage elms of any dead wood 1” or larger (sizes that can support the bark beetle).The pruning work is done in the winter, as the cut end of the branch is attractive to the elm bark beetles during the growing season, and could spread the disease.
The major problem we have with sanitation on campus is that American elm is spread throughout the Champlain Valley as a native tree, and, as it generally doesn’t succumb to DED until about 30-40 years old (after reproducing age), will always be around, albeit as small trees. We can’t remove all the elm surrounding campus, like in the woods of Ridgeline, say, or along the edge of St. Mary’s Cemetery, so we can’t count on sanitation as being as effective like in other more urban locations.
The most effective preventative we have to prevent DED is by fungicide, something to kill the spores of the disease causing fungus. There are only a couple of approved fungicides to treat trees, and we use a product called Arbotect. This kills the fungus that causes DED, and lasts up to three years. It does this by the unique ability to translocate into new sapwood, and persisting through environmental challenges. If symptoms of DED do appear, we use another fungicide called Alamo. This needs to be injected every year if used as a preventative treatment. The constant wounding of the tree to do that is worrisome.
Both fungicides are applied by a process called macro-injection. Holes are drilled into the base (root flare) of the tree, and tubes attached. The fungicide is then pumped to the base of the tree, and taken up by the tree’s natural process of transpiration. This is done in the beginning of July.
The final preservation technique is replacement of the American Elm in the landscape. Many agencies are actively breeding resistant strains of Elm, and we’ve planted several varieties over the years, including ‘Accolade’ and ‘Discovery’. We also have specimens of ‘Princeton’, ‘Liberty’, and an early variety called ‘Christine Buisman’. Naturally, we plant a wide diversity of tree species, so we never run into such a historic loss of canopy cover like what happened to the elms of the past.