How to

NA-PR-03-00

Identify, Prevent, andControl Oak Wilt

United States Prepared by Northeastern AreaDepartment of Forest Service State & PrivateAgriculture Forestry

AuthorsJoseph G. O’Brien, Plant Pathologist

USDA Forest ServiceNortheastern Area State and Private ForestryForest Health Protection

Manfred E. Mielke, Forest Health SpecialistUSDA Forest ServiceNortheastern Area State and Private ForestryForest Health Protection

Dale Starkey, Plant PathologistUSDA Forest ServiceSouthern RegionForest Health Protection

Jennifer Juzwik, Research Plant PathologistUSDA Forest ServiceNorth Central Research StationForest Disease Research Unit

Illustrations byJulie Martinez, Scientific Illustrator

Minneapolis, Minnesota

AcknowledgmentsThe authors thank Dr. Frank Tainter and Dr.David Appel for their reviews of the manuscript.We are also grateful to Steve Cook, Steve Kunde,Linda Haugen, and Jill Pokorny for providinghelpful suggestions concerning content, and toRoberta Burzynski for editing the manuscript.

The U.S. Department of Agriculture (USDA) prohibitsdiscrimination in all its programs and activities on the basis ofrace, color, national origin, sex, religion, age, disability,political beliefs, sexual orientation, or marital or family status.(Not all prohibited bases apply to all programs.) Personswith disabilities who require alternative means for communi-cation of program information (Braille, large print, audiotape,etc.) should contact USDA’s TARGET Center at (202) 720-2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director,Office of Civil Rights, Room 326-W, Whitten Building, 1400Independence Avenue, SW, Washington, D.C. 20250-9410or call (202) 720-5964 (voice and TDD). USDA is an equalopportunity provider and employer.

How toIdentify, Prevent, and ControlOak Wilt

Introduction ................................................... 1

Distribution .................................................... 1Hosts ............................................................. 2

How Infection Occurs: The Disease Cycle of Oak Wilt ................................................... 4 Local spread of oak wilt ............................... 4 Long-distance spread of oak wilt .................. 6

Symptoms ..................................................... 9 Red oak group ............................................ 9 White oak group ........................................ 10 Texas live oak ............................................ 10

Diagnosis ..................................................... 11

Other disorders of oaks ................................ 11 Oak Anthracnose ....................................... 13 Oak Decline ............................................... 13 Twolined Chestnut Borer ........................... 14

Integrated Management of Oak Wilt .............. 14 1. Preventing new infection centers ............ 14 Remove infected trees ............................ 15 Avoid injuring healthy trees ................... 15 2. Controlling existing infection centers ..... 16 Trenching and vibratory plowing ........... 17 Chemical root disruption ....................... 18 3. Chemical control using fungicides ......... 20 Summary of integrated oak wilt management strategies ...................... 20

References .................................................... 21

Glossary ....................................................... 22

Pesticide Precautionary Statement:

Pesticides used improperly can be injurious to humans,animals, and plants. Follow label directions and heed allprecautions on the labels. Store all pesticides in originalcontainers, out of reach of children and foodstuffs. Applypesticides selectively and carefully. Do not apply apesticide when there is danger of drift to other areas.After handling a pesticide, do not eat, drink, or smokeuntil you have washed. Dispose of empty pesticidecontainers properly. It is difficult to remove all traces of aherbicide (weed killer) from equipment. Therefore, toprevent injury to desirable plants do not use the sameequipment for insecticides that you use for herbicides.

NOTE: Some States have restrictions on the use ofcertain pesticides. Check your State and local regula-tions. Also, because registrations of pesticides are underconstant review by the Federal Environmental Protec-tion Agency, consult your county agricultural agent orState extension specialist to be sure the intended use isstill registered.

Introduction

Oak wilt is an aggressive disease that affectsmany species of oak (Quercus spp.). It is one ofthe most serious tree diseases in the EasternUnited States, killing thousands of oaks eachyear in forests, woodlots, and home landscapes.

Distribution

Oak wilt was first identified in 1944. The fungalpathogen that causes the disease, Ceratocystisfagacearum, is thought by most to be native tothe Eastern United States, but difficulty inisolating and identifying the fungus delayedrecognition of the extent of its impact until the1980’s. Some plant pathologists think that oakwilt is an exotic disease, arriving in NorthAmerica in the early 1900’s, but the fungus hasnever been reported from any country otherthan the United States. The disease has alsobecome much more apparent in some localareas since the 1980’s because of increased treewounding, due primarily to home constructionin oak woods. The current known distributionof oak wilt is shown in red in figure 1.

Figure 1.—In 1999, oak wilt was distributed over much of the Eastern United States.

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Hosts

Oaks can be organized into three main groupsbased on leaf shape: red oaks, white oaks, andlive oaks. Trees in the red oak group have fan-shaped leaves with sharply pointed tips; thosein the white oak group have fan-shaped leaveswith rounded or blunt tips; and trees in the liveoak group have oval leaves with rounded tips(fig. 2). Oaks most commonly killed by thedisease are listed in table 1.

All species in the white oak group are moder-ately resistant to oak wilt, but if infected, treescan be killed over a period of one to severalyears. Resistance in white oaks appears to berelated to characteristics of physiology andmorphology. Upon wounding, infection, or as apart of the natural aging process, white oakstend to form minute plugs called tyloses intheir sapwood vessels. These plugs make thewood of white oaks impermeable to water andappear to prevent the fungus from movingthroughout the vascular system of the tree.

The tendency for white oaks to form tyloses alsoexplains why these are the species of choice forwood used in cooperage for storing wine andwhiskey. The presence of tyloses ensures thatbarrels made from white oak wood will not leak.

Throughout the range of oak wilt in the UnitedStates, red oaks are the most important hosts,but susceptibility varies somewhat by species.Mortality in red oaks can occur within 3 weeksafter infection by the oak wilt pathogen undersome circumstances. Recovery from oak wiltinfections in red oaks can occur, but is rare.Texas live oak (Q. virginiana) is moderatelysusceptible to the disease, but because of itstendency to form large, root-connected clonesthrough which the disease can spread, it is alsoconsidered to be an important host.

Although the disease is not known west ofTexas, inoculation studies have shown thatmost oaks in the red oak group, including

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A. White oaks

Bur oak

White oak

B. Red oaks

Northern pin oak

Northern red oak

Texas red oak

C. Live oaks

Texas live oak

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Figure 2.—The three main groups of oaks are organized by leaf shape.

several western species, are susceptible to thedisease, and are at risk should the fungus everbe transmitted to them in their native habitat(Appel 1994).

How Infection Occurs: The Disease Cycleof Oak Wilt

The oak wilt fungus moves from tree to tree intwo ways: underground through the roots oroverland by insect vectors.

Local spr ead of oak wilt

Most new tree infections occur as a result of thefungus moving from an infected tree to a nearbyhealthy tree through connected root systems, aprocess called “local spread” (fig. 3, lowerpathway). The roots of trees in each oak groupcommonly graft to roots of other trees in the

Common name(s) Scientific name

Northern speciesBlack oak Q. velutinaBur oak2 Q. macrocarpaNorthern pin oak Q. ellipsoidalisNorthern red oak Q. rubraWhite oak2 Q. alba

Southern speciesBlackjack oak Q. marilandicaScrub live oak Q. fusiformisShumard oak Q. shumardiiSouthern red oak Q. falcataTexas live oak Q. virginianaTexas red oak (Spanish oak) Q. buckleyi

1 All red oaks in the Eastern United States are consid-ered susceptible to oak wilt.2 Infections are less common in these species and maytake years to run their course.

Table 1.—Oak species commonly killed by oak wilt1

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Figure 3, upper pathway.—Long-distance spread of oak wilt occurs when nitidulid beetles carry spores of the fungus from spore mats on infected trees to wounds on healthy trees, causing infection and death of the tree. Time from infection to mortality may be very short for red oaks and Texas live oak, or many years for members of the white oak group.

Figure 3, lower pathway.—Local spread of oak wilt occurs when the fungus travels through the interconnected roots of infected and healthy trees.

Figure 3.—The disease cycle of oak wilt.

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same group, forming a continuous undergroundnetwork. When one tree in a group becomesinfected and dies, the fungus spreads throughthe connected root systems, killing more treesand creating an “infection center.”

Root grafts do not commonly occur betweentrees of different species groups, althoughexceptions occur. Usually a mix of species in agiven location will retard local spread and limitthe impact of the disease. However, root graftsoften do occur between Texas live oaks and redoaks in mixed stands.

Depending upon soil type and the mix of treespecies in a forest or yard, infection of healthytrees through root grafts can occur at somedistance (up to 100 feet or more) from an in-fected tree. Sandy soils are conducive to theformation of widespread root systems, increas-ing the likelihood of root grafts occurring fartheraway from a diseased tree. Some oak species,including northern pin oak and Texas live oak,often grow in large groups of similar-aged treesthat share a common root system. Such situa-tions can lead to rapid expansion of oak wiltcenters if even one tree in the group becomesinfected.

Long-distance spread of oak wilt

New infection centers can occur if the fungus iscarried from an infected tree to a fresh woundon a healthy tree by an insect, a process called“overland spread” (fig. 3, upper pathway).

Under certain moisture and temperature condi-tions, compact masses of spore-producingfungal material, variously called “spore mats,”“spore pads,” “pressure mats,” or “pressurepads” are sometimes formed on oak trees thathave been killed by oak wilt (fig. 4). These matsform just under the bark, in contact with boththe bark and the infected sapwood of the tree.As the mats mature, they produce specializedstructures that exert outward pressure on the

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bark (the “pressure pads”) and cause it to split,providing a route for insects to reach the mats(fig. 5).

Oak wilt spore mats emit a strong fruity orwine-like odor that attracts many differentspecies of nitidulid beetles (fig. 6), also knownas sap beetles. As they feed on or tunnelthrough the spore mats, nitidulid beetles oftenaccumulate fungal spores on the surface oftheir bodies.

Oak trees often sustain wounds caused byconstruction equipment, storms, pruning tools,or vandalism. Fresh wounds usually leak sap.The sap attracts insects, including nitidulidbeetles that have visited oak wilt spore mats.The overland movement of nitidulid beetles fromspore mats on infected trees to wounds onotherwise healthy trees thus creates most newinfection centers.

Not all nitidulid beetles are vectors of the oakwilt pathogen. In the North, nitidulids in thegenera Carpophilus, Colopterus, and Epurea aremost often associated with both oak wilt sporemats and fresh wounds on healthy oaks. Thecommon picnic beetle in the genusGlischrochilus (the larger beetle in fig. 6) hasoften been implicated in the oak wilt diseasecycle, but does not appear to be an importantvector of the disease.

Spore mats can form only within a year aftertree death, and only when air temperature andwood moisture are within a certain range. In theNorthern United States, this combination ofwood moisture and temperature commonlyoccurs in spring of the year after the tree dies orsometimes in autumn of the year the tree dies.The period of time during which mats areformed increases with decreasing latitude. InTexas, mat formation occurs at any time duringthe year, but is most common in late fall andwinter when the weather is cooler and wetter.

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Figure 4.—Bark section showing an exposed spore mat on both surfaces.

Figure 5.—Bark split caused by pressure pad of the oak wilt fungus.

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Spore mats usually do not form on trees smallerthan 6 inches in diameter at breast height,although smaller trees can occasionally supportmats. In Texas, spore mats are formed only onTexas red oak and blackjack oak, and never onTexas live oak. For this reason, the red oaks areimportant for establishing new infection centersin Texas.

Another group of insects, oak bark beetles (notpictured), can also carry spores of the oak wiltpathogen and help to create new infectioncenters. These beetles acquire spores of thefungus while feeding on infected branches, andsubsequently transmit them when feeding onhealthy trees.

Symptoms

Oak wilt disease symptoms progress differentlyin red oaks, white oaks, and Texas live oak.

Red oak group

Oak wilt is usually identified in red oaks by thesymptoms of rapid leaf discoloration and wilt-ing. Often the initial symptom is a subtle off-green color that may be visible in the upper

Figure 6.—Nitidulid beetles are primarily respon- sible for overland spread of oak wilt.

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portion of the tree crown. This symptom isapparent in the northern part of the diseaserange in late June to early July. Shortly afterthis initial color shift, the leaves begin to wiltfrom the top of the crown downward. As thedisease progresses, individual leaves quicklydiscolor, taking on a “bronzed” appearance. Thediscoloration progresses around the margins ofthe leaf from the tip to the base (fig. 7B). Theprogressing discoloration may be interrupted bythe leaf veins, as shown in the white oak leaf infigure 7A, or may affect the entire upper portionof the leaf, as shown in the red oak leaf in figure7B.

Leaves are cast rapidly as the infectionprogresses. Commonly, infected trees are almostentirely defoliated within a few weeks of symp-tom onset. Fallen leaves usually are brown atthe tips and margins, and sometimes green atthe base and along the lower veins. Trees areoften killed in groups or disease “centers,” wheninfection occurs through grafted roots.

Occasionally the outer ring of vessels of dis-eased trees will be plugged with a brown sub-stance that may be visible in cross sections as aring or a series of dark spots through the outersapwood, and in tangential cuts as longitudinalstreaking of wood exposed after removing thebark. However, this is not always obvious to anuntrained observer, especially in the red oaks.The discoloration may be very light or appear asflecks in such sections. Discoloration is mostreadily seen in tangential cuts on branches.

White oak group

White oaks usually die slowly, one branch at atime, over a period of one to many years. Wiltingand death of leaves on individual branchesoccurs in a similar fashion to the disease in redoaks, but usually progresses much more slowly.Affected leaves exhibit a pattern of discolorationsimilar to that seen in red oaks, with discolora-tion proceeding from the margins to the base,

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sometimes interrupted by the leaf veins (fig. 7A).Brown streaking in the outer growth rings isoften readily apparent, even to an untrainedobserver, in infected white oaks and bur oaks,but may be missing.

Texas live oak

Texas live oaks can wilt and die rapidly orslowly, depending on the timing of infection andweather conditions, but generally succumbwithin 1-6 months of infection. Diagnostic leafsymptoms are usually produced somewhere onthe tree (especially in spring and fall). Leavesdevelop yellow (“chlorotic”) veins that eventu-ally turn brown (“necrotic”), a symptom termedveinal necrosis (fig. 7C). Affected leaves fall,and the tree crown progressively thins out untilthe entire tree is dead. Fallen leaves under thetree may show darker brown veins for months.Sometimes just the tips, margins, or interveinalportions of leaves will turn yellow or brown, butthese symptoms are not necessarily the result ofoak wilt, and not as useful in diagnosing thedisease. A small percentage of Texas live oaksmay survive oak wilt infection indefinitely, whilesuffering varying degrees of crown loss.

Diagnosis

Accurate diagnosis of oak wilt is essential beforecostly control efforts are begun. Foresters,arborists, or pathologists experienced with oakwilt can often diagnose the problem in the fieldusing host species, symptoms, and mortalitypatterns. Properly sampling suspect trees andculturing in a qualified laboratory may benecessary in some cases. See the publication“How to Collect Field Samples and Identify theOak Wilt Fungus in the Laboratory” for addi-tional information.

Other Disorders of Oaks

These oak disorders may sometimes be con-fused with oak wilt: anthracnose, decline, andinfestation with twolined chestnut borer.

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Figure 7.—Symptoms of oak wilt in A. white oak, B. red oak, and C. Texas live oak.

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A

B

C

Oak anthracnose

Trees in the red or white oak group are suscep-tible to a group of fungal leaf diseases collec-tively called “anthracnose,” which may belocally severe if weather conditions in the springare cool and wet for an extended period duringleaf expansion, which favors development ofsuch diseases. Anthracnose diseases are usu-ally more severe in the lower portions of thecrown, often causing the affected leaves to fallearly in summer. Leaves in the upper portion ofthe crown may remain attached. Leaf symptomsusually include brown spots or patches thatexpand outward to the leaf margins. Althoughthe leaves may be curled and distorted, theyusually do not wilt.

Oak decline

Oaks throughout the Eastern and SouthernUnited States are susceptible to a “decline”syndrome, which is defined as a disease causedby the interaction of several injurious agentsworking simultaneously. In the case of oakdecline, these factors can include drought,defoliation, fungi that cause stem cankers orroot diseases, and wood-boring beetles. Theinteraction of these factors may result in thedecline and death of oak trees over a local orregional area.

Symptoms vary greatly, and differentiating oakdecline from oak wilt can be quite difficult. Theabsence of the typical leaf symptoms of oak wiltand the retention of dead leaves on the tree areindicators of oak decline. Trees killed by oakwilt are usually completely defoliated and retainfew living or dead leaves, but this may not be aconsistent symptom for red oaks in the South.

Oak wilt usually occurs in discrete, spreadingpockets of mortality, with trees on the marginsof the infection center becoming infected overtime. Declines may occur in discrete pockets, orover a fairly large area, but do not typicallyspread outward from an initial infection center.

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Trees in decline may die over a period of yearsor may ultimately survive a decline episode withonly dead branches in the crown.

Twolined chestnut borer

The twolined chestnut borer (Agrilus bilineatus)is an insect that attacks oaks, especially thoseweakened by drought or defoliation. Larvae ofthe insect make tunnels in the inner bark,causing branch or even tree mortality. Symp-toms usually begin in the upper portion of thecrown and proceed downward, but this patternis variable. The insects leave a distinctive “D-shaped” exit hole about 1-2 mm in diameterwhen they mature and leave the tree. Initialsymptoms are usually single or scattered dyingbranches that often retain brown leaves untilautumn.

Integrated Management of Oak Wilt

Oak wilt can be managed by a variety of strate-gies that prevent new infection centers and limitthe expansion of existing infection centers. Agood management program for oak wilt willinclude all of these strategies for combating thedisease.

1. Preventing new infection centers

Once an oak tree becomes infected with oakwilt, there is no known chemical treatment thatis capable of “curing” the disease; however,fungicide research is continuing. The develop-ment of new oak wilt pockets can be avoided,however, either by preventing the developmentof spore mats of the fungus on diseased trees orby preventing the transfer of fungal spores bybeetles to healthy trees. In practice, this in-volves removing dead or diseased trees andavoiding injury to healthy trees.

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Remove infected trees

Trees that are infected with or have died fromoak wilt should be removed and properly treatedto prevent development of spore mats. Thesetreatments include debarking, chipping orsplitting, and drying the wood. Covering deadwood with plastic, burying the edges for 6months, and then air-drying for a similar timewill kill the fungus and any associated insects.Trees that die in summer should be removedand treated before the following spring, whennew spore mats can develop. If the wood issufficiently dried, however, spore mats will notdevelop.

A word of caution: Removing a diseased treethat is still living may actually spread theinfection by accelerating the movement of thefungus into adjacent trees that are grafted to itby the roots. To avoid this problem, beforeremoving living diseased trees, disrupt intercon-nected roots as described in the section on“Controlling existing infection centers.”

Avoid injuring healthy trees

Trees with fresh wounds outside existing oakwilt centers are visited by beetles transportingspores of the fungus. Because open woundscreate avenues for infection, damage to treesfrom construction, pruning, or severe stormsmay lead to new infection centers. Avoid injuryto oaks during favorable conditions for infection,which occur in spring and early summer in theNorth, when spore mats are present and thebeetles are flying. Favorable conditions usuallyoccur between April 15 and July 1 in the LakeStates, and over a correspondingly longer periodof time to the South. In Texas, avoid damage tooak trees from February through June.

Preventing injury caused by human activity isespecially effective in avoiding the establishmentof new infection centers. In particular, pruning

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or construction activities in or near oakwoodlots during the susceptible period oftenresult in injury to oak trees that leads to infec-tion.

If construction activity or pruning are unavoid-able, or if storms injure oak trees during thecritical period, the wounds should be treatedimmediately with a commercial tree paint orwound dressing. Tree paints are normally notrecommended for general use, but in thisinstance, use of these products can protecttrees from oak wilt. In the North, if trees arewounded during the dormant season tree paintsare not necessary, but judicious use during therest of the year is acceptable. From Missouri toTexas, tree paint should be used immediatelyafter trees are wounded at any time of the year.

2. Controlling existing infection centers

Once the oak wilt fungus becomes establishedin a stand that includes a high proportion ofoak, it will often continue to spread through thegrafted root systems of the trees, causing infec-tion in healthy oaks.

Disrupting the connections between roots ofinfected and healthy trees limits the spread ofoak wilt and is an effective control measure.Infected trees and their roots will usually diebefore root grafts can be reestablished. Thefungus that causes oak wilt does not survive inthe root systems of dead trees for more than afew years.

The potential for spread of oak wilt throughgrafted roots is especially high after a diseasedtree is removed or dies. While a diseased tree isstill living and intact, there is some resistanceto fungal spores moving through root grafts intoroots of healthy trees. Either removal or deathof a diseased tree removes this natural resis-tance to spore movement, and spores may thentravel more freely through interconnected roots.Therefore, timing of root disruption treatment is

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critical. Roots should be disrupted before aninfected tree dies or is removed, or within ashort time of tree death for maximum protectionof healthy trees.

Interconnected root systems can be disruptedwith a trencher, vibratory plow, or other equip-ment.

Trenching and vibratory plowing

Cutting roots by using a trenching or cuttingtool effectively controls the expansion of oak wiltpockets. In the Lake States, using a vibratoryplow with a 5-foot blade (fig. 8) is the mostcommon method of disrupting grafted rootsystems. The vibratory plow consists of amechanical shaker unit with an attached bladethat is pulled behind a tractor. The blade pen-etrates to a depth of about 5 feet and cutsthrough the roots of oaks that may be graftedtogether. While oak roots may extend deeperthan 5 feet in the soil, most root grafts aredisrupted by trenching or plowing to that depth.Standard trenching tools do considerably moredamage to the site, and the result is a muchmore apparent plow line than that caused bythe vibratory plow. In Texas, shallow, rocky

Figure 8.—The vibratory plow is sometimes used to disrupt grafted roots to prevent local spread of oak wilt.

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soils and even layered rock often make the useof a rock saw necessary for disrupting oakroots. A chain trencher, backhoe, or ripper barcan sometimes be used. Trench depth should beat least 3 feet, although this may be difficult toachieve.

The lines cut by these trenching implements areusually referred to as “barrier lines” (fig. 9).Successful disruption of root grafts to protecthealthy trees close to an oak wilt infectioncenter often requires that two or more parallelor intersecting lines be made. Primary barrierlines are those expected to have a good chanceof protecting trees outside the lines. In addition,secondary barrier lines are often used to helpensure that the root graft disruption is effective(fig. 9).

The efficacy of root graft disruption can beenhanced by removing all oak trees inside thebarrier lines following plowing or trenching.Removing these trees and optionally treating thestumps with an herbicide helps to ensure thatall of the oak roots inside the barrier will diebefore root grafts can be reestablished. Thispractice is sometimes referred to as “cut to theline.” Although this is a radical treatment, itmay be useful in areas where oak wilt eradica-tion is the goal. Assume that all trees removedare infected with the oak wilt fungus, anddestroy or treat them on site.

Chemical root disruption

Biocidal chemicals have been used in the pastto disrupt root grafts in trees, including oaks.These chemicals are very dangerous and diffi-cult to work with, but can sometimes be used inareas where vibratory plowing or trenching isnot an option because of buried utilities, septictanks, or steep slopes. Holes are drilled into thesoil at prescribed intervals, and the chemical ispoured into the holes, where it diffuses into thesoil and kills the roots in a localized area. Thesechemicals are restricted-use pesticides; they

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Figure 9.—This plan of a home landscape shows possible locations of vibratory plow lines for control of oak wilt. All trees are oaks. Brown trees are infected or dead; green trees are healthy. The solid red lines indicate primary barriers; the dotted lines are secondary barriers.

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must be applied by a licensed pesticide applica-tor, who has been trained in their use. In addi-tion, these chemicals are costly, may causedamage to the trees, and are effective only inuniform-textured soils where the chemicaldistribution is even and predictable.

3. Chemical control using fungicides

Fungicides have been developed that may beeffective in preventing oak wilt when injectedinto living oak trees without active symptoms.These fungicides are apparently unable to stopan already infected oak tree from dying. Thosecurrently available utilize some form of a chemi-cal called propiconazol in the formulation. Suchtreatments create their own problems, includingthe necessity of wounding the tree to inject thefungicides.

The cost of the fungicide is high, so only high-value trees should be considered for treatment.Contact your county extension office for currentadvice on the use of chemicals for control of oakwilt.

Summary of integrated oak wilt managementstrategies

Effective oak wilt management programs use avariety of strategies to limit the spread of oakwilt. Some of the practices and policies that canbe used in combination to effectively manageoak wilt include the following:

Avoid wounding oaks during critical infec-tion periods.

• If pruning is necessary, or if wounds occuron oak trees during the critical infectionperiod, use tree wound dressings or paintsto prevent transmission of oak wilt.

• Develop and enforce construction ordi-nances and utility pruning guidelines thatminimize wounding of oak trees.

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• Use public service announcements, bill-boards, and flyers to raise awareness of thedangers of wounding oaks during suscep-tible periods.

Use vibratory plow line, trench barriers, orchemical disruption of roots to isolate pock-ets of oak wilt.

• Communities and neighbors should jointogether to lower the cost of these tools andachieve more complete and effective localcontrol.

• Use root graft disruption, cut-to-the-linepractices, and treat stumps with herbicidesto greatly reduce or eradicate oak wilt inlocal areas.

Remove and properly treat oaks killed by oakwilt by debarking, chipping or splitting, anddrying the wood before the spring followingthe tree’s death.

• Do not move infected wood off-site withoutdebarking, chipping, or properly drying it.Do not move or store firewood from infectedstands near healthy oaks without propertreatment.

• Use and enforce city codes and ordinancesthat mandate removal and treatment of deadoak trees. Such ordinances can significantlyreduce the chances for long-distance trans-mission of oak wilt.

References

Appel, D.N. 1994. The potential for a Californiaoak wilt epidemic. Journal of Arboriculture20(2): 79-86.

Appel, D.N.; Cameron, R.S.; Wilson, A.D;Johnson, J.D. 1995. How to identify andmanage oak wilt in Texas. How-to SR-1. NewOrleans, LA: U.S. Department of Agriculture,Forest Service, Southern Research Station. 7 p.

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French, D.W.; Juzwik, J. 1999 (revised). Oakwilt in Minnesota. Ext. Publ. MI-3174-GO. St.Paul, MN: Minnesota Extension Service. 5 p.

Pokorny, J. 1999. How to collect field samplesand identify the oak wilt fungus in the labora-tory. NA-FR-01-99. St. Paul, MN: U.S. Depart-ment of Agriculture, Forest Service, Northeast-ern Area State and Private Forestry. 12 p.

Glossary

AnthracnoseA fungal disease of the leaves of many treespecies. On oak, this disease may be con-fused with oak wilt.

Chlorosis (chlorotic)A loss of green color in leaves that results ina yellow discoloration.

Cut-to-the-lineThe practice of removing all oaks, living ordead, inside the primary barrier line whenusing trenching for controlling local spread ofoak wilt.

Infection CenterA localized group of trees that has beenaffected by a disease. The disease mayspread from the margins of the infectedgroup.

Local SpreadSpread of oak wilt from diseased to healthytrees through root grafted, interconnectedroot systems.

Long-distance SpreadSpread of oak wilt by nitidulid beetles fromspore mats on infected trees to open woundson healthy trees. Oak bark beetles can alsofacilitate the spread of oak wilt.

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Nitidulid BeetlesBeetles in the family Nitidulidae, sometimescalled sap beetles. These beetles have beenimplicated as the primary carriers of fungalspores of the oak wilt pathogen to healthytrees.

Necrosis (necrotic)A brown discoloration of leaves that indicatesdead tissue.

Overland Spread(See “Long-distance Spread.”)

Pressure Mat, Pressure Pad(See “Spore Mat,” Spore Pad”)

Primary Barrier LineA trench cut to disrupt grafted roots of oak. Iftwo lines are used, the primary barrier line isthe one expected to have the better chance toprotect trees outside the line (see “SecondaryBarrier Line”).

Root GraftRoots that have grown together so that agraft union is made between the conductingtissues of both roots. The oak wilt pathogencan move through grafted roots betweeninfected and healthy trees to cause newinfections.

Secondary Barrier LineA trench cut to disrupt grafted roots of oak. Iftwo lines are used, the secondary barrier lineis the one expected to have the lesser chanceto protect trees outside the line (see “PrimaryBarrier Line”).

Spore Mat, Spore PadA structure produced by the oak wilt fungusat the bark-wood interface in oaks killed bythe disease. Development of the structurecauses the bark to split, exposing the matbelow. The mat is covered with the spores ofthe oak wilt pathogen, which are picked upby visiting nitidulid beetles.

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TylosesMicroscopic structures that are produced inthe conducting vessels of white oaks, whichblock the movement of water and fungalspores within the tree. The rapid develop-ment of tyloses may explain the difference insusceptibility between white oaks and redoaks.

VectorAn organism such as an insect, mite, nema-tode, or a higher animal such as a bird orrodent that carries a pathogenic agent to asusceptible host.

Veinal necrosisDark yellow or brown discoloration thatoccurs along the veins of leaves.

Vibratory PlowA shaker unit with a 5-foot blade pulledbehind a tractor, that is used to disrupt thegrafted root systems of oaks to preventspread of oak wilt.

Companion Publication

Pokorny, J. 1999. How to Collect Field Samples and Identify the Oak Wilt Fungus in the Laboratory. NA-FR-01-99. St. Paul, MN: U.S. Department of Agriculture, Forest Service, Northeastern Area State and Private Forestry. 12 p.

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Northeastern Area State and PrivateForestry Offices:

Headquarters OfficeNortheastern Area State and Private ForestryUSDA Forest Service11 Campus Boulevard, Suite 200Newtown Square, PA 19073

Durham Field OfficeNortheastern Area State and Private ForestryUSDA Forest ServiceLouis C. Wyman Forest Services LaboratoryP.O. Box 640Durham, NH 03824-0640

Morgantown Field OfficeNortheastern Area State and Private ForestryUSDA Forest Service180 Canfield StreetMorgantown, WV 26505-3101

St. Paul Field OfficeNortheastern Area State and Private ForestryUSDA Forest Service1992 Folwell Ave.St. Paul, MN 55108-1099

North Central Research Station:North Central Research StationUSDA Forest Service1992 Folwell Ave.St. Paul, MN 55108-1099

Forest Service Southern Region State andPrivate Forestry Offices:

Headquarters OfficeSouthern RegionState and Private ForestryUSDA Forest ServiceForest Health Protection1720 Peachtree Road, NWRoom 862 SAtlanta, GA 30367-9012

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Asheville Field OfficeSouthern RegionState and Private ForestryUSDA Forest ServiceForest Health ProtectionAsheville Field OfficeP.O. Box 2680Asheville, NC 28802

Pineville Field OfficeSouthern RegionState and Private ForestryUSDA Forest ServiceForest Health ProtectionPineville Field Office2500 Shreveport Hwy.Pineville, LA 71360

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Notes:

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For further information, contact:

How To Identify, Prevent, andControl Oak Wilt

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Publications and Other Info.