MYRTLE WILT AND ITS POSSIBLE MANAGEMENT IN ASSOCIATION WITH HUMAN DISTURBANCE OF RAINFOREST IN TASMANIA

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1 256 MYRTLE WILT AND ITS POSSIBLE MANAGEMENT IN ASSOCIATION WITH HUMAN DISTURBANCE OF RAINFOREST IN TASMANIA G.A. KILE CSIRO Division of Forestry nd Forest Products, Stowell Avenue, Bttery Point, Tsmni 7004, Austrli J.M. PACKHAM Ntionl Rinforest Conservtion Progrmme, C/o Tsmnin Forestry Commission, G.P.O. Box 207B, Hobrt, Tsmni 7001, Austrli nd H.J. ELLIOTT Tsmnin Forestry Commission, G.P.O. Box 207B, Hobrt, Tsmni 7001, Austrli (Received for publiction 19 July 1989; revision 28 September 1989) ABSTRACT Chlr ustrlis Wlker & Kile is lethl primry pthogen of Nothofgus cunninghmii (Hook.) Oerst. (myrtle) in Tsmnin rinforests. It is mjor cuse of gps in myrtledominted stnds but mortlity is often most severe in rinforest subject to humn disturbnce. Possible strtegies could be developed to minimise disese in res of disturbed rinforest. Keywords: rinforest; disturbnce; Nothofgus cunninghmii; Chlr ustrlis. INTRODUCTION Nothofgus cunninghmii (myrtle), the most common of the dominnt tree species of cool temperte rinforest in south-estern Austrli, is suffering widespred nd loclly severe mortlity (Elliott et l 1987; Kile & Wlker 1987). Tree deth results from infection nd necrosis of root nd outer stem wood by the hyphomycete Chlr ustrlis. The pin-hole borer Pltypus subgrnosus Schedl ttcks the stems of dying trees. The wilt symptoms which develop in the crowns of disesed trees hve led to the doption of the term "myrtle wilt" to describe the disese nd distinguish it from less specific syndromes of morbidity ffecting other Nothofgus species (Arentz 1983; Skipworth 1983). Myrtle wilt, however, is considered to be vsculr stin disese s the pthogen is not confined to the conducting elements of the xylem in the initil phses of infection nd it does not become systemic in the host plnt, fetures ssocited with vsculr wilt pthogens (Green 1981). Myrtle wilt is mjor cuse of gps in Tsmnin rinforests. Initil reserch on disese impct, etiology, nd the reltionship of the pthogen to the potentil vector P. New Zelnd Journl of Forestry Science 19(2/3): (1989)

2 Kile et l. Myrtle wilt nd its possible mngement 257 subgrnosus is now being expnded to ssess the significnce of the disese in the ecology nd conservtion of N. cunninghmii. This pper reviews current informtion on myrtle wilt nd the possibilities for limiting its impct in cool temperte rinforest disturbed by humn ctivity. DISEASE SYMPTOMS IN INDIVIDUAL TREES Crown symptoms include chlorosis, necrosis, nd bscission of the oldest folige prior to the sudden generl wilting nd necrosis of residul folige (Kile & Wlker 1987). Initil symptom expression my vry between trees; individul brnches of the upper crown sometimes brown off leving green brnches surrounding or below, while in other trees extensive folige necrosis nd bscission cn occur leving few distl tufts of green folige prior to tree deth. Hosts my tke from 1 to 3 yers to die from the time of initil C. ustrlis infection. Root nd stem wood infected by C. ustrlis is discoloured drk brown. In crosssection this is visible s rdil streks of vrible width nd length or ptches in the spwood or outer hertwood which my be interspersed with cler wood. Discolortion ttenutes upwrds in the stem nd rrely reches crown brek. Irregulr, blck, sporulting mycelil felts of C. ustrlis my develop on the brk of the lower stem of infected trees nd these re probble source of ir- or wter-borne inoculum. Pltypus subgrnosus ttck is initited on trees with helthy crowns nd is relible guide to C. ustrlis infection in the underlying wood. Infesttion is initilly often concentrted in one sector of the stem before becoming more widespred on the lower stem nd buttress roots in subsequent flight sesons. Tunnelling by P. subgrnosus could ccelerte disese expression by ssisting the internl spred of the pthogen within host vsculr tissues. INCIDENCE AND SPATIAL PATTERN OF DISEASE IN UNDISTURBED RAINFOREST Trnsect surveys of 20 rinforest sites undisturbed by humn ctivity or recent fire showed cumultive mortlity from 9.4% to 53.4% with n verge disese incidence of 24.6% (Elliott et l. 1987). Incidence decresed with incresing ltitude nd, when djusted for this prmeter, the cllidendrous rinforest subtype (see rinforest clssifictions by Jrmn et l. 1984) hd higher incidence thn thefloristicllymore diverse thmnic-implicte subtypes, respectively. In mixed forest (rinforest with euclypt overstorey) incidence incresed s both reltive nd bsolute mesures of myrtle density incresed but this trend ws not evident in the other forest types. Lrger-dimeter trees hd the highest disese incidence s did trees with stem or crown dmge. However, significnt vrition remined unexplined by the site nd stnd vribles which were mesured. Disesed trees were clumped with the degree of ssocition resonbly consistent cross sites but dependent on nerest neighbour distnces within sites. Dying trees occurred t n verge rte of 2.4 trees/h or 1.6% of live trees cross the sites.

3 258 New Zelnd Journl of Forestry Science 19(2/3) INCIDENCE IN DISTURBED RAINFOREST Although high levels of the disese occur in undisturbed stnds, roding nd logging ctivities hve been observed to led to rised incidence levels. Results from series of logging, regenertion, nd thinning trils, confirm these observtions (J. Hickey pers. comm.). All logging methods which were tested on old rinforest incresed disese incidence nd mortlity levels of the remining myrtles, compred with undisturbed forest, i.e., selective logging, strip logging, logging leving shelterwood nd seed trees, nd lso pre-logging soil scrifiction to improve regenertion. Thinning even-ged myrtle stnds t 40 nd 65 yers significntly (p < 0.05) incresed mortlity. In some of these logged nd thinned res there ws evidence tht elevted mortlity levels due to myrtle wilt eventully dropped to levels more "norml" in undisturbed forest. Where this hppened it took n verge of 9 yers (rnge yers). Trnsect dt from severely ffected rodside re in north-est Tsmni showed tht the proportion of disesed trees decresed with distnce from the rod, the fitted model explining 53% of the devince (Fig. 1). 1-oh V ^0-8 Q LU CO < LD 0-6 Q CO LU LU Ql h-0-4 z O 50.2h L < er V- H L V > i V L V f V D n D D \ D %N o-o h D D D DDD i i i i.i 1_. i i DISTANCE FROM ROAD EDGE (D) FIG. 1 Sctter plot of dt from multiple trnsects showing proportion (P) of disesed (ded or symptomtic) trees of Nothofgus cunninghmii in reltion to metres from rod edge (D) in north-estern Tsmni. Log it (P) = D, percentge devince explined =53%. THE PATHOGEN Chlr ustrlis grows redily nd specultes profusely in culture on vriety of medi. The optimum temperture for growth is pproximtely 20^. The single-celled conidi re produced in chins from the distinctive philides. Scleroti (suspected perithecil initils) develop on gr but no teleomorph hs been identified for the species (Kile & Wlker 1987). D

4 Kile et l Myrtle wilt nd its possible mngement 259 While the fungus shows similrities to other Chlr species such s C. quercin Henry the nmorph of Certocystis fgcemm (Bretz) Hunt (cuse of ok wilt) nd the Chlr nmorphs of Certocystis virescens (Dvidson) C. Moreu (cuse of spstrek of sugr mple), it most closely resembles Chlr neocledonie Kiffer & Delon, species pthogenic on coffee (Coffe robust Linden) nd guv (Psidium gujv L.) in New Cledoni (Ddnt 1950; Kiffer & Delon 1983). Chlr neocledonie hs smller philides nd higher optimum temperture thn C ustrlis. Koch's postultes were estblished for C. ustrlis on seedlings, splings, nd lrge trees (Kile & Wlker 1987). Wood nd folige symptoms were reproduced in hosts of vrious size nd ge even though nturl infection is not observed in seedlings. These inocultion studies showed evidence of vrition in host resistnce nd in pthogenicity, both of which require further study. HOST RANGE Inocultion studies indicte tht C ustrlis is reltively host specific. Of 18 Tsmnin ntive nd four exotic species inoculted with C ustrlis (Kile 1989), only the rinforest understorey species Trochocrp gunnii (Hook, f.) Benth. (Epcridcee) ws killed while Nothofgus gunnii (Hook, f.) Oerst. ws extensively infected nd my hve died with longer inocultion period. Other species showed only minor vsculr discolortion or were essentilly resistnt. Nturl infection hs not been observed in either T. gunnii or N. gunnii. The mrked susceptibility of T. gunnii is puzzling s there is no close reltionship between the Fgcee nd Epcridcee. Inocultion of limited number of seedlings of exotic Nothofgus species showed N gluc (Phil.) Krsser, N. leoni Espinos, nd, to lesser extent, TV. obliqu (Mirbel) Blume, TV. lpin Poepp. & Endle., nd TV. lessndrii Espinos to be susceptible but no deths occurred (Kile 1989). These species nd the Tsmnin endemic TV. gunnii re ll deciduous species grouped in the section Nothofgus of the genus. The results indicte some susceptibility to C ustrlis in other members of the genus, prticulrly the deciduous species, but more stisfctory ssessment must wit inocultion studies in mture trees. DISEASE DISPERSAL Superficilly there is close ssocition between ttck of the stems of myrtle trees by P. subgrnosus nd the development in their crowns of the chrcteristic wilt symptoms. Howrd (1973) hypothesised tht the deth of myrtle ws cused by pthogenic fungus trnsmitted by this borer. Kile & Wlker (1987) estblished the vercity of this first element of Howrd's hypothesis. The clumping of disesed trees shown by the survey of Elliott et l. (1987), together with the initil observtions of Kile & Wlker (1987) on the pprent reltionship between P. subgrnosus ttck nd C. ustrlis infection, suggested three hypotheses which lone, or in combintion, could explin dispersl nd infection by C ustrlis: () direct vectoring by P. subgrnosus (or unknown insect/s), (b) ir- nd/or wter-borne infection of wounds with below-ground spred by root grft or root contct (or

5 260 New Zelnd Journl of Forestry Science 19(2/3) unknown below-ground vectors), or (c) rndom ttcks by P. subgrnosus on helthy trees or on trees rendered susceptible by stress, followed by infection of the wood vi the borer tunnels with ir-, wter-, or insect-borne inoculum of C. ustrlis. Kile & Hll (1988) provided detiled ssessment of the possible role of P. subgrnosus in pthogen dispersl from which they concluded tht the beetle rrely, if ever, ws direct vector but rther secondry gent ttcking trees lredy infected by the fungus. This conclusion ws bsed on () the very infrequent isoltion of C. ustrlis from the beetles, (b) lck of infection in disese-free myrtle billets exposed to beetle ttck, (c) frequent beetle ttck of rtificilly stressed or wounded trees without infection, (d) evidence of infection of trees in the forest prior to beetle ttck, (e) lck of convergence in the life-cycle of the fungus nd the beetle in the infected host which would llow emergent beetles to become contminted with the fungus. Exmples of wound infection, pprently without insect involvement, hve been documented (Kile & Wlker 1987; Kile & Hll 1988) nd this is probbly the min mens for the estblishment of new infection foci. Two potentil sources of ir-, wter-, or insect-borne inoculum occur in rinforest () conidi from mycelil felts on the brk of infected trees or other wood surfces (Kile 1989), (b) wind-borne frss contminted with conidi nd philides from beetle tunnelling in infected tissue of living trees. Inoculum-trpping experiments using fresh billets of stem wood (5-6 cm dim, x 30 cm long) of myrtle nd rinwter collection hve confirmed the presence within rinforest of ir- or wter-borne inoculum but quntifying its sesonl vilbility hs proved difficult (Kile unpubl. dt). Trpping on wood illustrtes strong summer pek in inoculum levels but it hs not been possible to determine whether this indictes greter vilbility of inoculum propgules or secondry infection between billets during the 1-month exposure periods within the forest (Fig. 2). Sporulting felts re produced most bundntly during the utumn-winter period (Kile unpubl. dt), nd pek in summer could indicte the presence of frss-borne inoculum during the beetle flight seson. It is evident, though, tht there is the potentil for inoculum to be vilble during most of the yer. There is no evidence tht insects re involved in trnsfer of inoculum to wounds. Clumping of disesed trees nd reltionship between incidence nd stnd density hs been considered indictive of below-ground spred (Elliott et l 1987). Although no investigtions of the frequency or mechnism of underground trnsmission hve yet been undertken, it is likely to be mjor mens of locl dispersl. The third hypothesis is more difficult to estblish. Tretments to induce stress in myrtle indicte tht C. ustrlis renders trees more susceptible to continued P. subgrnosus ttck thn sp-ringing or scorching (Tble 1). Artificil wounds which expose spwood result in loclised nd trnsient ttrctiveness of trees to P. subgrnosus (Kile unpubl. dt). These results explin why C. usfr/zs-infected trees re fvoured for beetle ttck in the forest. Thus, while P. subgrnosus is not direct vector of C ustrlis it hs the potentil to be of significnce in disese spred through libertion of contminted frss, cretion of wounds (pin holes) in stressed trees, nd promotion of spred within trees lredy infected.

6 Kile et l. Myrtle wilt nd its possible mngement Q C8 O LD U_ Z \2 0-6 LU _J GO O0-4 z o H o < Little Florentine River Arve Loop \ /\ A / \ K \\ / 1 \\\\V / / / / - // / \ / / 1 \ / / / i... 1_ <00^, m00m " s r / 1 / i /. <- r^i - 1I *' i / / / / / / is i/ My 1985 July Sept. Nov. Jn MONTH OF SAMPLING i i i. i i i A i i i i... Mr. FIG. 2 Percentge of billets of Nothofgus cunninghmii stem wood infected by Chlr ustrlis during 1 -month exposure periods t three rinforest sites in Tsmni during Presence of infection in individul billets ws determined by isoltion of the fungus from the wood on 3% mlt extrct gr. TABLE 1 Number of mle nd femle Pltypus subgrnosus trpped on sticky trps compred with the number of tunnels initited on the sme trees in five tretments on Nothofgus cunninghmii during the beetle flight seson (December My 1986)* Tretment of trees Chlr ustrlis inoculted Sp ringed Scorched Sterile wter injected Control Totls P. subgrnosus Mle Femle : Figures re totls for the seven trees in ech tretment Totl trpped Totl tunnels OTHER DISEASES AFFECTING STAND STRUCTURE OF NOTHOFAGUS COMMUNITIES Myrtle wilt ppers to be unique s n often severe nd sustined stnd-level disese cused by primry pthogen in Nothofgus forests. Other diseses in Nothofgus spp. ffecting stnd structure hve been recorded from New Zelnd nd Ppu New Guine but mortlity hs usully been relted to environmentl stress, prticulrly drought, followed by infections of secondry pthogens or infesttions of pests

7 262 New Zelnd Journl of Forestry Science 19(2/3) (Crtledge et l 1975; Arentz 1983; Skipworth 1983; Hosking & Kershw 1985; Jne & Green 1986; Hosking & Hutcheson 1986). Fulds (1977) showed tht fungus, tenttively identified s Sporothrix species, ws pthogenic when inoculted into N fusc (Hook.f.) Oerst. This fungus cused somewht similr effects in the wood nd the crowns of this species to those cused by C. ustrlis in N. cunninghmii. Although infection ws ssocited with ttck by Pltypus spp., it ws not estblished whether the beetles were direct vectors. Tree killing by the fungus, however, ppers quite restricted but my be ggrvted by stress or forest disturbnce (Fulds 1977). The role, if ny, of C. neocledonie s pthogen in forests of Nothofgus spp. in New Cledoni is unknown. As it ttcks exotic species in the Myrtcee nd Rubicee (Ddnt 1950) nd is probbly indigenous to the islnd, infection of ntive hosts is possible. No Chlr species hve been recorded on Nothofgus spp. in Ppu New Guine (Shw 1984). MANAGEMENT OF MYRTLE WILT IN ASSOCIATION WITH DISTURBANCE A mortorium on rinforest logging in Tsmni is in force until 1990, lthough mixed forests with more thn 5% euclypt crown cover re logged for regenertion to euclypt species. It seems likely tht significnt rinforest res will eventully be reserved, nd tht ny silviculture of N cunninghmii will be restricted to res of limited extent. Myrtle wilt my be nturl cuse of gps which llow stnd regenertion in N cunninghmii rinforest. If the host-pthogen reltionship is stble one, then there is little cuse for concern bout the level of disese expression in lrge res of otherwiseundisturbed rinforest. There re, however, other hypotheses which could explin the current levels of disese such s the development of new, more ggressive strins of the pthogen or promotion of disese development through greter humn ccess to nd disturbnce of rinforest. These re being investigted s prt of n ssessment of the need for disese mngement. For the foreseeble future, though, ny ttempts t mngement or meliortion of disese will focus on isolted rinforest remnnts, nd reserves such s ntionl prks nd conservtion res disturbed by mn for ccess, or coupes mnged for wood production. In such res ggrvted disese my reduce esthetic vlues nd timber yield nd crete hzrdous trees. As some nturl infections will lwys occur, mngement will in the min be directed to minimising disese levels. Prevention of wounds on N. cunninghmii in ny opertions in rinforest is n essentil element in ny strtegy to minimise disese development, whether this be in silviculturl tretments or in cretion of ccess. Assessment of the sesonl vilbility of inoculum nd the period for which wounds re susceptible to infection is necessry to determine if there re periods when rinforest could be disturbed with miniml risk of infection. The significnce of myrtle wilt in N cunninghmii silviculture will depend on the future scle of such ctivities. In logging trils in rinforest to obtin N cunninghmii regenertion vi retined seed trees, the seed trees often die soon fter stnd tretment

8 Kile et l Myrtle wilt nd its possible mngement 263 without the opportunity for seed shed (J. Hickey pers. comm.). The mjority of trees probbly die from C. ustrlis infection; during hrvesting opertions, creful selection of unwounded trees or trees seprted from existing infection foci could promote seedtree longevity. Thinning my be n option for the intensive mngement of densely stocked N. cunninghmii stnds. Retined trees my be wounded during thinning opertions, nd infection of stumps ws observed in recently thinned pole stnd (Kile unpubl. dt). The implictions of the ltter source of infection for survivl of crop trees re unknown but mngement to limit entry of C. ustrlis to thinned stnds ppers necessry. Options could include the timing of opertions nd the tretment of wounds nd stump surfces, but chemicl nd biologicl tretment of wounds to prevent C. ustrlis infection hs not been investigted. An importnt pproch to disese mngement my be delinetion of buffer zones to protect core N. cunninghmii stnds. Disturbnce for ccess increses mortlity, probbly initilly by creting new disese foci through infection of wounds on border trees nd lter on continuing bsis by below-ground spred between djcent trees. Severe mortlity lso hs the potentil to rise inoculum levels nd thereby increse the chnces of estblishing new infections in djcent helthy stnds. Observtions t severl loctions in Tsmni suggest continuing, lbeit slow spred into contiguous "helthy" myrtle more thn 15 yers fter roding (Fig. 1). Buffer zone size will depend on the rte of disese spred. Plots to mesure this rte hve only recently been estblished in Tsmnin rinforest. Within buffers, however, loss of existing trees could be compensted for by regenertion nd n initil pproch to defining buffer could be to consider it s zone in which ll existing mture myrtle re not killed within, for exmple, period of 50 yers of its estblishment. By this time myrtle regenertion could form n identifible forest. Using n estimte for nnul below-ground spred of 1-5 m, bsed on the liner growth rte of the fungus in N cunninghmii stem tissue of up to 1 cm/dy (Kile 1989), such buffer my need to be m wide. Disese development within the buffer zone nd the width of the zone might be considerbly reduced if the boundry could be crefully formed to minimise infection of the border trees of such zone. The integrity of buffer zone my lso be ffected by ir-borne spred which my or my not be relted to disturbnce in djcent res, but there is no bsis on which to predict such infections. Snittion tretment in buffer zones by removl of ded or infected trees could lso be considered in especilly vluble rinforest res if the opertions could be done without dmge to djcent trees. As P. subgrnosus is secondry gent it seems unlikely tht ttempts to mnge beetle popultions would hve significnt effect on disese development, except possibly if frss contminted with C ustrlis is importnt s inoculum. CONCLUSIONS Myrtle wilt does not threten the survivl of N. cunninghmii but it is n importnt disese in Tsmnin rinforests. Chlr ustrlis is potentil hzrd for Nothofgus species growing elsewhere in the world. Infection develops through root

9 264 New Zelnd Journl of Forestry Science 19(2/3) or stem wounds vi ir- or wter-borne inoculum nd through locl below-ground spred. Pltypus subgrnosus is secondry gent ttcking trees lredy infected by the fungus but it my liberte inoculum from infected trees. The disese my rdiclly lter stnd structure nd in public ccess res crete trees which re hzrdous for people nd property. There is the potentil to develop strtegies to minimise disese development in remnnt nd disturbed rinforest res. ACKNOWLEDGMENTS We would like to thnk Steve Cndy, Tsmnin Forestry Commission, for the nlysis represented in Fig. 1. REFERENCES ARENTZ, F. 1983: Nothofgus diebck on Mt. Giluwe, Ppu New Guine. Pcific Science 37: CARTLEDGE, E.G.; SHAW, D.E.; STAMPS, D.J. 1975: Studies in reltion to ded ptches of Nothofgus in Ppu New Guine. Deprtment of Agriculture, Stock nd Fisheries, Port Moresby, Reserch Bulletin 13: DAD ANT, M.R. 1950: Sur une nouvelle mldie du Coffe robust en Nouvelle Cledonie. Revue Generle de Botnique 57: ELLIOTT, H.J.; KILE, G.A.; CANDY, S.G.; RATKOWSKY, D.A. 1987: The incidence nd sptil pttern of Nothofgus cunninghmii (Hook.) Oerst. ttcked by Pltypus subgrnosus Schedl in Tsmni's cool temperte rinforest. Austrlin Journl of Ecology 12: FAULDS, W. 1977: A pthogenic fungus ssocited with Pltypus ttck on New Zelnd Nothofgus species. New Zelnd Journl of Forestry Science 7: GREEN, R.J. 1981: An overview. Pp in Mce, M.E.; Bll, A.A.; Beckmn, CH. (Ed.). "Fungl Wilt Diseses of Plnts". Acdemic Press, New York. HOSKING, G.P.; HUTCHESON, J.A. 1986: Hrd beech (Nothofgus trunct) decline on the Mmku Plteu, North Islnd, New Zelnd. New Zelnd Journl of Botny 24: HOSKING, G.P.; KERSHAW, D.J. 1985: Red beech deth in the Mrui Vlley, South Islnd, New Zelnd. New Zelnd Journl of Botny 23: HOWARD, T.M. 1973: Accelerted tree deth in mture Nothofgus cunninghmii Oerst. forests in Tsmni. Victorin Nturlist 90: JANE, G.T.; GREEN, T.G.A. 1986: Etiology of forest diebck res within the Kimi Rnge, North Islnd, New Zelnd. New Zelnd Journl of Botny 24: JARMAN, SJ.; BROWN, M.J.; KANTVILAS, G. 1984: "Rinforest in Tsmni". Ntionl Prks nd Wildlife Service, Tsmni. 201 p. KIFFER, E.; DELON, R. 1983: Chlr elegns {.Thielviopsis bsicol) nd llied species. II. Vlidtion of two tx. Mycotxon 18: KILE, G.A. 1989: Infection of exotic nd Tsmnin ntive tree nd shrub species by the vsculr stin fungus Chlr ustrlis. Europen Journl of Forest Pthology 19: KILE, G.A.; HALL, M.F. 1988: Assessment of Pltypus subgrnosus s vector of Chlr ustrlis, cusl gent of vsculr disese of Nothofgus cunninghmii. New Zelnd Journl of Forestry Science 18: KILE, G.A.; WALKER, J. 1987: Chlr ustrlis sp. nov. (Hyphomycetes), vsculr pthogen of Nothofgus cunninghmii (Fgcee) in Austrli nd its reltionship to other Chlr species. Austrlin Journl of Botny 35: SHAW, D.E. 1984: Microorgnisms in Ppu New Guine. Deprtment of Primry Industry Port Moresby. Reserch Bulletin 33: SKIPWORTH, J.P. 1983: Cnopy diebck in New Zelnd mountin beech forest. Pcific Science 37: