G3590

Oak wilt management—what are the options?Jane Cummings Carlson, A. Jeff Martin and Kyoko Scanlon

Trees at risk

Thousands of oaks in woodlandand urban settings die from oakwilt every year. Widespread inWisconsin, Minnesota andMichigan, the disease is causedby the fungus Ceratocystis

fagacearum. Figure 1 shows the extent ofoak wilt in Michigan, Minnesota andWisconsin.Trees from the white and red oakgroups, both found commonly in theLake States, are susceptible to oak wilt.Because trees in the red oak group fallprey to the disease most often, this pub-lication focuses on the red oak group.

Biology and spread

of oak wiltMats of fungus, known as “pressurepads,” develop under the bark of treesthat have died from oak wilt (example1a). Mats form most often in spring,approximately 9-10 months after a treedies from oak wilt. These mats force thebark to crack open. The fungus pro-duces a sweet odor that attracts sap-feeding beetles such as Nitidulids(example 1b). The beetles pick up fungalspores by crawling on the mats. Thenthey fly to healthy oaks to feed on sapflowing from fresh wounds, thus infect-ing new trees. As fungus invades a tree’s water con-ducting system, the leaves turn dullgreen, bronze or tan, and wilt from thetop of the tree downward (example 1c).Leaves fall rapidly after wilting.Infected trees are bare in 4-6 weeks.Underground spread of oak wilt frominfected to healthy trees occurs throughroot grafts. Most root grafts formbetween oaks of the same species; redoak roots graft more commonly than dowhite oak roots, and grafts between redand white oaks are very rare.

Example 1c.The fungus invades the treeʼswater-carrying system, causing leaves to wiltand fall. Wilting occurs most often in July andAugust, and occasionally in spring or fall.

Example 1b. Fungal mats force the barkto crack open. The matsʼ odor attracts sap-feeding beetles which spread the disease to healthy trees.

Example 1a. In April, May and June,fungal mats (“pressure pads”) grow underthe bark of trees that wilted the previoussummer. Mats are sometimes present inlate summer or fall.Figure 1. This map shows the county dis-

tribution of oak wilt in Michigan, Minnesotaand Wisconsin in 2010. Map produced bythe USDA Forest Service, NortheasternArea–Forest Health Monitoring GIS Group.

Prevention, sampling

and management

Prevention

You can take two precautions to lessenthe chances of oak wilt invading youryards and woodlands. 1. Do not harvest, prune or other-

wise wound oak trees from bud

swell up to two or three weeks

past full leaf development.

During this time, fungal mats aremost abundant and oak trees mostvulnerable to oak wilt. In the urbansetting, remember not to prune oaktrees from April through July. If thespring comes early with unusuallywarm temperatures, pruning mayneed to stop before the beginning ofApril in some years. Infection is lesscommon later in the summer but canoccur after July. If you decide to takea very cautious approach—limit anycutting activities from April 1 toOctober 1. If wounding does occur,

apply a tree wound paint immedi-ately. These products normallyinhibit proper wound closure solimit your use of tree paint to thesesituations.In the forest setting, the WisconsinDepartment of Natural Resourcesdeveloped new oak harvestingguidelines. The guidelines are site-specific, and based on factors thataffect the risk level of introductionand spread of this disease. For moreinformation about oak harvestingguidelines in the forest setting,please visithttp://dnr.wi.gov/forestry/fh/oakWilt/guidelines.asp

2. Do not move infected trees

with the bark still attached (as

firewood or logs) into your

woodlands. Pressure pads mayform on the transported wood,attracting insects that spread oak wilt.

Note: The research on thetransmission of oak wiltthrough tools is minimal.Transmission of the pathogenwas observed via selectedtools through artificial inocu-lation; however no evidenceof natural spread through avariety of tools has been con-firmed. At this time, it doesnot appear necessary to disin-fect the tools.

Sampling

You may need a laboratory analysis toconfirm the presence of oak wilt. Toobtain an analysis, collect three twigs(about 1⁄2 in. diameter and 4 in. long)from three different branches withwilting leaves. You must send sampleswhich still have live tissue. Scratch thesample branch with your fingernail. Ifthe wood under the bark is a light color(white to green), the sample is fresh. Ifthe wood is brown or dark, it is too oldto be useful. Wrap the samples in waxpaper and keep them cool until youmail them. In Wisconsin, mail your samples to:

Plant Disease Diagnostic ClinicDept. of Plant PathologyUniversity of Wisconsin-Madison1630 Linden DriveMadison, WI 53706(608) 262-2863www.plantpath.wisc.edu/PDDC

In Minnesota, send them to: Minnesota Dept. of Agriculture Plant Protection Laboratory90 West Plato St. Paul, MN 55108 (651) 296-4749www.mda.state.mn.us

In Michigan, mail to:State Pathologist Michigan Dept. of AgricultureLaboratory Division 1615 Harrison Rd. East Lansing, MI 48823 (517) 337-5091

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Example 2a. Don’t confuse oak wilt with the

two-lined chestnut borer. Galleries, or feedingtunnels of the two-lined chestnut borer are visible in thisphoto. The insect causes symptoms resembling that ofoak wilt with one notable difference—trees infested withthe chestnut borer usually keep their dead leaves, whiletrees with oak wilt lose them.

Example 2b.This aerial view shows the characteristic pocket ofdead trees, a hallmark of oak wilt.

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A fee may be charged for testing. Callthe appropriate lab to determinecharges.

If sampling results are negative

If your samples test negative for oakwilt, check for the presence of the two-lined chestnut borer which harms treesby tunneling between the bark andwood (example 2a). This insect causesleaves to wilt and turn brown in mid-summer when larvae feed between thebark and wood of infested trees. Deadleaves generally remain on the tree. Afew branches or the whole crown maybe affected. (Oak wilt always affects theentire crown in red oaks.)

If sampling results are positive

There are two management strategiesfrom which you can choose.Passive management. If you allowthe disease to progress, it will spread tohealthy oaks through the roots of dis-eased trees. In the Lake States, oak treesmay grow in pure or relatively homoge-neous stands. You may also find themmixed with species such as white pine,sugar maple, basswood and white birch(common in the northern hardwoodforest type), or hickory, elm, ash, aspen,and black cherry (species common inthe oak-hickory forest type).

In pure oak stands, the disease movesoutward from the original infected treesthrough root grafts, killing more treeseach year. This creates the pocket ofdead trees that characterizes oak wilt. Inmixed stands, where oaks are scatteredamong other species, root grafts mayoccur less often than in pure oak stands;thus, spread may be slower or less con-spicuous. New pockets can appear inyour woodlands from overland spreadby Nitidulid beetles feeding on freshwounds (example 2b). If you allow the disease to progress, youshould consider the impact on thehealth of neighboring trees. Even if root-graft spread to your neighbor’s trees isunlikely, overland spread to a neigh-bor’s yard or woodlot is a real possibil-ity. Proper removal and utilization ofinfected trees reduces the hazard ofoverland spread. You can find guide-lines for removing and using dead andinfected trees in the section on activemanagement. As oaks die, the open spaces and deadtrees can provide valuable feeding andnesting sites for wildlife. When over-story trees die, the site often becomesbrushy for about ten years. Warblers,grosbeaks, cuckoos, cardinals, grouse,rabbits, deer and shrews will beattracted to the brushy area. Browncreepers may nest under the sloughingbark on dead trees. Dead trees willfurnish insects for birds, and largerspecimens may provide perches forraptors.

Example 3a.Thevibratory plow is atracked or rubber-tired vehicle with avibrating headattached at the rear.A knife-like plowblade with a slighthook at the bottom isattached to thevibrating head. Theblade is pulled hori-zontally through thesoil, slicing the rootconnections.

Example 3c. Crown wilt indicates thepresence of oak wilt.

Example 3b. Oak wilt trenching.

Active ma nagement. Since oak wiltspreads both underground and aboveground, you must combat it with a two-pronged approach. To contain thedisease:1. install a root graft barrier; and

2. remove and properly use trees

inside the barrier.

Installing a root graft barrier. Rootgraft barriers break the root connectionsbetween infected and healthy trees. Abarrier should be at least four, and prefer-ably five feet deep. Trenchers and vibra-tory plows are the tools most commonlyused for this job (example 3a, 3b, 3d).Locating the root graft barrier.

Properly locating the root graft barrier iscritical to stopping the undergroundspread of oak wilt. Incorrectly placingthe barrier may cancel out your effortsto halt the disease (figure 2). A forest pest specialist, forester orarborist trained in oak wilt managementshould work with you to plan thebarrier location. Choose a location forthe barrier in early August, after most ofthe year’s infected trees have wilted.Barrier installation should take placebefore the soil freezes.

To determine where to place the barrier,measure the diameters at 41⁄2 feet abovethe ground (or diameter at breastheight, DBH) of an infected tree(example 3c) and a nearby apparentlyhealthy tree; add these figures to calcu-late the combined diameter. A sproutclump is represented as the sum of thediameters of all its stems. Measure thedistance between the same two trees.Consult table 1 to determine theminimum inter-tree distance listed forthe combined diameters.If the two trees are closer than the dis-tance listed in the table, the barriershould be placed outside the apparentlyhealthy tree. If the two trees are fartherthan the distance listed in the table, thebarrier should be placed inside theapparently healthy tree.Research shows that three factors signif-icantly determine the likelihood of twotrees grafting: 1) diameter; 2) distance between the trees; and3) soil and drainage characteristics. In general, root grafts spread oak wiltfarther in shallower than in deeper soils,and in sandier than in loamier soils. Theexact underground location of the oakwilt pathogen is unknown.

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wilt pathogen is unknown.

Figure 2. Based on table 1, tree A must be atleast 34 feet away from the infected tree onloamy sand, and 42.7 feet away on sandy soil,to assume that it has not been infected throughroot graft. Table 1 forecasts with 95% accuracy.Since tree A is farther than the recommendedguidelines for loamy sand, a barrier can beplaced between the healthy and diseased tree.On sandy soil, tree A is less than the recom-mended safe distance, so you can concludewith 95% certainty that the tree has beeninfected and that a barrier must be placedoutside it.

Combined Inter-tree distance

DBH loamy sandy

(inches) sandy sand loam/loam

2 3.9 3.1 2.24 7.8 6.2 4.56 11.6 9.3 6.78 15.5 12.4 8.9

10 19.4 15.4 11.212 23.3 18.5 13.414 27.2 21.6 15.616 31.0 24.7 17.918 34.9 27.8 20.120 38.8 30.9 22.322 42.7 34.0 24.624 46.6 37.1 26.826 50.4 40.2 29.128 54.3 43.2 31.330 58.2 46.3 33.532 62.1 49.4 35.834 66.0 52.5 38.036 69.8 55.6 40.238 73.7 58.7 42.540 77.6 61.8 44.742 81.5 64.9 46.944 85.4 68.0 49.246 89.3 71.1 51.448 93.1 74.1 53.6

Table 1. Distances for loamy sand andsandy soils were based on 95% confi-dence level model of Bruhn et al. (1992);Distances for sandy loam and loam soilswere extrapolated from sandy loam modelat 80% confidence level.

Model developed by Johann Bruhn from data colleted in Menominee County, MI.Bruhn, J.N., and Heyd, R.L., 1992, Biology and Control of Oak Wilt in MichiganRed Oak Stands, Northern Journal of Applied Forestry 9(2): 47-51.

The method shown in figure 2 attemptsto include, inside the root graft barrier,all apparently healthy oaks with a 5% orgreater chance of being infected in thefirst year after installation of the rootgraft barriers. Additional lines closer todiseased trees can be installed in aneffort to save more trees, but suchattempts often fail. This model wasdeveloped for sites with sand andloamy sand soils. Use of the model forroot graft barrier placement on siteswith sandy loam and heavier texturedsoil will predict longer root grafting dis-tances than what actually occurs. Thus,if this model is used on sandy loam orloamy sites, the number of healthy treesremoved to the line increases. The inter-tree distances to use for such sites wereextrapolated accordingly based on arecent study in Minnesota(Arboriculture and Urban Forestry, inpress), and are provided in table 1 whensuch sacrifices are unacceptable to thelandowner.

Removing and using infected trees

You must install root barriers

before trees in the pocket of

wilting trees are removed. Watertension released when live wilting treesare felled permits fluids to move rapidlyto grafted healthy trees beyond theintended barrier. Remove all dead

trees with the bark attached and

all apparently healthy trees from

inside the barrier (all trees over 3 in.in diameter should be removed). Applyan herbicide registered for treating cutoak stumps to prevent sprouting andminimize the chance of roots regraftingacross the barrier. If you do not use anherbicide, sprouts may continue to keepthe disease active in the pocket, posing arisk to trees outside the barrier. Removetrees and treat stumps after placing thebarrier and before the following April(when fungal mats may form). You donot need to remove trees with loosebark since they can no longer producefungal mats.

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WHITE OAKS

Types: White oak, bur oak,swamp white oakOaks in the white oak grouphave greater resistance to oakwilt. They often remain healthyafter surrounding black and redoaks succumb to the disease.Symptoms: Wilted andbronzed leaves appear on scat-tered branches; leaf fall isusually light. More branchesmay die each year until the treedies—or the tree may survive.

RED OAKS

Types: northern red oak,northern pin oak, black oakOaks in the red oak group aremost susceptible to oak wilt.Trees die rapidly after leaveswilt.Symptoms: Leaves wilt fromthe top of the tree downward.Leaf fall is usually heavy andoccurs during and shortly afterleaves wilt. Wilting takes placemost often in July and August.

Example 3d. A trencher is a rubber-tired vehicle with a digging chain attached tothe back. The chain moves along a boom, digging a trench and breaking root graftsas it is pulled through the soil.

white oak

black oak

red oak

bur oak

Firewood. Dead trees with the barkattached should be debarked, chipped,or cut and split to hasten drying. Stackthe cut pieces and place them in the sun;tarp the stack with 4 mil plastic. Placingthe tarped pile in the sun will hastenbark deterioration and loosen the barkquicker. Seal the plastic around the baseto be as airtight as possible to preventinsects from reaching the fungal mats. Ifthe wood is not burned over the winter,leave the tarp on until the end of the fol-lowing summer (approximately oneyear after cutting). Moving infected firewood withoutcareful attention to tarping can con-tribute to overland spread of oak wilt.Wood from dead trees with loose bark,and from apparently healthy trees withno wilting symptoms, does not pose adanger and does not require tarping. Wisconsin has developed a number ofrequirements pertaining to the move-ment of firewood coming into the stateand the movement of firewood withinthe state’s borders. This is to limit thespread or introduction of invasiveinsects and diseases that are easilytransported on firewood. For moreinformation about firewood restrictions,please visit the Wisconsin Departmentof Agriculture, Trade and ConsumerProtection website athttp://datcp.state.wi.us/arm/environment/insects/firewood_restrictions/

Other wood products. Wood frominfected trees may be sold to a sawmillor chipping facility—preferably onewhich is several miles away from thenearest red oak. Advise the purchaserthat the infected trees with attachedbark must be used over the comingwinter. Regenerating oak in the pocket.

Though information on the likelihood ofoak wilt transferring to regeneratedgrowth is sparse, it is clear that thor-ough tree removal and stump treatmentminimize the risk. Planting oakseedlings or encouraging natural oakseeding in the pocket can be tried afteryou remove the infected trees and treatthe stumps. You may need toapply herbicides to sprouts for two tothree years after cutting if sprouts con-tinue to form inside the barrier.Chemical treatment. Fungicideshave recently been developed and maybe added to the management toolbox.Currently available products contain theactive ingredient propiconazol. Thisproduct may be effective in preventingoak wilt when injected into living oaktrees without disease symptoms. If youare considering using chemical treat-ment, contact an arborist specializing inoak wilt management. The cost andintensity of this level of management istypically reserved for high-value urbanoaks.

Herbicides as a tool to stop root graft transmission of oak wilt

Several studies have been conducted totest the effectiveness of various herbi-cides in stopping the transmission ofoak wilt by root grafts in the LakeStates. Three different combinations ofherbicides were tested. Products testedincluded various formulations and com-binations of Garlon 3A, Garlon 4,Arsenal AC, Stalker and Tordon RTU. The results showed that some productswere effective in killing the above-ground portions of the trees; some werenot. None of the products killed rootscompletely in a timely manner. This lackof complete root death indicates herbi-cides may not be effective in stoppingroot graft transmission of oak wilt.There is a similar study in progress inWisconsin and the test sites are beingmonitored. Herbicides continue to be an importanttool for killing or at least weakeningtrees and sprouts inside a trenched area.However, at this printing, herbicidesshould not be presented as a provenmeans of creating effective root graftbarriers.

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© 2010 by the Board of Regents of the University of Wisconsin System doing business as the division of CooperativeExtension of the University of Wisconsin-Extension. All rights reserved. Send copyright inquiries to: Cooperative ExtensionPublishing, 432 N. Lake St., Rm. 227, Madison, WI 53706, pubs@uwex.edu.Authors: Jane Cummings Carlson is a forest pathologist with the Wisconsin Department of Natural Resources, A. Jeff Martinis a retired professor of forestry with the College of Agricultural and Life Sciences at the University of Wisconsin–Madisonand the University of Wisconsin–Extension, Cooperative Extension, and Kyoko Scanlon is a forest pathologist with WisconsinDepartment of Natural Resources, Division of Forestry, Forest Health Protection. Technical advice also from USDA ForestService. Cooperative Extension Publications are subject to peer review.University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture andWisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914, Acts of Congress. AnEEO/AA employer, the University of Wisconsin-Extension, Cooperative Extension provides equal opportunities in employ-ment and programming, including Title IX and ADA requirements. If you need this information in an alternative format,contact Equal Opportunity and Diversity Programs, University of Wisconsin-Extension, 432 N. Lake St., Rm. 501, Madison,WI 53706, diversity@uwex.edu, phone: (608) 262-0277, fax: (608) 262-8404, TTY: 711 Wisconsin Relay.This publication is available from your county UW-Extension office (www.uwex.edu/ces/cty) or from Cooperative ExtensionPublishing. To order, call toll-free: 1-877-947-7827 (WIS-PUBS) or visit our website: learningstore.uwex.edu.Lake States Woodlands: Oak Wilt Management—What are the Options? (G3590) SR-5/2010