Research

Trench Inserts as Long-term Barriers to Root Transmissionfor Control of Oak Wilt

A. D. Wilson and D. G. Lester, USDA Forest Service, Forest Insect and Disease Research, Southern Research Sta-tion, Center for Bottomland Hardwoods Research, Southern Hardwoods Laboratory, Stoneville, MS 38776-0227

ABSTRACTWilson, A. D., and Lester, D. Ci. 2002. Trench inserts as long-term barriers to root transmissionfor control of oak wilt. Plant Dis. 86: 1067-1074.

Physical and chemical barriers to root penetration and root grafting across trenches were evalu-ated for their effectiveness in improving trenches as barriers to root transmission of the oak wiltfungus in live oaks. Four trench insert materials were tested, including water-permeable Typarand Biobarrier, and water-impermeable Geomembranc of two thicknesses. Systemic fungicidetreatments of trees immediately outside of trenches also were tested. In the first several yearsfollowing trench installation, an abundance of small adventitious roots commonly formed fromroots sevcrcd by trenching. These roots provided opportunities for initiation of root grafts acrosstrenches in subsequent years. Although trench inserts did not significantly improve trenchesduring the first 3 years following trench installation, water-permeable inserts did effectivelyimprove the performance of trenches beyond the third posttrenching year, when trenches arenormally effective, and extended trench longevity indefinitely. The water-permeable insertswere more effective root barriers because they did not direct root growth from the point of rootcontact. The water-impermeable materials, however, did tend to direct root growth around thesebarriers, leading to the development of new root graft connections and associated oak wilt roottransmission across the trench. The additional cost of trench inserts above trenching costs wasjustified in urban and rural homestead sites, where high-value landscape trees required moreprotection and additional retrenching costs were avoided.

Additional keywords: Cer-trtoc~stis egucrcrrunz, cultural control, propiconazole, Quercus ,fic,ri-,fbnnis, Quer-cus ~i~~inirrrra, tritlurahn herbicide

Oak wilt, caused by Cmtrocystis jir-,~trcetrr~~r~~ (T.W. Bretz) J. Hunt, is a majorvascular wilt disease that continues toshape the ecology of hardwood forest eco-systems of the eastern United States. Sinceoak wilt was first discovered in Wisconsinin 1942 (I), it has been considered bymany to be the most serious disease of oak(Q~KXXS sp.) in North America (2,16,20).The oak wilt fungus is potentially the mostdestructive of all forest pathogens becausefew phytopathogenic microbes havegreater capacity to kill their tree hosts withsuch rapidity (16,23,3X). The impact of thedisease on oak forests in the United States

Publication no. D-2002-0809-01 RThis article is in the public domain and not copy-rightable. It may be freely reprinted with custom-ary crediting of the source. The American Phyto-pathological Society, 2002.

has been exacerbated by changes in foreststand composition and forest managementpractices that have resulted in oak standswith greater proportions of susceptible redoak species ( 16,29).

Oak wilt probably was first observed inTexas in the 1930s within the Hill Countryor Edwards Plateau region. Unusually highlive oak mortality was reported during thisperiod in the Austin area (24,25). Thesemievergreen live oaks, Qrrercus $&

,fortni.s Small (plateau live oak) and Quer-cus vir;i’itGnrm Miller (coastal live oak), areconsidered the most valuable woodlandand urban tree species in central Texas(2 1). It appears that populations of the oakwilt fungus gradually developed withinsusceptible live oak stands over the next 40years until they reached critical mass in the1970s (29). The oak wilt epidemic thatensued continues to cause increasinglydevastating losses to oak resources inTexas. Cumulative economic losses hith-erto have been very conservatively esti-mated to be in the hundreds of millions ofdollars statewide (2930). Such losses arceasily rationalized given that a single largelive oak can be worth up to $20,000 toresidential property values in metropolitanareas ( 13). Confirmed diagnoses of the

disease have been reported in oaks from atleast 61 of 254 Texas counties.

The practice of mechanically cuttingroot connections to control root transmis-sion of the oak wilt fungus has been rec-ommended for many years (18). Trenchingto sever root connections between healthytrees in advance of the visible expandingedge of infection centers has long been thecornerstone of oak witt suppression effortsboth in Texas and in midwestern stateswith active control programs (29). Since1988, the Texas Forest Service has admin-istered the Texas Oak Wilt SuppressionProject (TOWSP), which has installed over650,000 m of trench to combat this disease(4). Trenching has been a particularly im-portant tool for dealing with the disease inhighly valued live oak stands because rootgrafts result in exlensively interconnectedroot systems. This tendency is furthercompounded by the growth habit of liveoaks in forming root sprouts from mothertrees that often give rise to large clusters ofclonal trees or “motts” with common rootsystems (12,22,27). These natural growthtendencies increase the predisposition oflive oaks to root transmission and haveoften resulted in dramatic mortality oververy large areas. Red oaks such as Texasred oak or Spanish oak (Q. ttr~~lnu Buckley= Q. buckleyi Dorr & Nixon), blackjackoak (Q. ttmrilrttzrlica Milnchh.), and Shu-mard oak (Q. .shumurdii Buckley) are themost susceptible species to oak wilt inTexas, but disease incidence is greater inlive oaks due to their growth-form predis-positions and shallow, extensive root sys-tems. The higher incidence of the diseasein live oaks also may be attributed to theirabundance in both urban and rural forestsof central Texas. The predominance of liveoaks has resulted from landscape-management practices involving fire sup-pression, preferential thinning, overgraz-ing, and selective protection of live oaks inthis region (2,7,12).

A 7-year study was initiated in 1993 toevaluate the efficacy of adding physicaland/or chemical barriers to trenches forlong-term control of root transmission ofC. ,fir~ctcrrrrrrm in live oaks. The primaryobjectives of this study were to (i) test theusefulness of trench insert materials inpreventing root penetrations and the devel-opment of new root graft connections

Plant Disease / October 2002 1067

across trenches, (ii) evaluate fungicide(.propiconazole) treatments of trees imme-diately outside of trenches in preventingroot transmission of the oak wilt fungus,and (iii) assess the capacity of all thesebarriers to provide long-term control ofoak wilt root transmission. Since the timerequired for maturation of this study was

unknown, interim results providing peri-odic assessments of barrier performancewere reported previously (32-36).

MATERIALS AND METHODSResearch site and plot installation.

Research was conducted on the l,lOO-haCircle C Ranch Land Development Tractlocated at the southern limits of Austin,TX, in Travis County. Plots were estab-lished in a mature natural stand of liveoaks growing near a residential develop-ment site with a predominantly rocky,sandy clay-loam soil. Soil depth to bedrockranged from 1.0 to I .7 m at the test site.Test trees were selected approximately 25to 30 m beyond the expanding edge of alarge oak wilt infection center, previouslydetermined to be a minimum buffer zone(15,29). A roughly linear trench, estab-lished 27 July 1993, was cut approximately1.6 km long and 1.5 m deep with a Ver-meer turbo II trencher immediately adja-cent to test trees and between test trees andthe infection center. Selection of trenchdepth was based on soil depth, insert avail-ability, and root penetration of soil (27,37).The experimental design consisted of 18

sequential plots approximately 46 to 157 mlong containing 12 to 18 test trees eachsituated along the full length of the trench.Trees within research plots were mappedfor spatial calculations using a Criterion400 survey laser (Laser Technology, Inc.,Englewood, CO) and sequential-targetmapping algorithms with polar Y plottingmethods (28). Seven barrier treatmentswere applied to separate plots on 13 De-cember 1993 in a completely randomizedlinear order along the trench with threereplicate plots per treatment. The treat-ments included trenches with one of fourtrench inserts, no insert (trench only),trench with fungicide treatments of testtrees outside of the trench, and no trench asuntreated controls (Fig. 1). The threetrench-only plots were established as semi-circular bubbles around no-trench seg-ments to maintain continuity to the trenchbarrier. Four trench-insert materials weretested, including water-permeable Typarpolypropylene spunbonded fabric at 4 oz.(1 x) weight; Biobarrier or Typar withtritluralin-impregnated I O-mm-diameter,controlled-release hemispherical pellets(54% polyethylene, 18% carbon black, and28% trifluralin by weight) bonded to poly-propylene fabric with uniform 3%cmspacing or 688 pellets per square meter(Reemay Inc., Old Hickory, TN); and wa-ter-impermeable polyethylene Rufco Ge-omembrane liners (Raven Industries,Springfield, OH) of two thicknesses (20and 30 mil). Trench inserts were placed

into trenches in 15.2- or 30.5-m lengths,mounted with 15-cm steel or aluminumpins to the wall of the trench on the sideclosest to the infection center, and addi-tionally supported by backfilling the trenchwith soil removed during construction ofthe trench, followed by leveling with abackhoe scoop blade (Fig. 2A to E).

Individual live oak trees within fungi-cide-treated plots received one of fourfungicide applications: high-volume boleinjections, low-volume bole injections withtwo types of microinjectors, and soil appli-cations. All four fungicide applicationmethods utilized the microencapsulated(blue) 14.3% EC formulation of propi-conazole (Alamo) without xylene. Fungi-cide treatments were applied 23 to 27 Au-gust 1993 in a completely randomizedlinear sequence within fungicide-treatedplots, each containing two to three repli-cate trees per treatment. All bole-injectionmethods applied the fungicide under pres-sure at 1.5 psi through injection ports, oneport per 15.4 cm of tree circumference.High volume injections utilized a Turfcomodel 490 Injector (Turf Industries, Inc.,Austin, TX) pressurized with CO? andconnected in a continuous series around thebole with tygon tubing and polyethylene T-injection ports. The ports, inserted into 7-mm holes drilled into exposed root flaresapproximately 10 to 12 cm below the soilsurface, were used to apply the fungicide ata rate of 3 ml/liter H,0/6.4 cm tree diame-ter at breast height (dbh). Microinjections

Fig. 1. Experimental d e s i g n a n d l a y o u t o f sewn p l o t t r e a t m e n t s p l u s h e a l t h y a n d i n o c u l a t e d c o n t r o l s i n one o f th ree rep l ica tes ad jacent to an expandingoak w i l t in fec t ion cen te r . Tes t t rees assoc ia ted w i th t rench t rea tments were loca ted jus t ou ts ide o f the t rench on t h e s i d e o p p o s i t e t o t h e i n f e c t i o n c e n t e r .T r e a t m e n t s a p p l i e d t o p l o t s i n 199.1 i n c l u d e d : H C = h e a l t h y c o n t r o l s ; I C = i n o c u l a t e d c o n t r o l s ( c i r c l e d ) ; N T = n o t r e n c h ; T = t r e n c h o n l y ; T + B = t r e n c h +Bioharrier insert; T + F = tl-ench + fungicide; T + G20 = trench + Geomembrane 20 mil insert: T + G30 = trench + Geomemhrane 30 mil insert; and T + T= trench + Typar inser t . Dash l i n e i n d i c a t e s v i s i b l e f r o n t e d g e o f i n f e c t i o n c e n t e r i n 1993. a n d b o l d wrows i n d i c a t e d i r e c t i o n t h e f r o n t w a s m o v i n g . O p e nand closed tree symbols indicate asymptomatic and symptomatic trees in 1993, respectively. while circled, closed tree symbols indicate asymptomatictrees inocula ted in 1994 .

1068 Plant Disease / Vol. 86 No. 10

(strain SHL-TX-36; ATCC 200432), iso-lated from the adjacent infection center(3(l), and incubated for 1 week withoutshaking in 0.5% neopeptone-glucose broth(26). The inoculum was ground with ablender for I to 2 min within the brothprior to quantitation and inoculation.

Soil excavations of trench inserts.Root growth in relation to trench insertswas assessed the fifth year following chal-lenge inoculations by random spot rootexcavations in plots of each treatment tosample soil to a maximum depth of 1.5 mon the inside of the trench for each of thefour trench insert types. Determinationswere made as to whether root growth oc-curred over the inserts in cases where thebarrier material was buried too deeplybelow the soil surface. Root growth at thebottom of the trench was assessed only incases where trench breakouts (symptomatictrees found outside of the trench) occurredin sections with trench inserts and when itwas suspected that growth of roots mayhave occurred under the barrier material.Insert materials also were examined forroot contacts and root penetrations in eachsampled trench section. The effects ofinsert materials on root growth were scoredbased on whether root growth was direc-tionally diverted upon contact with insertmaterials, exhibited dichotomous branch-ing, grew around insert barriers. was inhib-ited by chemical action, and whether rootshad swollen apices.

Data collections and analysis. Datafrom test trees within all research plotswere collected and evaluated on an annualbasis during a b-year sequential period(1995 to 2000), with the exception of year5 (I999), when no data were collected.These data were compared against inocu-latcd control trees inside of the trench,healthy control trees well outside of thetrench, and infected breakout trees thatbecame infected by root transmission be-yond test trees outside of the trench.Trench breakouts, percent tree infection,and percent tree mortality were measured.

The incidence of trench breakouts wasnoted per 183 m of barrier within each ofthree trench segments. The mean distanceof symptomatic trees from the trench wasrecorded when trench breakouts occurred.Crown symptom ratings, percent branchmortality, percent defoliation, and crownlight transmission were recorded as indica-tions of disease severity. Crown symptomswere rated using the following scale: 1 =crown dead, totally defoliated, or with onlynecrotic leaves attached, 2 = thinningcrown with leaves having diagnostic oakwilt symptoms, 3 = crowns containingfoliage with chlorosis or reduced leaf size,but lacking diagnostic symptoms of oakwilt. and 4 = full, healthy crown with no

apparent foliar symptoms. Veinal necrosisis considered the most diagnostic foliarsymptom of oak wilt in Texas live oaks.Veinal necrosis is often accompanied bymarginal necrosis in later slages of leafsymptom development. Crown light trans-mission, indicating the percentage of totalavailable sunlight passing through thecrown, was calculated from lux units re-corded with an Extech light meter (ExtechInstruments Corp., Waltham. MA) underthe crown relative to direct sunlight. Sap-wood water content was measured with aProtimeter Digital Timbermaster moistureprobe (Protimeter Inc., Commack, NY).Disease severity percentage values werearcsine transformed prior to analysis. Sig-nificant differences among means weredetermined according to Fisher’s LSD testsafter GLM analysis.

RESULTSTrench insert and fungicide barrier

tests. During the first 2 years, disease pro-gressed slowly toward the trench frominoculated and naturally infected trees inthe adjacent infection center. Only inocu-lated control trees located inside of cofi-tainment trenches, used to provide addi-tional challenge to test barriers, expresseddiagnostic symptoms of oak wilt during thefirst year after inoculation. Almost 60% of

inoculated control trees had oak wilt symp-toms, and 44% were dead due to oak wiltafter I year (Table 1). Inoculated controlshad considerable decline in crown densitydue to effects of the disease on foliage aswell as associated decreases in crownsymptom ratings and considerable in-creases in branch mortality the first year(Fig. 3A and B). An appreciable decreasein crown symptom rating and increase indefoliation and branch mortality also oc-curred in trees within no-trench plots dur-ing the first year, but no diagnostic oakwilt symptoms were as yet detected (Fip.3A to C).

The advancing front of the infectioncenter moved unimpeded via root trans-mission into plots lacking trenches (notrench controls) during the second year,although only trees in one of these plotsexhibited leaf veinal necrosis. Neverthe-less, trees in no-trench control plots devel-oped crown injury at levels that ap-proached those observed in inoculatedcontrols during the first year (Fig. 3A toD). Almost a third of all trees within plotswithout trenches were infected based onconsiderable defoliation and crown de-cline, and almost 14% mortality was ob-served in these trees by the end of the sec-ond year (Table 1). Diagnostic symptomswere found in leaves on the ground, butthere were limited numbers of sympto-matic leaves on living trees due to droughtconditions during the evaluation. However,inoculated controls developed oak wiltsymptoms much more rapidly and hadhigher disease incidence and severity thantrees that become infected by root trans-mission. Drought conditions that prevailedthroughout the summer months during thesecond evaluation year exacerbated diseasedevelopment, especially branch death.

The advancing front of the infectioncenter continued to move unimpeded intotwo no-trench plots in year 3, resulting inincreased incidence of infection and treemortality relative to year 2. The occurrenceof diseased trees outside of containment

Table 1. Disease progress associated with irench trcatn~ents indicated by the incidence of trench breakou&, percent infection. and mortality of test treesduring the first four years following inoculations of challenge trees insidc conlainmcnt harriers)

Treuch breakouts” Percent symptomatic Percent mortalityYears Years Years

Treatment n 1 2 3 4 1 2 3 4 1 2 3 4

Healthy control 84 - - - - 0 0 0 0 0 0 0 0Trench + Typar 3 4 0 0 0 0 0 0 0 0 0 0 0 0Trench + Gco 30 mil 3 2 0 0 0 0 0 0 0 0 0 0 0 0Trench + Bioharrier 3 7 0 0 0 0 0 0 0 0 0 0 0 0Trench only 2 x 0 0 0 I 0 0 0 3.6 0 0 0 0Trench + fungicide 26 0 0 0 I 0 0 0 7 . 7 0 0 0 0Trench + Geo 20 tnil 3 2 0 I’:, ii1 3 0 0 9 . 4 12.5 0 0 3 . 1 6 . 3No trench 29 0 121 0 3 I .o 41.4 4 x . 3 0 13.X 2 0 . 7 2 4 . 1inoculated control 46 - - - - S6.5 7 3 . 0 7 x . 3 X 2 . 6 43.5 4 3 . 5 4.5.7 45.7

! Field plots were established with trench inslallation on 27 July 1993, trench insert inst:rllations on I3 December 1993, and inoculations of chnllcngeLrecs within trenches on 5 M:ty 1994. Year5 I lo 4 aflet- challenge inoculation\ correspond to annual data coIIccGons from lc)% ((1 1%)X. respectively.I’crcent values :Irc the portion of test tl-ees exhibiting oak wilt sytnproms zrnd morlality: n = number ol’tesl trees per trealment.

’ Three replicated plots per treatmen(. Breakour\ refer lo the number 01‘ plots (out of three) with sympton%nic nccs beyond test trees posirioned immedi-ately ou&idc of barrier (rc:nnlcnts. The prescncc of aymp(omatic nccs beyond Lest trees within plots con(ainin, ~7 no Irenches is indicated by j 1 .

1070 Plant Disease / Vol. 86 No. 10

trenches (trench breakouts) was not seen intreatment plots with barriers until the thirdyear. A trench breakout was observed onlyfor the thinner, water-impermeable Ge-omembrane 20 insert. However, the inci-dence of infection among test trees withinthe trench breakout of this plot was lessthan IO%, with only 3% mortality by theend of the third year (Table 1). Infectedtrees in breakout areas exhibited signifi-cantly lower crown ratings and signifi-cantly higher branch mortality and defolia-tion than healthy controls (Fig. 3A and B).The level of disease in inoculated treesincreased only slightly the third year, asmost of the trees had already been symp-tomatic (almost SO%), and tree and branchmortality as well as defoliation appeared toreach a plateau (Table 1, Fig. 3B and C).Approximately 20% of inoculated treescontinued to show a progression of declinebut remained alive for the duration of thestudy.

Differences in disease incidence anddisease progress among treatments weregreater in the fourth year. Infected trees inbreakout areas exhibited significantlylower crown ratings and significantlyhigher branch mortality and defoliationthan healthy controls (Fig. 3A to C). Newtrench breakouts occurred in trench onlyand trench + fungicide plots, but no mor-tality was observed in these plots. No sig-nificant difference in disease incidence wasfound among fungicide treatments withintrench + fungicide plots. Therefore, allfungicide application results were com-bined. No apparent phytotoxicity was ob-served with any fungicide treatments. Ad-ditional trench breakouts also occurred inthe Geomembrane 20 plots in year 4 (TableI), but no breakouts were evident in theother barrier treatments. Disease incidenceand severity also continued to increase inthe no trench plots. Crown light transmis-sion continued to increase in year 4 due tothinning of crowns, particularly in inocu-lated controls, no trench, and Gcomen-brane 20 plots (Fig. 3D).

Treatment plots examined at the end ofthe fifth year exhibited no significantchanges in trench breakouts among treat-ments from those observed for the fourth-year evaluation. However, treatment ef-fects were well differentiated by the end o!the sixth year. No trench breakouts wererecorded for the water-permeable trenchinsert materials, Typar and Biobarrier. norwith the water-impermeable insert Ge-omembrane 30 (Table 2). Nevertheless. allof the replicate plots containing the water-impcrmeztble insert Geomembrane 20 hadbreakouts. Disease incidence was highest(86%) in no trench plots, and this valuewas comparable to inoculated controls. Thetrench + Geo 20 plots had intermediatelevels of tree infection, but the trench +fungicide and trench only plots similarlyexhibited low levels of disease incidence,less that half of that found in the trench +

Gco 20 plots. Symptomatic breakout trees plots. Symptomatic trees were more dia-were relatively close to the trench (mean persed and further from the trench (meandistance = 4.1 m; range = 1 .S to 12.4 m) in distance = 18.1 m; range = 6.3 to 43.3 m)the trench only and trench + fungicide in the trench + Ceo 20 plots.

1

70

s 60

zr”

50

0E 40

5 305; 20

10

090

80@ ‘;; 70

E 60.; 505

L!i

40

3020

100

?L 80

g 70‘5.u, 60E 506E 40

g 30

5 20

g 10

02 3 4

Time (years)

Fig. 3. Ch;lnge5 in disease ptugrcss and symplom severity for test trees during the tirst 4 yexs Mowinginoculation of challenge trees within trentment plots. Indicators of symptom severity included: A, crownsymptom rating on ii I to 4 sc&: 13, branch mortality measured it a percentage of n~jor branches thatwzrc dcntl; C, defoli;ltion l~~-ccntqc, a visual :isscsstncnt of leaf drop relative to healthy trees; and D,XW~ light trrrnsmia\ion indicated by the percentage of ambient light that passed through the crownrelative to direct sunlight. Error bnra rcprcsent standard deviaiions of means.

Plant Disease / October 2002 1071

Measures of disease severity were inclose agreement with disease incidence in

providing clear distinctions after 6 years.Treatment effects on disease severity were

highly significant (F = 62.5, P < 0.001).Crown symptom ratings provided the most

reliable indicators of infection status anddisease deve lopmen t s i nce d iagnos t i c

symptoms were included in this measure of

disease severity. Tree mortality was notobserved in Spar, Geo 30 mil, and Biobar-rier plots. Relatively few trees died intrench only plots (I 1 o/o), but higher mortal-ity occurred in trenched plots with fungi-cide treatments (15%) and with Geo 20inserts (2X%) (Table 2). Tree mortality wasabout 45%~ in no trench plots and in inocu-lated trees. Branch mortality developedmore slowly than other symptoms. Somebranch mortality (~10%) was found innonsymptomatic trees in plots with effec-tive barriers to root transmission. However,branch mortality generally correlated withincidence of breakouts, infection, tree mor-tality, and symptom ratings.

Crown defoliation was the most appar-ent symptom of oak wilt in latter stages ofdisease development. Crown light trans-mission increased proportionally with in-creased defoliation in medium to large-sized trees (>30 cm dbh), but branch mor-

tality contributed more significantly toincreased crown light transmission insmaller trees (~20 cm dbh). Many inocu-lated trees rapidly defoliated and appeareddead, yet some apparently dead trees stillhad small amounts of living foliage. Crownlight transmission underestimated defolia-tion, as dead leaves on some dying treestended to remain attached to limbs forprolonged periods of time. Nonsympto-matic trees in plots having trench insertsshowed rates of defoliation up to 200/o,probably due to the effects of drought im-mediately prior to the end of the sixth-yearevaluation. Defoliation rates were corre-

lated with increasing tree and branch mor-tality in infected breakout trees in plotswith barrier failure. However, defoliationoccurred more rapidly than branch mortal-ity in infected breakout trees as diseaseincidence and disease severity increased.

Soi l excavat ions of trench inserts .When examined after 5 years, roots en-countered in the loose backfill soil withintrenches were predominantly small adven-titious roots less than 2 cm in diameter.Root contacts occurred with the Typar andGeomembrane inserts of both thicknesses,but root contacts were absent with theBiobarrier insert, perhaps because of thepresence of the controlled-release triflu-

ralin herbicide (Table 3). Root penetrationswere not observed for any trench insertmaterial in spot tests along the full lengthof the trench, even in situations wheretrench breakouts occurred. All breakoutswere associated with segments where rootsgrew either over or under the trench insertmaterial. Root growth around the insertmaterials was only observed with the wa-ter-impermeable Geo 20 and Geo 30 mate-rials. Adventitious roots commonly grewthrough the soil over these insert materialswhere the inserts were accidentally buriedtoo deeply, generally <7 cm below the soilsurface. Similarly, roots occasionally grewunder the trench in situations where rootscontacted the water-impermeable Gee 20inserts near the bottom of the trench andwere diverted downward and under thebarrier.

Adventitious roots found in contact withthe water-impermeable Geomembranematerial of both thicknesses were usuallystrongly diverted perpendicularly upward,downward, or sideways along the face ofthe material. Linear growth extended up to50 cm or more from the point of root con-tact with the material, and the rootsshowed no appreciable dichotomousbranching (Table 3). However, roots thatcame in contact with the water-permeable

Table 2. Effects of trenching, fungicide, and trench insert barriers on root transmission of Cr,-crrocysti.s,~~,~rr~errnlnz in live oaks (, years after inoculationsof challenge trees inside containment barriers”

Disease incidenceX Disease severitys

Treatment

Healthy controlTrench + TyparTrench + Gee 30 milTrench + BiobarrierTrench onlyTrcltch + fungicideTrench + Geo 20 milNo trenchInoculated control

DBH” (cm)

8 4 2 5 . 43 4 2 3 . 63 2 2 3 . 33 7 27.62X 35.62 6 3 1.63 2 2X.22 9 3 5 . 546 34.X

Trench Treesbreakouts Symptomatic

- 00 00 00 0

I 4 (14.3)1 7 (26.9)

r:,IX (56.3)2s (X6.2)40 (87.0)

Distancefrom Symptom

trench (m) r a t i n g ”

- 3 . 9 4 a- 3.56 b- 3 . 4 4 hc- 3.41 b c

4 . 9 3.21 c3 . 2 3 . 1 9 c

18.1 2 . 2 5 d- 1.69 e- 1.67 e

Mortality (%)

Tree Branch

0 1.5 e0 2.1 e0 6.3 d0 4.3 e

10.7 11.6de15.4 19.9 d28.1 4 0 . 1 c44.8 65.0 b4 5 . 7 69.6 b

Defoliation(%)

Crown lighttransmission

(%)

2.6 g9.7 sg

I O . 2 fg16.4 ef23.4 e24.6 e55.X d6 9 . 4 cXl.2 b

2 X . 3 g45.1 de36.0 f3 9 . 9 e f52.1 d47.3 de6 4 . 7 c75.1 b8 1.2 ab

Infected breakouts s o 30.0 - 50 ( 100) - 1.34 f 66.0 x 0 . x a 95.1 a X 7 . 3 a

“Data taken in 2000, 5 years after challenge inoculations, from three replicated plots per treatment, with the exceptions of control and breakout trees.Plots were established with trench installation on 27 July 1993, trench insert installations on 13 December 1993, and inoculations of challenge trees on SMay 1994; n = number of test trees per treatment.

Y Disease incidence in trees outside of barrier treatments. Breakouts refer to the number of plots (out of three) with symptomatic trees beyond the barrier,or plots indicated by [ 1 where no barrier existed. Values in parentheses are the percentage of infected trees within all plots for each treatment. Distancesrefer to the mean distances of symptomatic trees from the trench in breakout plots.

y Percent values were arcsine transformed prior to analysis, although values presented arc actual percentages. Means with different letters within eachcolumn are significantly different (P < 0.001) according to Fisher’s LSD tests.

’ Crown symptom ratings used the following scale: I = crown dead, totally defoliated, or with only necrotic leaves attached; 2 = thinning crown withleaves having diagnostic oak wilt symptoms, including veinal chlorosis or veinal necrosis; 3 = crowns containing foliage with chlorosis or reduced leafsiLe, but lacking diagnostic symptoms of oak wilt; and 4 = full. healthy crown with no apparent foliar symptoms.

Table 3. Root growth in the vicinity of trench insert materials S years after challenge inoculations’

Trench insertRoot

contactRoot

penetrationDichotomous

branchingDirectionaldiversion

Growth aroundtrench insert

Growthinhibition

Swollenapices

TyparBiobarrierGee 20 milGco 30 mil

+ - + + - -- - - - - + ++ - ti + - --+ - - + + + -

’ Scoring of effects and interactions with root growth: (-) no effect or interaction; (+) moderately positive effect or interaction; (+t) strong effect or inler-action.

1072 Plant Disease / Vol. 86 No. 10

Typar insert were diverted only slightly(~20 cm length) from the point of contactand tended to exhibit considerable di-chotomous branching. Adventitious rootscoming in contact with trifluralin-treatedsoil around Biobarricr slow-release pelletsshowed some effects of herbicide expo-sure, including reduced growth (growthinhibition) relative to nonherbicide treat-ments, swollen apices, and greatly attenu-ated root branching. None of the otherinsert materials caused significant growthinhibition of roots or swollen apices.

D I S C U S S I O NSmall adventitious roots <2 cm in di-

ameter commonly formed from the ends otlateral live oak roots severed by trenching.These small roots grew and accumulatedwithin the trench in the loose backfill soil,which favored root growth relative to thehard, compact undisturbed soil. The proc-ess of root regeneration within trenchbackfill soil took at least 3 to 4 years aftertrenching before it was sufficient for rootgrafting. The slow growth of regeneratingroots was attributed to the dry edaphicconditions in the semiarid Texas HillCountry.

Interactions between newly regeneratedadventitious roots and trench-insert materi-als appear to explain the cause of trenchbreakouts. Root contacts occurred with allinsert materials except Biobarrier, but noroot penetrations were observed with anyinsert material. This implied that trenchbreakouts were due to root growth aroundthese physical barriers. Biobarrier chemi-cally inhibited root growth with the herbi-cide trifluralin. Root diversion by the wa-ter-permeable Typar was limited bydichotomous branching of root apices.which reduced linear elongation. Dichoto-mous branching may be a growth responseof roots receiving moisture through thebarrier. However, the water-impermeableinserts (Geo 20 and Geo 30) diverted rootsto the upper and lower edges of insertswhere root grafting may occur. Root graft-ing can occur either because the insert wasburied too deeply in the trench (soil aboveinsert) or because the soil depth to bedrockis greater than the width of the insert (soilbelow insert). Several instances were dis-covered where short sections of the Geo 20material were buried too deeply, and thesesections were in very close proximity totrench breakouts.

Trench inserts did not significantly im-prove on trenches alone as barriers to roottransmission during the first 3 years aftertrench installation, but certain inserts didimprove trench eff‘ectiveness beyond thethird year. The water-impermeable insertsappeared to direct root growth around thesebarriers, potentially leading to root graft-ing. Although Geomembrane is a tough,heavy, and durable material that serves asan excellent barrier to root penetrationowing to its pliability and resistance to

puncture, these properties contribute toroot diversion and difficulty of installation.The thicker Geomembrane 30 mil alsodiverted root growth, but no breakoutswere observed. With Biobarrier, roots gen-erally do not come in contact with the fab-ric itself because root growth is inhibitedin the herbicide-treated zone immediatelyadjacent to the fabric. The use of the herbi-cide trifluralin (Dow Elanco, Indianapolis,IN), which specifically prevents root-tipcell division, in controlled-release pelletshas several advantages, including very lowwater solubility, negligible groundwatercontamination, low minimum effectiveconcentrations (<IO ppm), rapid initialrelease rates, long activity time, and lackof uptake by tree roots and translocationwithin the plant (8,9,11). The restrictedmovement of trifluralin in soil due to verylow water solubility and strong adsorptionto soil particles limits the distance of her-bicidal activity to <5 cm from the pellets(8,14,17). Thus, there are very limitedadverse effects on roots not immediatelyadjacent to the barrier.

The results here suggest that Typar per-formed as well as Biobarrier in providingsufficient protection against root penetra-tions without the need for the additional(chemical) barrier provided by the triflu-ralin. The Typar material also is availablein three thicknesses (ix, 1.5x, 2x, or 4, 6,and 8 oz., respectively), which provideincreasing levels of resistance to root pene-trations as needed. Both Biobarrier andTypar are composed of lightweight poly-propylene and were easily installed, butunlike the heavy Geomembrane material(polyethylene), both degrade with pro-longed exposure to sunlight. Typar is gen-erally priced 70 to 80% less than Biobar-rier at comparable widths and thicknesses.

The results presented here strongly sug-gest that fungicide injections are ineffec-tive in preventing root transmission of theoak wilt fungus, confirming previous ob-servations (3). The ineffectiveness of fun-gicide injections is largely explained by thepredominantly upward movement of tria-zole fungicides such as propiconazole(Alamo) in the vascular system of injectedplants (19). Consequently, insufficientquantities of fungicide are translocateddown into the root system to prevent viableinoculum From being translocated throughroot grafts into adjacent noninfected trees.Applications of propiconazole as a chemi-cal control to prevent root transmissioncould be modified so that the material isintroduced at the distal ends of the rootsystem, as with soil application treatments.The most effective results with propicona-zole are achieved when it is applied annu-ally as a soil drench over several consecu-tive years (31).

Improvements in trenching methods areneeded by oak wilt suppression programsin alf states affected by this disease. Instail-ing trench inserts into primary trenches to

preclude breakouts due to root regraftingshould significantly reduce incidences ofoak wilt root transmission. Water perme-able trench inserts provide cost-effectiveinsurance against breakouts because theymay be installed at a fraction (less than10%) of the cost of expensive backuptrenches that are required when primarytrenches fail. Developing methods for re-ducing overland transmission of C. fil-Jp”“U’“m and improving on existingmethods for optimizing trench placement(5,6,10) also would provide useful tools tofacilitate control of this malady.

ACKNOWLEDGMENTSWe thank Steven P. Barrletr, Bradley Dcvelop-

mat, Circle C Land Corporation, for providingland for this work. W C thank Carey Cornelius,Circle C Ranch landscaping crewmen, rind LynnSilverman, who assisted in the installation ofresearch plots for this study. We particularly ap-preciate the financial expenses incurred by Brad-ley Developlnenl in association with Ihe installa-tion and backfilling OF the test trench, brushmaintenance around test trees, and assistanceprovided in the USC of heavy excavation equip-ment for collecting roo1 samples and for subsur-face examinations of trench insert performancewithin t renches. We thank Carl Schattenberg,former forester for the City of Austin in TravisCo., TX, who provided initial contacts and assis-tance in locating suitable sites for this study. WC

appreciate the donations of trench insert materialsfrom Reemay Inc. and Raven Industries. We aregrateful to Jerry A. Dunaway, who provided assis-tance in the installation of trench insert materialsfor this study. We are indebted to Ciba (Nova&),J. J. Maugct Co., Inc., and Monterey Chemical Co.for donating materials for the fungicide applica-tioms. We thank James B. Briggs for assistance inroot-flare soil excavations and high-volume fungi-cide applications. Finally, we acknowledge Lisa EWilson and Bryce Burke for assistance in datacollecGons.

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