Sudden oak death epidemiology

Transcription

1 Reserch Article t Phosphonte controls sudden ok deth pthogen for up to 2 yers y Mtteo Grelotto nd Dougls J. Schmidt Since its emergence in the lte 1990s, sudden ok deth hs killed mture ok trees nd tnoks in 14 Cliforni counties. Tretment options re now ville to sfegurd these trees from infection y Phytophthor rmorum, the ggressive nd exotic pthogen responsile for sudden ok deth. We provide n updte on current knowledge regrding this emergent disese in Cliforni, nd present results from three controlled experiments of two chemicl tretments to mnge the disese in oks nd tnoks. Phosphonte tretments, leglly registered in Cliforni to control sudden ok deth, were effective in slowing oth infection nd growth rtes for t lest 18 months. Conversely, n lterntive method consisting of n zomite soil mendment nd rk lime wsh ws lwys ineffective, nd did not reduce either growth or infection rtes. Sudden ok deth (SOD), the emergent forest disese plguing four ok species nd tnoks in 14 Cliforni counties, is cused y the introduced, nonntive pthogen Phytophthor rmorum (Grelotto et l. 2001; Rizzo et l. 2002). Trees re infected y the pthogen through intct rk, nd deth is cused y lesions girdling the tree s cmium nd prtly occluding its vsculr system. The time from infection to deth cn rnge from few months to few yers, depending on environmentl conditions nd genetics of the host nd pthogen. Eventully, the mjority of girdled trees experience sudden rowning of the entire crown without thinning or the loss of folige, giving the disese its nme (Rizzo nd Grelotto 2003). Oks nd tnoks in 14 Cliforni counties re plgued y sudden ok deth, including, ove, tnoks in the Big Sur region. The tree disese is cused y the exotic pthogen Phytophthor rmorum. Sudden ok deth epidemiology Host plnts. Although the disese kills lrge numers of oks nd tnoks, P. rmorum is rod generlist cple of infecting more thn 100 plnt species nd ferns (USDA APHIS 2007). On most hosts other thn oks nd tnoks, infection results in lef lotches nd rnch diecks tht only occsionlly kill infected plnts. However, infected non-ok hosts re responsile for the disese s spred. In Cliforni woodlnds, for instnce, the surfces of Cliforni y lurel leves nd tnok twigs produce lrge numers of infectious irorne P. rmorum propgules, clled sporngi. In commercil nurseries nd Europen wildlnd settings, cmellis nd rhododendrons pper to ply key epidemiologicl role in spreding P. rmorum. Oks re defined s ded-end hosts ecuse they re seldom if ever infectious to other plnt species (Grelotto et l. 2003). Infection propgules. While most Phytophthors found in temperte forests re soilorne nd wterorne (Erwin nd Rieiro 1996), P. rmorum is chrcterized y predominnt eril phse (Dvidson et l. 2005). During the riny seson, infectious sporngi re produced on the lef surfces or twigs of infectious hosts, nd re relesed to the environment in wind or rinstorms. Once sporngi lnd on n re of suitle host tht is covered y film of wter, infection occurs y the relese of motile zoospores 10 CALIFORNIA AGRICULTURE VOLUME 63, NUMBER 1

2 (see photo, pge 12) tht swim until they cn penetrte the host tissue. Epidemiologicl oservtions suggest tht infection success my e highest when tempertures re wrm. In unfvorle dry conditions, nd t ny time on some plnt hosts, P. rmorum produces chlmydospores, which re survivl or resting propgules chrcterized y thick protective outer lyer. Dispersl. Lrge numers of P. rmorum sporngi, zoospores nd chlmydospores eventully end up either in the soil or in wterwys. While the epidemiologicl role of pthogenering soil nd wter hs not een documented, their potentil roles rnge from eing short-distnce infection sources (such s y infested soil splshing onto the min ole of tree), to eing gents of long-distnce dispersl (such s y infested wterwys Top left, Cliforni y lurel leves typiclly ecome infected with P. rmorum on the tips of their leves, where wter nd dew collect; y leves re elieved to ply significnt role in the disese s spred to ok trees. Top right, sudden ok deth infections often produce weeping or leeding cnkers nd my emit thick, drk ooze from tree rk; this dischrge is the tree s ttempt to defend itself ginst infection. Left, in mture cost live ok tree trunk, the lck line shows the edge of the infection nd mrks the zone etween living nd ded woody tissue. crrying the pthogen downstrem for tens or hundreds of miles). Although the pthogen s exct introduction dte nd its re of origin re not known, recent genetic nlysis reconstructed the history of sudden ok deth in Cliforni nd elucidted the pthogen s dispersl ility (Mscheretti et l. 2008). P. rmorum ppers to hve escped the plnt nursery environment in Snt Cruz nd Mrin counties, proly in the 1980s. Humns further enhnced dispersl of the pthogen in the 1990s round the Sn Frncisco By Are, presumly y moving infected plnts, s indicted y the fct tht identicl pthogen genotypes were found t distnces up to 100 miles, rnge tht exceeds the nturl dispersl ility of this microorgnism. Nturl spred of the pthogen my e occurring very efficiently t distnces up to 200 yrds, nd presumly in the presence of strong winds, up to 3 miles. Nturl movement over 6 to 7 miles ppers to e extremely rre. These inferences of the pthogen s spred re congruent with rtes of spred for symptoms. Individul forest ptches few hundred yrds long cn e colonized y P. rmorum within one seson, while movement of the pthogen etween forest ptches 0.5 to 3 miles prt cn lso occur t lest once during the course of riny spring (Meentemeyer et l. 2004; Prospero et l. 2007). Long-distnce spred is not well understood, ut my rely on rre long huls of propgules in strong winds or e linked to humn movement. Oks nd tnoks. Although ny plnt suject to high levels of infection y P. rmorum my perish, oks nd tnoks normlly die in lrge numers. The verge mortlity of oks nd tnoks vries etween 10% nd 40%, ut t the locl level mortlity levels over 50% for dult oks nd up to 100% for dult tnoks hve een recorded (Dvidson et l. 2005; Mloney et t. 2005). Besides their ecologicl importnce, oks nd tnoks enhnce residentil lndscpes nd cn significntly improve property vlues. As ckyrd trees, oks nd tnoks hve ecome prt of the culturl nd socil heritge of Cliforni. As keystone species of costl ecosystems, indigenous people revere them s scred trees. Chemicl tretments tested Phosphonte. Chemicl phosphonte tretments re registered to control sudden ok deth in oks nd tnoks, nd re most effective if dministered preventively efore trees re infected (Grelotto et l. 2007). Phosphontes re environmentlly friendly, nrrowspectrum fungicides tht cn e pplied either y injection in the outer wood or topiclly on the rk. The topicl tretments involve dding the orgnosilicte surfctnt Pentrrk, which does not ply ny direct role in disese control ut llows phosphonte to e sored through the rk. Phosphontes re not toxic to nimls, including fish nd invertertes, nd do not ffect eneficil microorgnisms. Insted, these simple derivtives of phosphorous cid enhnce the Jnury Mrch

3 Left, UC Berkeley postdoctorl resercher Noh Rosensweig uses DNA sequencer to nlyze DNA from P. rmorum; middle, UC Berkeley stff reserch ssocite Lydi Bker extrcts DNA from P. rmorum nd, right, uses rel-time PCR to nlyze it. Aove, three stges of motile, swimming P. rmorum zoospores re relesed from infectious nd irorne sporngi, which re normlly produced y P. rmorum on the surfce of Cliforni y lurel leves during the spring riny seson. production of secondry metolites tht ct s ntiiotics (Grelotto et l. 2008), helping treted plnts to fend off microil infections, including those y P. rmorum. The efficcy of phosphonte tretments hs lredy een proven (Grelotto et l. 2007), ut such tretments were developed only reltively recently nd no informtion is ville on the durtion of their efficcy. Azomite nd lime wsh. Severl unregistered nd unproven tretments re lso ville to purportedly control sudden ok deth in oks nd tnoks. In prticulr, one lterntive tretment involves mending soil with zomite nd using rk lime wsh (see www. suddenoklife.com). Azomite is n orgnic trce minerl dded to soil s fertilizer, while lime wshes re ment to supply clcium to the plnt. Supporters of this pproch clim tht these tretments decrese soil cidity in ffected res nd provide nutrients to infected trees, therey strengthening their defense mechnisms. The leglity of this tretment is questionle ecuse it lcks n officil registrtion in Cliforni or elsewhere in the United Sttes; furthermore, the sence of pulished scientific experiments mkes it difficult to evlute not only its merit nd efficcy, ut lso the presence of potentil side effects. In this study, we present dt from the first controlled experiment investigting the longevity of phosphonte tretments. We lso present novel wy to test tretment efficcy on dult trees in the field without inoculting their min stems, nd descrie results of n experiment compring the efficcy of the registered phosphonte tretment with tht of the lterntive zomite nd lime wsh tretment. Longevity nd efficcy studies Potted ok experiment. We used 45 potted cost live ok trees tht were 6 yers old, 7 to 10 feet tll, nd grown in 15-gllon pots filled with 50:50 mix of snd nd fir mulch. Trees were kept onsite in lthe house t UC Berkeley for 2 months efore the experiment strted. On Nov. 30, 2005, 15 trees were injected with Agrifos (Agrichem, Queenslnd, Austrli), the only phosphonte currently registered to tret sudden ok deth in Cliforni. Another 15 trees were treted with rk ppliction of Agrifos nd the orgnosilicte surfctnt Pentrrk (Quest Products, Louisurg, KS), which is lso registered for sudden ok deth in Cliforni. The surfctnt helps to even ppliction of the phosphonte nd enhnces its dhesion on the tree trunk, so tht the phosphonte cn e progressively sored into the cmium. The remining 15 trees were untreted controls. Injections were pplied with Chemjet injectors (Chemjet Trding, Queenslnd, Austrli) ech contining 10 milliliters of the phosphonte formultion t concentrtion of 217 milligrms per milliliter. For the rk pplictions, we spryed solution of 310 milligrms per milliliter Agrifos nd 1% Pentrrk on 1-yrdhigh nd of the tree min stems, until the rk surfce ws completely wet. 12 CALIFORNIA AGRICULTURE VOLUME 63, NUMBER 1

4 Approximtely 150 milliliters of solution per squre meter of rk ws used for this ppliction. Pr102, widespred P. rmorum genotype (Ivors et l. 2004), ws inoculted under rk of the min stem of treted nd positive control splings s descried in Grelotto et l. (2007). As negtive control, ech tree ws lso mock-inoculted y plcing plug of sterile gr on the opposite side of the stem nd 5 inches higher thn the pthogen inocultion point. The mock inocultion llows reserchers to determine the size of lesions cused y wounding during the inocultion process, in the sence of the pthogen. For ech tretment nd control, five trees were inoculted 6, 12 nd 18 months post-tretment. Six weeks fter ech inocultion, we removed the outer rk nd mesured the extent of the necrotic lesions. To confirm tht lesions were cused y P. rmorum, t lest four isoltions were tken y excising the mrgins of ech visile lesion nd plting them on Phytophthor-semiselective culture medi clled PARP (Erwin nd Rieiro 1996). For ech spling, we clculted the cumultive liner lesion size y dding the mximum liner extent of the lesion long the stem to the mximum lesion extent cross the stem s circumference. The dt ws then logtrnsformed nd nlyzed y ANOVA; multiple comprisons were performed with t-tests t lph = This experiment ws repeted for phosphonte injections, ut the results were nlogous to the first experiment nd re not shown. Field study. We selected 15 mture cost live ok trees in Novto, Clif., whose trunks with dimeters t rest height (DBH) rnged from 24 to 48 inches nd verged 31 inches. On Fe. 14, 2005, five trees were treted s recommended y the mnufcturer with phosphonte nd orgnosilicte surfctnt on the rk (pproximtely 1.35 gllons of tnk mix per tree; mix composed of Agrifos t 0.07 ounce per gllon nd 1% Pentrrk). Another five trees were treted s recommended y the distriutor y dding zomite (143 pounds per tree) to the top orgnic lyer of soil from the tree trunk to the cnopy drip line, nd pplying lime wsh to the rk from UC Berkeley stff reserch ssocite Ellen Crocker uses spring-loded mechnicl syringes to inject phosphonte compounds into Mrin County cost live ok. UC Berkeley stff reserch ssocite Brett Voss pplies topicl phosphonte spry to cost live ok in Almed County to study the tretment s effectiveness. the root collr to 5 to 6 feet up the trunk t pproximtely 2 gllons per tree. The remining five trees were left untreted to serve s controls. The efficcy of these two chemicl tretments ws tested t 6, 12 nd 18 months post-tretment y inoculting 10 cut rnches per tree with P. rmorum s descried in Dodd et l. (2005). Terminl rnches (0.8 to 1.2 millimeters dimeter) were cut, immeditely plced in wter nd trnsported to the UC Berkeley greenhouse. Cuttings were trnsferred to fresh wter nd kept in mist chmer. The rk ws slit with sclpel, nd plug of gr recently colonized y P. rmorum isolte Pr102 ws plced under the rk. Mock control inocultions were performed on one rnch per tree y plcing plug of sterile gr under the rk. All inocultions were wrpped with prfilm (Pechiney Plstic Pckging, Mensh, WI) nd luminum foil. Three weeks fter inocultion, we removed the outer rk nd mesured the extent of necrotic lesions. To confirm tht these lesions were cused y P. rmorum, t lest two isoltions were tken from the edges of ech visile lesion nd plted on PARP. For ech cutting, the extent of the lesion upwrd towrd the tip of the rnch ws dded to the extent of the lesion downwrd towrd the rnch se. Dt ws then logtrnsformed nd nlyzed y ANOVA; multiple comprisons were performed y t-tests with lph set t Efficcy testing. For most plnt diseses cused y Phytophthor spp., there is good correltion etween the size of lesions cused y under-rk inocultions nd susceptiility of the individul plnt (Huerli et l. 2002). By wounding the inocultion site, underrk inocultions ypss the infection process nd thus test the rte of pthogen spred rther thn tht of infection. Becuse it could e rgued tht some tretments effectively prevent infection ut do not necessrily slow disese progression, we designed wy to directly test the efficcy of tretments in preventing infection y P. rmorum. To circumvent the limittions of under-rk inocultions, we used more relistic inocultion test on 40 potted oks like those used in the first experiment. Before tretment, the rk ws gently scrped t the selected inocultion points to ese direct infection y zoospores. We treted 10 trees with rk ppliction of phosphonte nd orgnosilicte surfctnt (Agrifos nd Pentrrk) nd 10 with the zomite soil mendment nd rk lime wsh; 20 were left untreted. Fourteen dys lter, the 20 treted trees nd 10 Jnury Mrch

5 A plstic device developed t UC Berkeley is used to rtificilly infect trees with zoospores of P. rmorum. of the untreted trees were inoculted with zoospores of Pr102 isoltes. The other 10 untreted control trees were mock-inoculted using wter without zoospores. To produce P. rmorum zoospores, ten 1-squre-centimeter plugs were cut from the mrgins of 2-week-old colonies nd floted on sterile deionized wter in 100-millimeter-dimeter Petri dishes. The pltes were incuted in the drk t room temperture for 72 hours (Erwin nd Rieiro 1996). At the end of the incution period, the wter used to flood ech of the 10 pltes ws poured into plstic ottle with 250-milliliter-wide mouth (Nlgene, Rochester, NY). To otin dditionl zoospore solution, ll pltes were rinsed second time with Phosphonte tretments reduce oth coloniztion nd infection rtes of the sudden ok deth pthogen. 10 milliliters of sterile deionized wter, nd second rinses were dded to the first ones in the sme ottles. The ottle cp ws then secured without tightening it, to llow for ir flow. We induced sporngi to relese zoospores y plcing the ottle on ice for 30 minutes, followed y 1 hour t room temperture. The drk lesion in this Almed County cost live ok my continue to grow until it girdles nd eventully kills the tree. In order to visully verify tht zoospore relese hd occurred, zoospore counts were performed using hemcytometer (Husser Scientific, PA). Once zoospores were produced nd their concentrtion djusted to per milliliter, 1.5 milliliters of zoospore suspension ws dded to n inocultion lock plced round the inocultion point. The locks, mde out of polyurethne resin, hd concve surfces tht conformed to the round rk surfce. Ech inocultion lock hd two openings: one in the concve side to llow delivery of zoospores to the inocultion point, nd nother on top to llow the opertor to plce the desired liquot of zoospore suspension in the lock. Grfting wx ws used to ensure perfect sel etween the inocultion lock nd tree surfce. Blocks were left for 24 hours with the upper opening corked. After the locks were removed, the inocultion points were entirely wrpped with prfilm nd foil. Six weeks fter inocultion, we removed the outer rk nd mesured the extent of necrotic lesions. To confirm tht these lesions were cused y P. rmorum, t lest four isoltions were tken from the edges of ech visile lesion nd plted on PARP. For ech spling, we clculted the cumultive liner lesion size y dding the mxi- mum liner extent of the lesion long the stem to the mximum lesion extent cross the stem s circumference. The dt ws then log-trnsformed nd nlyzed y ANOVA; multiple comprisons were performed y t-tests with lph set t Tretment comprisons Phosphonte nd potted oks. Compred to the untreted trees, cumultive lesions were significntly smller in the potted trees treted with phosphonte injections nd rk pplictions of phosphonte nd orgnosilicte surfctnt (Agrifos nd Pentrrk) for ech post-tretment dtset (df = 2; 25 < F < 76.3; < P < ), s indicted y ANOVA performed seprtely on the 6-, 12- nd 18-month dtsets (fig. 1). Tretment efficcy ws lso detected 1 nd 3 months post-tretment (dt not shown). In ll cses, P. rmorum ws isolted t lest once from ech inoculted tree, independent of tretment, confirming tht the lesions visile in the phloem were cused y the inoculted pthogen. Negtive mock inocultions never cused ny lesions eyond the wound-induced one, nd were excluded from the nlysis. Field study. The rnch cuttings tht we plced in wter nd kept in mist chmer survived well during the length of ech inocultion tril. P. rmorum ws reisolted t lest once from ech inoculted rnch, confirming tht lesions visile in the phloem were cused y the inoculted pthogen. There ws never significnt difference etween the size of lesions in untreted trees nd in those treted with the zomite soil mendment nd rk lime wsh, s indicted y ANOVAs performed on ech individul dtset of inocultions t 6, 12 nd 18 months fter tretment (fig. 2). Conversely, the verge lesion size in tree rnches treted with rk pplictions of phosphonte nd orgnosilicte surfctnt ws lwys significntly smller thn those in untreted tree rnches (df = 2; 56 < F < 79.4; P < ). Phosphonte efficcy ws lso detected 1 nd 3 months posttretment (dt not shown). Efficcy testing. Potted trees were successfully infected y zoospore suspensions pplied through the inocul- 14 CALIFORNIA AGRICULTURE VOLUME 63, NUMBER 1

6 Lesion size (mm) C PI TP + O Lesion size (mm) C AZ + L TP + O Lesion size (mm) c Months Months 0 C AZ + L TP + O NC Tretment Fig. 1. Cumultive liner size of lesions (long nd cross stem) cused y under-rk inocultion of Phytophthor rmorum onto the min stem of potted trees t 6, 12 nd 18 months fter tretment. Tretments included injections with phosphonte (PI), topicl rk ppliction of phosphonte nd orgnosilicte surfctnt (TP + O) nd untreted controls (C). Averge lesion size nd stndrd error in wounded ut uninoculted negtive controls were 16.6 nd 0.6 millimeters, respectively. tion locks. Lesion sizes in trees treted with the zomite soil mendment nd rk lime wsh were not sttisticlly different from those in untreted trees (fig. 3). In contrst, lesions in trees whose rk ws treted with topicl ppliction of phosphonte nd orgnosilicte surfctnt were oth significntly smller thn those of untreted trees (df = 2; F = 37.6; P < 0.001) nd significntly lrger thn those of the mockinoculted trees. Recommendtions for SOD tretment Introduced plnt diseses represent one of the most serious threts to endemic plnt communities in Cliforni nd cn led to the decimtion or even extinction of ffected plnt hosts (Wingfield et l. 2001). The high levels of mortlity cused y sudden ok deth mong some ok species nd tnoks re vivid exmple of the intensity nd scle of dmge tht cn e cused y n introduced microorgnism. Genetic nlyses confirm the exotic nture of the sudden ok deth pthogen (Ivors et l. 2006; Mscheretti et l. 2008), intensifying the need for pproches imed t oth slowing down the epidemic nd protecting tree species lethlly ffected y P. rmorum. Fig. 2. Totl liner size of lesions long stems of rnch cuttings cused y under-rk inocultion of Phytophthor rmorum t 6, 12 nd 18 months from tretment of prent trees from which cuttings cme. Tretments included topicl rk ppliction of phosphonte nd n orgnosilicte surfctnt (TP + O), zomite soil mendment nd rk lime wsh (AZ + L) nd untreted control (C). Averge lesion sizes (stndrd error) in wounded ut uninoculted negtive controls were 14 (0.3), 13 (0.2) nd 18 (0.3) millimeters, respectively, for the 6-, 12- nd 18-month trils. Phosphontes hve long een used in griculturl situtions, including vocdo orchrds in Cliforni to control other Phytophthor species (Guest et l. 1995), s well s in nturl ecosystems, including the Jrrh forests of Western Austrli (Hrdy et l. 2001). Phosphontes re fungicides trgeting smll ut importnt group of plnt pthogens clled oomycetes. The ction of phosphontes is often dul, with direct contct effect ugmented y n incresed defense response in the treted plnt, which my include rnge of physiologicl chnges from cell-wll thickening to the incresed production of ntimicroil secondry metolites (Guest nd Grnt 1991). However, P. rmorum is not prticulrly susceptile to direct contct with phosphonte (Grelotto et l. 2008), nd most of this tretment s efficcy hinges on the ugmented production of defensive secondry metolites. While registered in Cliforni to tret sudden ok deth in oks nd tnoks, phosphonte tretments re not necessry for lue ok, vlley ok, Oregon ok or interior live ok, ll of which hve never een reported s infected y P. rmorum. Becuse sudden ok deth is n emergent disese with Fig. 3. Cumultive liner size of lesions (long nd cross stem) cused y pplying zoospores of Phytophthor rmorum onto min stem of potted trees 42 dys fter tretment. Tretments included topicl rk ppliction of phosphonte nd orgnosilicte surfctnt (TP + O), zomite soil mendment nd rk lime wsh (AZ + L), untreted controls (C) nd negtive control in which rk ws gently scrped ut no zoospores were pplied (NC). only recent history in Cliforni, the longevity of efficcy for registered tretments is unknown, lthough controlled studies hve shown tht tretments re most effective if dministered preventively efore trees re infected (Grelotto et l. 2007). Phosphonte tretments hve een effective for 2 to 6 yers in other Phytophthor species, depending on the host species (Hrdy et l. 2001). Our first potted tree experiment nd the field experiment indicted tht phosphonte tretments hd significnt effect t 6, 12 nd 18 months postppliction; future testing will deter- The zomite nd lime rk wsh showed no efficcy whtsoever. mine whether they re effective for longer. Becuse most P. rmorum infections occur in lte winter nd spring, 18 months of coverge will dequtely protect trees for 2 yers. However, in order to otin the highest level of control using phosphonte tretments, recommended schedule of preventive pplictions for oks nd tnoks includes two pplictions the first yer to ensure Jnury Mrch

7 Top left, UC Berkeley stff reserch ssocite Alex Lundquist uses pole pruner to collect rnches of Mrin County cost live ok. Top right, smll plug of gr is used to infect the rnches with P. rmorum. Aove, UC Berkeley stff reserch ssocites Ellen Crocker nd Lundquist inoculte ok rnches in the lortory. This study method llows reserchers to sfely infect ok wood with P. rmorum in the lortory nd test vrious sudden ok deth tretments. full coverge (one in lte fll nd one in erly spring), followed y one fll tretment every yer. Fll tretments re recommended ecuse they llow the plnt to develop full defensive response efore infection spreds in the wrm nd riny spring. The choice of phosphonte ppliction method (injection or rk spry) is to some extent personl one. Injections relese the totlity of the product inside the tree without the orgnosilicte surfctnt, presum- growth rings of the wood. Asorption of the chemicl ppers to e fstest on wrm sunny dys, from the lte morning to midfternoon. Brk topicl pplictions re rpid nd esy; however, post-ppliction efficcy tkes longer, the orgnosilicte surfctnt must e used in conjunction with phosphonte, the chemicls cn disperse into the environment, nd ll types of folige including moss re t risk of phytotoxicity upon contct with the chemicl. It is est to mix the phosphonte nd orgnosilicte surfctnt on tretment dy. Applictions cn e done with ny kind of pplictor or spryer. The lower 10 feet of trunk must e thoroughly wetted, possily including some of the lower min rnches in the cnopy. The field experiment in which dult trees were treted ut efficcy ws tested on cut rnches rther thn on the entire tree ws successful. This pproch my provide new tool to study tretment options for sudden ok deth nd other forest diseses without killing plnts or using dngerous inoculum in the field. P. rmorum grew significntly slower in trees inoculted with phosphontes t 6, 12 nd 18 months fter tretment, while the pthogen s growth rte with the zomite nd lime wsh tretment ws lwys the sme s tht of untreted controls. We found tht the phosphonte topicl tretment ws effective up to 18 months from ppliction, while the zomite nd lime rk wsh showed no efficcy whtsoever t ny of the three time periods tested. The second potted tree experiment ws designed to directly ssess the efficcy of tretments on infection rther thn coloniztion rtes of the pthogen. The results indicted tht the phosphonte topicl ppliction significntly reduced the size of infection lesions cused y zoospores, ut the zomite nd lime wsh tretment hd no effect. Besides this lck of efficcy, we found tht the sheer mount of mteril needed to tret ech tree with zomite nd lime wsh ws cumersome nd estheticlly unplesnt due to the lrge mounts of zomite on the ground round treted trees. Although this tretment is eing touted s holistic fertilizing pproch imed t enly chieving efficcy fster; ut specilized equipment nd trining re needed to drill multiple holes in the tree, nd certin numer of injections my fil, especilly in gnrly trees with undnt decy pockets. Although single injection ws pplied to our potted trees, lrger trees require multiple injections, pproximtely one for every 6 inches (15 centimeters) of tree circumference. Holes re drilled t slight downwrd ngle through the rk nd cmium into the outer three to six 16 CALIFORNIA AGRICULTURE VOLUME 63, NUMBER 1

8 hncing the overll helth of the tree, it is often nd illeglly prescried to directly control sudden ok deth. While true fertiliztion tretments do not require registrtion, those used to directly control disese do; s such, the zomite nd lime wsh tretments pper to e in legl grey zone. Our results highlight the importnce of continued testing pproches oth to verify the efficcy of tretments nd to etter understnd their rel potentil to control sudden ok deth. Mny spects of phosphonte tretments re still unknown, including their longerterm efficcy nd the percentge of ok nd tnok tree popultions tht will respond positively. Phosphonte tretments re one component of n integrted disese mngement pproch tht lso includes snittion nd the elimintion of infectious plnt hosts. Becuse phosphonte is most effective when pplied preventively efore tree is infected, one of the gretest chllenges is identifying good cndidte trees. Unfortuntely, trees undergo phse in which P. rmorum infection is ltent. Our improved understnding of sudden ok deth epidemiology cn help with the selection of good cndidte trees, which my include symptomtic oks locted in disese-infested groves. In tnoks, the disese my ffect lrge numers of contiguous trees, while pprently helthy trees re most likely lredy infected. To circumvent this prolem, we recommend extending the tretment of tnoks t lest 328 to 656 feet (100 to 200 meters) into the surrounding re tht is free of the pthogen. Finlly, given the novelty of the phosphonte tretment nd limited knowledge of sudden ok deth, tretment recommendtions nd guidelines will likely chnge s more reserch dt ecomes ville. M. Grelotto is Associte Extension Specilist in Forest Pthology nd Mycology nd Adjunct Associte Professor of Forest Pthology, nd D.J. Schmidt is Stff Reserch Associte, oth in Deprtment of Environmentl Science, Policy nd Mngement, UC Berkeley. This reserch ws mde possile y competitive grnt from the USDA, nd with support from the Gordon nd Betty Moore Foundtion. References Dvidson JM, Wicklnd AC, Ptterson HA, et l Trnsmission of Phytophthor rmorum in mixedevergreen forest in Cliforni. Phytopthol 95: Dodd RS, Huerli D, Douhovnikoff V, et l Is vrition in susceptiility to Phytophthor rmorum correlted with popultion genetic structure in cost live ok (Quercus grifoli)? New Phytoptholog 165: Erwin DC, Rieiro OK Phytophthor Diseses Worldwide. Americn Phytopthologicl Society. St. Pul, MN. 592 p. Grelotto M, Dvidson JM, Ivors K, et l Nonok ntive plnts re min hosts for sudden ok deth pthogen in Cliforni. Cl Ag 57(1): Grelotto M, Hrnik TY, Schmidt DJ Efficcy of phosphonic cid, metlxyl-m, nd copper hydroxide ginst Phytophthor rmorum in vitro nd in plnt. Plnt Pthol. In press. DOI: /j x. Grelotto M, Schmidt DJ, Hrnik TY Phosphite injections nd rk ppliction of phosphite + Pentrrk control sudden ok deth in cost live ok. Aroricult Urn Forest 33(5): Grelotto M, Svihr P, Rizzo DM Sudden ok deth syndrome fells three ok species. Cl Ag 55(1):9 19. Guest DI, Grnt BR The complex ction of phosphontes s ntifungl gents. Biolog Rev Cmr Philosoph Soci 66: Guest DI, Pegg KC, Whiley AW Control of Phytophthor diseses of trees crops using trunk-injected phosphontes. Horticult Rev 17: Hrdy G, Brrett S, Sherer BL The future of phosphite s fungicide to control the soilorne pthogen Phytophthor cinnmomi in nturl ecosystems. Austrls Plnt Pthol 30: Huerli D, Tommerup IC, Colquhoun IJ, Hrdy GE St J Evlution of resistnce to Phytophthor cinnmomi in seed-grown trees nd clonl lines of Euclyptus mrgint inoculted in lterl rnches nd roots. Plnt Pthol 5: A Mrin County cost live ok ws used to test n lterntive sudden ok deth tretment, in which rk is covered with lime wsh nd n zomite soil mendment is pplied round the tree. In this study, the unregistered tretment ws not effective in stemming the disese s spred. Ivors K, Grelotto M, Vries ID, et l Microstellite mrkers identify three lineges of Phytophthor rmorum in US nurseries, yet single lineges in US forest nd Europen nursery popultions. Molecul Ecol 15: Ivors KL, Hyden KJ, Bonnts PJ, et l AFLP nd phylogenetic nlyses of North Americn nd Europen popultions of Phytophthor rmorum. Mycolog Res 108: Mloney PE, Lynch SC, Kne SF, et l Estlishment of n emerging generlist pthogen in redwood forest communities. J Ecol 93: Mscheretti S, Croucher PJP, Vettrino A, et l Reconstruction of the sudden ok deth epidemic in Cliforni through microstellite nlysis of the pthogen Phytophthor rmorum. Molec Ecol 17: Meentemeyer R, Rizzo DM, Mrk W, et l Mpping the risk of estlishment nd spred of sudden ok deth in Cliforni. Forest Ecol Mnge 200: Prospero S, Hnsen EM, Grunwld NJ, et l Popultion dynmics of the sudden ok deth pthogen Phytophthor rmorum in Oregon from 2001 to Molec Ecol 16: Rizzo DM, Grelotto M Sudden ok deth: Endngering Cliforni nd Oregon forest ecosystems. Frontier Ecol Env 1: Rizzo DM, Grelotto M, Dvidson JM, et l Phytophthor rmorum s the cuse of extensive mortlity of Quercus spp. nd Lithocrpus densiflorus in Cliforni. Plnt Dis 86: [USDA APHIS] US Deprtment of Agriculture Animl Plnt Helth Inspection Service Phytophthor rmorum: Qurntine nd regultion. Fed Reg 72: Wingfield MJ, Slippers B, Roux J, Wingfield DB Worldwide movement of exotic forest fungi, especilly in the tropics nd southern hemisphere. Bioscience 51(2): Jnury Mrch