A Threat to Red Oaks
White Oaks Species
Michigan State University Extension
Disease Cycle White Oak & Oak Wilt The Pruning Connection White Oak Leaves Distribution Red Oak Leaves Field Diagnosis of Oak Wilt Oak Wilt Images Management of Oak Wilt Stop the Chain Saws Now! The Value of Oaks Press Release Site Directory Oak Wilt Research Winter Pruning Oak Wilt: Identification & Management Guide.pdf USDA Forestry Leaflet #29 Oak Wilt Resolution Adopted by Bloomfiled Township, MI
OAK WILT: A Threat to Red Oaks
David L. Roberts, Ph.D.
Michigan State University Extension
Oak wilt, caused by Ceratocystis fagacearum, is a lethal disease of oak (Quercus sp.). The disease is most serious on members of the red oak family, including red oak, scarlet oak, black oak, pin oak, etc. Members of the white oak family are generally not nearly as susceptible: white oak, swamp white oak, burr oak, etc. The disease is analogous to and potentially as serious as Dutch Elm Disease on American Elm. Both are introduced, exotic diseases caused by vascular wilt fungi; both diseases are transmitted by insects; both diseases can be transmitted through root grafts, and both fungi kill their host plants rather quickly.
Once a red oak becomes infected with the oak wilt fungus, it usually dies within several months. The most common predisposing factors are injury; and human activity is among the leading causes of infection. Symptoms in recently infected trees include the following: leaf margin browning progressing inward and from the leaf tips to the leaf bases; leaves on branches may appear to be wilting: some leaves may defoliate green or brown while others remain attached; vascular discoloration in the sap may or may not be present. These symptoms may be descriptive of other problems and hence misdiagnosis is not uncommon. Fungal mats, called pressure pads, may develop beneath the bark by the following spring; the fungal mats serve as sources of infection for transmission by the picnic beetles to healthy trees, thus completing the cycle.
Once established in a tree, the oak wilt fungus becomes systemic within the tree. Hence neighboring trees which happen to be root grafted to the diseased tree are now at risk. In this manner, the disease may affect many trees in an area, moving radially from the point source. Urban and rural/native forests can be affected from these point source infections.
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ThePruning ConnectionThe health of oaks becomes high risk during the warm season months when they are injured through human practices, especially pruning, and through storm damage. April, May and June are the months posing the greatest risk but other warm season months are also suspect. Several species of picnic beetles are attracted to the fresh wounds caused by trimming and storm damage, and their activity transmits the fungus to healthy, recently pruned oaks from nearby diseased trees. Trees which come into contact with the fungus in the Spring are often dead by August or September.
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Although not widely known, oak wilt is widely distributed throughout Michigan and can be found in any areas where oaks are found or propagated. I have often been asked whether oak wilt exists in a county, township or region of the state. My common reply has become, “prune a red oak in May and wait”. Anyone who prunes oaks during April, May and June is definitely taking some serious risks, even though such practices might have been acceptable previously. (Back to top)
Field Diagnosis of Oak Wilt
There have been a substantial number of misconceptions and misinterpretations of oak wilt. Many oaks have suddenly died and individuals are perplexed with regard to the cause of death of their oaks. Submissions to diagnostic labs often result in negative tests for oak wilt and the interpretation is that oak wilt is not the cause of death. In several instances, the lack of confirmation of oak wilt resulted in extensive but unnecessary deaths of oak trees due to continued advance of oak wilt. On a good day, with highly expert plant diagnosticians analyzing excellent sample submissions, the oak wilt fungus may only be confirmed about 40% of the time from actual oak wilt affected trees. Unreliable lab tests are why I developed the following criteria for helping us to diagnose oak wilt.
Be suspicious of oak wilt if all or most of the following apply:
1) Identity of the oak species as a member of the red oak family
2) Presently, wilting symptoms are evident in live trees
3) Sudden death, generally on the order of months
4) Recent pruning or storm damage (months to a year)
5) Proximity to other oaks (root graft transmission from nearby dead oaks)
6) Elimination of other causes of oak death: gas leaks, root injury, other diseases, etc.
If laboratory diagnosis is desired, consider the following recommendations: collect samples during the growing season from live symptomatic trees (dead trees are practically valueless for lab diagnosis unless the above mentioned pressure pads are evident); branch segments should be at least one inch in diameter; bole (trunk) samples, preferably with vascular staining, can be submitted-do not include bark; all samples should be kept cool and clean-the oak wilt fungus does not compete well with other fungi and bacteria, and submitted as soon as possible to a reputable diagnostic lab. Remember, a negative lab report for oak wilt does not guarantee that oak wilt is not present in the tree from which the sample was collected.
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Management of Oak WiltThere are several procedures to prevent oak wilt and to minimize the effects of oak once detected. Clearly, the first one is to prevent oak wilt from becoming established in your oaks. Once oak wilt is detected, or perhaps even strongly suspected, several important steps need to be followed. Some of these steps depend on the value of the trees and the extent that the plant owner may want to invest in their oak trees.
1)Pruning: DO NOT PRUNE during the warm season; trimming is a major cause of oak wilt infections in Michigan today. If oaks need to be pruned, the dormant period is best - November through February. If storm damage occurs during the warm season, clean-prune the branch 1-2 feet below any visible injury and seal with a pruning paint. Repair of storm damage should occur as quickly as possible-optimally within hours or within a couple of days of the damage.
Storm damage which has occurred during the dormant (winter) months is not of particular concern for oak wilt. Nevertheless, winter storm damage should be repaired during the winter months and not during the warm season. It is not advised to use pruning paints during winter storm repair.
Except for emergency situations, cessation of pruning during the warm season, is probably our most effective tool for preventing oak wilt at this time. Compared to Dutch Elm Disease, in which elm bark beetles can easily transmit the fungus from diseased trees to healthy trees, the oak wilt insect vector is inefficient at transmitting the oak wilt fungus when no wounds on oaks are present.
2)Removal: Infected trees should be removed promptly. Once a tree has become infected, there is little chance to save it. Tree removal should take into account disposal. Wood may be used for firewood provided it is debarked or covered and sealed during the spring/summer months (remember-diseased trees with bark serve as a source of further infection for neighboring trees). In some situations of dense stands of trees, a tier of healthy oaks surrounding an oak wilt-affected tree may need to be sacrificed to “save the forest”. Remember to trench before removal if other oaks are nearby and there is a possibility of root grafts between trees (see below).
3)Trenching: If there is a possibility of root grafts, trenching at least 3-4 feet deep is recommended midway between the diseased and healthy trees. This effort will hopefully prevent transmission of the deadly fungus between diseased and healthy trees which may be root grafted if in close proximity.
4)Injection: Micro- and macro-injection of fungicides is a protective measure against infection. It should be understood that injection may be expensive and is often not a guarantee of tree survival. In a landscape or urban situation, injection of apparently healthy” oaks in the vicinity of dying oaks might be a recommended practice if economics condone the treatments. Injection should not be attempted on an oak wilt-affected tree except as a last ditch, fruitless effort. Research has generally shown that once oaks become infected with the oak wilt fungus, there is little that can be done to save the infected trees. Hence, efforts should be directed toward saving non-infected trees.
5)Vigilance: Because of the size, grandeur and value of our oak trees, a constant vigilance must be maintained about the threat of oak wilt. If any unusual problems or symptoms are associated with oaks, obtain help. It is imperative that we spread the words- “DO NOT PRUNE!” during the warm season. Storm damage should be assessed and repaired promptly. It is highly recommended that community efforts be established to thwart the threat of oak wilt. Any time an oak tree succumbs to oak wilt in a community, it eventually becomes everyone’s problem.
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Prevent Oak Wilt: Stop the Chain Saws Now!!March UpdateLate March to early April is the time to stop pruning oak trees. As publicized previously, oaks wounded during the warm season attract sap beetles which transmit the deadly oak wilt fungus. Members of the red oak family usually die within a month or so of coming into contact with the fungus.
What is the date to cease pruning?
I have often been asked for exact dates when pruning of oak trees should stop and when pruning can resume. In general, April 1 (fool’s day), is the time to cease pruning for the coming warm season; anyone pruning after this date is probably taking too much of a risk and may be jeopardizing the health of the oak trees they are maintaining. We must remember that insects and diseases do not recognize artificial dates created by Homo sapiens. As more information is learned about the nature of the disease and the insect vectors, we will undoubtedly determine that the insect vectors recognize temperature and heat units as the factors important in their activity....or inactivity.
Are you confused?
There has been literature in the past that suggested oak trees should not be pruned in April, May and June. In fact, it may be implied from a new publication from MSU in 2001 that oak trees may be safely pruned after June 30. Again, the insect vectors do not recognize artificial dates, and we know that sap beetles, such as picnic beetles, remain active from spring through fall. As individual plants, oak trees are among the most valuable plants in Michigan. I strongly recommend that because of the property value loss associated from oak wilt, because of the costs of removing large trees and because of the cost of containing oak wilt, it is prudent that oak trees only be pruned dormantly; other scientists and arborists have also joined me in this view. Trees which must be pruned during the warm season, for reasons of storm damage or otherwise, should be treated with a sealant on the wounded surface immediately after the wound is created; this treatment will hopefully prevent transmission of the oak wilt fungus to the tree.
The Value of Oaks The Cost of OAK WILTOaks comprise a significant percentage of our forests, both urban and natural, in terms of population and canopy. Oak wilt has, perhaps, become an emotional issue to many of us. Like some human communicable diseases, we are sometimes reluctant to admit or inform others that we have a problem. I know of several oak wilt sites where home/property owners have requested anonymity. And for good reason. The presence of oak wilt can dramatically reduce property value because of the cost of containment of the disease or because so many oak trees have been eliminated by the disease. A once highly valued and lovely home site is no longer as desirable as it once was. Perhaps the site is even a liability when some of the following costs are considered (all are general estimates):*Tree Value Losses: Potentially several $1000's per tree.
*Tree Removal Costs: $2,000- $7,000 or more per tree depending on tree size and location.
*Alamo Injection: $300-600 per tree depending on size.
*Trenching: $5-10 per linear foot for a 3-5 foot deep trench.
Between two trees only, the cost may be $300 (60ft. x $5=$300).It is not difficult to imagine that the cost of a single incidence of oak wilt at a single urban site can easily exceed $5,000-10,000. Many of the oak trees we value are older trees, which beenPress Release:
growing on the site at least a 100 years, long before most of us, even our older CAT readers, came into this world. Some of the even larger oak trees were growing long before President
Lincoln signed the Emancipation Proclamation. If we plant a small oak seedling today, it is possible that our great, great grand children may enjoy its mature, magnificent beauty.... if it
survives the human onslaught. I value these large trees; they are our friends. If you’ve ever played golf on a new golf course carved out of a soybean field or if you seen new housing
developments in a cornfield, you understand my sentiments. Like air and water, perhaps we’ve taken these large trees for granted by expecting them to require no care- we expect them to
remain healthy regardless of what we do to them. Like oak wilt, we sometimes only react or care about health when we contract that serious disease such as cancer, and then we realize the true
cost. Obviously, it is far less costly to practice preventative health care than to try to correct problems after they occur. (Back to top)Detroit Free Press: 2/18/00 "Something is killing the majestic oak..."
Associated Press: 9/25/00 "Experts worry oak wilt ...spreading"
Announcing the new Oak Wilt Brochure:
As an outgrowth of my oak wilt efforts and in conjunction with Bloomfield Township, an Oak Wilt brochure has been developed. The title of the brochure is Oak Wilt: Identification and Management Guide. The purpose of the brochure is to provide commercial plant health care providers, townships, neighborhood associations, etc. with an educational tool in an easy-to-read, easy-to-understand and easy to reproduce tri-fold format, complete with color pictures. The brochure can be distributed to your clients or to other interested parties. The brochure alerts the public about oak wilt by providing some of the high points about the disease without all of the minutia which sometimes prompts readers to reach for their favorite novel.Oak Wilt Resolution Adopted by Bloomfield Township:
The brochure (saved in pdf format) can be viewed, printed and down-loaded using Acrobat Reader. If you do not have Acrobat Reader a free version is provided at the Adobe web site. The brochure may be copied in color or black and white and distributed in any manner deemed necessary. A space for your business name or logo is provided on the back cover of the brochure. Click to get a 'free' Acrobat Reader 5
(Back to top)The Bloomfield Township Trustees have adopted an Oak Wilt Resolution. Recognizing that oak wilt knows no boundaries, the adoption of this resolution is viewed as a progressive effort to limit the escalation of oak wilt by providing educational materials and guidelines to the public and commercial enterprises (including homeowners, plant health care companies, developers and construction companies, power line trimmers, etc.) who/which may impact the health of oaks. As with many areas in Michigan, oaks comprise a significant and valuable natural resource in Bloomfield Township’s urban forest. The resolution provides guidelines for managing oaks and observing practices which prohibit the introduction and spread of oak wilt. The resolution also requires companies which work in Bloomfield Township to register with the township. The goal of registration is to encourage quality companies with knowledge of oak wilt to operate in the township. It is anticipated that other Michigan communities will eventually adopt resolutions similar to this and those adopted in other neighboring states. The Resolution is now available on my web site as a printable document file (.pdf) at Oak Wilt Resolution .
The Resolution (saved in pdf format) can viewed, printed and down-loaded using Acrobat Reader
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Oak Wilt Research
There are very few funds available for oak wilt research largely due to the fact that urban oak forests do not represent a traditional agricultural commodity. The information presented herein, has been gathered from other states’ research and through our own limited research here in Michigan. In 1999, limited funds have been competitively obtained by the author through the GREEEN Project at MSU. This research would not have been possible however, without the assistance of Mayor Mark Steenburgh, City of Warren, and George R. Lee of Branch Tree Service. If individuals or organizations wish to contribute to our ongoing oak wilt research, they may do so by contacting me at Michigan State University Extension at this address:
Oak Wilt Research, c/o Dr. David Roberts,
MSUE SE Region, 28115 Meadowbrook Rd., Novi, MI 48377-3128
For comments or questions email David Roberts
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