ORIGINAL PAPER

Ralstonia solanacearum, Ganoderma australe, and bacterialwetwood as predictors of ironwood tree (Casuarina equisetifolia)decline in Guam

C. M. Ayin1& A. M. Alvarez1 & C. Awana1 & F. M. Schleinzer1 & B. D. Marx2 & R. L. Schlub3

Received: 15 August 2018 /Accepted: 2 October 2019# Australasian Plant Pathology Society Inc. 2019

AbstractIronwood (Casuarina equisetifolia subsp. equisetifolia) trees on Guam have been in decline since 2002. This study appliedproportional odds logistic multiple regression modeling to a set of biological variables in order to find significant declinepredictors as a first step towards identifying pathogenic contributors. Based on the analysis of a set of 77 medium and largetrees, the bacterium Ralstonia solanacearum and the fungus Ganoderma australe species complex were found to be significantpredictors, with p values of <0.001 and < 0.008, respectively. Their respective values for symptomless trees were 18% and 0%,compared to 80% and 35% for those nearly dead trees of which 30% had both organisms and 15% had neither. More complexmodels were also fit, that included interactions. TheR. solanacearum byG. australe interaction was not a strong effect, indicatingadditive rather than multiplicative behavior of these effects on decline severity (p value>0.087). When nine covariates wereapplied univariately to a data subset of 30 tree cross-sections, the significance of R. solanacearumwas strongly upheld while thatof G. australe was moderately reduced. Also significant were percent cross-sectional area with bacterial wetwood and theformation of ooze within 24 h. Wetwood bacteria of Klebsiella spp. and K. oxytoca were found across all levels of declineand were not significant predictors. Other enteric bacteria identified included Kosakonia, Enterobacter, Pantoea, Erwinia, andCitrobacter.

Keywords Casuarina equisetifolia . Tree decline . Ralstonia solanacearum . Klebsiella sp. . Ganoderma australe . Wetwood .

TZCmedium

Introduction

Casuarina equisetifolia subsp. equisetifolia, often referred toas ironwood, is indigenous to Southeast Asia, Malaysia,Northern Australia, Oceania (Potgieter et al. 2014) and possi-bly to Guam and the NorthernMariana Islands (Safford 1905).Casuarina equisetifolia is considered an integral member ofGuam’s natural landscape (Stone 1970; Fosberg et al. 1979).

Pollen dating confirms its presence in Guam for hundreds ofyears (Athens and Ward 2004). A 2002 forestry inventoryreported ironwood among the healthiest tree species on theisland, based on a survey of over 115,000 ironwood treeslarger than 12.7 cm in diameter at breast height (DBH)(Donnegan et al. 2004).

However, in the same year, a local farmer reported that fiveof his 10-year old ironwood trees, planted as part of a wind-break, exhibited symptoms of rapid yellowing and death.Cross-sections of these trees exhibited areas of wetwood thatwere dark, water infused, and radiated from their centers(Mersha et al. 2009; Mersha et al. 2010a; Schlub et al.2011a). Droplets of bacterial ooze appeared inside and outsidethe wetwood stained areas. More numerous at the farm, andappearing elsewhere on Guam, were trees with the samecross-sectional symptoms but accompanied by thinning fo-liage and a lethal progressive dieback. This latter conditionis now referred to as ironwood tree decline (IWTD) (Mershaet al. 2009; Mersha et al. 2010a; Schlub et al. 2011a).

* R. L. Schlubrlschlub@triton.uog.edu

1 Department of Plant and Environmental Protection Sciences,University of Hawaii at Manoa, 3190 Maile Way,Honolulu, HI 96822, USA

2 Department of Experimental Statistics, Louisiana State University,Baton Rouge, LA 70803, USA

3 University of Guam Cooperative Extension Service,Mangilao, GU 96923, USA

https://doi.org/10.1007/s13313-019-00666-8Australasian Plant Pathology (2019) 48:625–636

/Published online: 12 November 2019

At the time, the presence of ooze was attributed entirely tononpathogenic endophytes associated with bacterial wetwoodand none to the bacterial wilt pathogen, Ralstoniasolanacearum. Though it was known that wetwood had beenreported in Casuarina in Mauritius as well as in other treespecies in Indonesia (Supriadi and Sitepu, 2001), it had notbeen reported as a disease of Casuarina nor a symptom ofbacterial wilt in China (Liang and Chen 1982) or India (Aliet al. 1991). Other than causing a reduction in lumber quality,the presence of wetwood is generally considered benign(Ward and Pong 1980; Jeremic et al. 2004) and the resulteither of microbial activity (principally bacteria), injury, ornormal aging (Ward and Pong 1980; Jeremic et al. 2004).

Due to the belief that the observed bacterial ooze was com-posed of one or more species of wetwood bacteria and notR. solanacearum, other possible explanatory variables forIWTD were investigated. As a result of an island wide surveyof 1427 trees, the level of management intensity, presence ofbasidiocarps, and termites were determined to be significantpredictive variables (Schlub 2010; Schlub et al., 2011a). Asurvey conducted in Guam and Saipan in 2010 led to theconclusion that the Ganoderma australe species complexwas the basidiocarp most likely responsible for decline(Mersha et al. 2011; Schlub et al. 2012a). In 2015, the prom-inent termite species infecting 93% of Guam’s ironwood treeswas determine to be Nasutitermes takasagoensis or a closelyrelated species from the N. takasagoensis complex (Park et al.2019).

The discovery in 2010 that wood drill shavings andooze f r om in f e c t ed t r e e s t e s t e d po s i t i v e f o rR. solanacearum using immunodiagnostic strips (Agdia,Inc.) (Mersha et al. 2010b; Schlub et al. 2011b; Schlubet al. 2012b) led researchers to suspect that this bacterialwilt pathogen may in fact play a role in IWTD. The richbacterial flora associated with wetwood made it difficultto obtain pure cultures of R. solanacearum (Ayin et al.2013; Schlub et al. 2013). Ayin et al. (2013, 2015)succeeded in purifying R. solanacearum as well as otherbacteria from infected tree tissues using a semi-selectiveRalstonia medium (mSMSA)(Engelbrecht 1994) and amodified Kelman’s tetrozolium chloride medium(TZC)(Norman and Alvarez 1989). Shortly thereafter,t h e s e a u t h o r s d e m o n s t r a t e d t h a t G u a m ’ sR. solanacearum strains causes wilting and death in iron-wood seedlings (Ayin et al. 2013; Ayin et al. 2015). Inaddition to R. solancearum, Klebsiella spp. also dominat-ed the cultures obtained from bacterial ooze emanatingfrom diseased ironwood trees and were further identifiedas K. oxytoca and K. variicola (Ayin et al. 2013; Ayinet al. 2015). In other forest tree species, Klebsiella spp.are thought to play a role in the formation of wetwood(Hartley et al. 1961; Jeremic et al. 2004). With the failureof strains of Klebsiella to cause wilt of inoculated

seedlings (Ayin et al. 2015), Klebsiella spp. weredismissed as poss ib le aggress ive pathogens ofC. equisetifolia but were not ruled out as secondary oropportunistic pathogenic contributors to IWTD.

The primary cause or causes of IWTD remains elusive, yetin previous studies several possible biological variables havebeen identified. The aim of this study was to apply propor-tional odds logistic regression modeling to evaluate the signif-icance of R. solanacearum and Ganoderma australe aspredicators of IWTD. Additionally, we report the analyses of30 tree cross-sections for K. oxytoca, ooze formation, andbacterial wetwood as predictors of decline.

Materials and methods

Tree selection, processing, and data analyses

Seventy-seven ironwood trees were visually inspected, sam-pled, and growth measurements recorded. Trunk diameter incentimeters was determined at breast height (1.3 m) (DBH).Also determined was tree height in meters. Through visualinspection, decline severity (DS) was recorded as one of fiveordinal categories based on fullness of branches and dieback(0 = symptomless, 1 = slight damage, 2 = distinctly damaged,3 = heavily damaged, and 4 = nearly dead) (Schlub et al.2011a). The estimated percentage of bare branches associatedwith categories of DS for trees larger than 32 cmDBH are 0, 9,49, 67, and 98, respectively (Schlub et al. 2011a). Trees wereselected randomly from 15 sites, of which 10 sites were usedin previous studies (Fig. 1) (Schlub 2010; Schlub et al. 2011a;Schlub et al. 2011b). Each tree was examined for presence/absence of the wood-rot fungus G. australe species-complexbased on basidiocarp morphology. Tree drill shavings wereassayed for R. solanacearum using R. solanacearum-specificimmunostrip tests kits manufactured by Agdia Inc. of Elkhart,Indiana, USA (catalog number: STX 33900 and ISK 33900)(Ayin et al. 2015). The test kit consisted of a sample extractbag containing 3 ml of BEB1 buffer and a test strip. Theprocedure involved placing 150 mg of wood shavings in theextract bag, mixing thoroughly, dipping the end of the teststrip into the solution, and then waiting for the appearanceof a colored test line.

Secondly, a 30-tree subset of the original 77 trees was se-lected for further analyses. Symptomless trees and those withIWTD symptomswere sampled among the windbreaks onMr.Bernard Watson’s farm that formed part of the initial declinereport in 2002. Site and soil descriptors included slope of 3-7degrees, parent material coralline limestone, map unit Guamcobbly clay loam, and family clayey, gibbsitic, nonacid,isohyperthermic Lithic Ustorthents (Soil Survey Staff 2015).Using a chainsaw, trees were felled with a transverse cut 51-76 cm above the ground, followed by an additional cut to

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flatten the stump and clean the blade of any debris from pre-vious cuts. Approximately 12 cm of bark was removed fromthe top of the stump. For analysis, a single 2.5-5 cm cross-sectional disc/slice was removed from the top of each stump.To remove debris, cross-sections were wiped with a clean,moist paper towel.

Under laboratory conditions, cross-sections werephotographed, sampled for bacteria, and examined for evi-dence of bacterial wetwood and G. australe wood-rot.Percent wetwood area was based on those areas that exhibiteddark discoloration that radiated from the center of the section.Additionally, these areas had one or more of the followingcharacteristics: higher moisture than adjacent tissue, a darkline of demarcation at its outer edge, and the presence of ooze.Cross-sections were examined for differences in ooze forma-tion as a means of distinguishing ooze from R. solanacearumfrom that from wetwood bacteria. The presence or absence ofooze within 24 h was determined (ooze initiation). Ooze wasrecorded as one of three types: i) a viscous white to off-white

substance (VO); ii) a watery, amber ooze (WO); or iii) a mix-ture of types i and ii (MO). Ooze quantity was recorded as theamount of ooze formed within 72 h: none, slight (less than 10single drops) or abundant (10 or more single drops). For sta-tistical purposes, the quantity score assignments were 0, 1, and4, respectively. Evidence of G. australe wood-rot includedsoft white rot or the presence of mycelial strands characteristicof G. australe.

Within 24 h, tree ooze and drill shavings were collectedfrom cross-sections for bacterial analyses. Drill shavings fromsix holes (2-3 cm deep and 0.5 cm in diameter, into the center,middle, and outer areas of each tree cross-section) were col-lected. Ralstonia solanacearum immunostrip testing was con-ducted on samples: either 150 mg of wood shavings, 80 μL ofwater from soaked drill shavings or a droplet of ooze.Shavings (2 g) from each hole were placed into 6 ml of sterilewater. Two 1 ml subsets were transferred to 1.5 ml Eppendorfmicrocentrifuge tubes for shipment to Hawaii. When present,ooze was scrapped from cross-sections and transferred to two

Fig. 1 Guam ironwood tree(Casuarina equisetifolia) surveymap, 15 sites in the current study(shaded), 10 were a part of aprevious survey of 38 sites, inwhich averages of declineseverity (DS) were determined,DS = 0 (symptomless) to 4(nearly dead) (Schlub et al.2011a)

Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree... 627

empty 0.1 ml Eppendorf microcentrifuge tubes for shipment.In both cases, the two tubes constituted a set for culturing andanother for LAMP assay. The LAMP assay sample sets weretreated in a simmering water bath for 10 min in an attempt tostop bacterial growth while preserving DNA.

The analysis of the ironwood decline data was performedusing proportional odds logistic regression. An excellent in-troduction to these models can be found in Agresti (2019).The choice of the proportional odds logistic regression modelwas based on the fact that there were five ordinal levels of theresponse category IWTD. The statistical models and analyseswere generated using SAS software, Version 9.4.

Isolation, purification, DNA extraction of bacteria

Culturable water samples were vortexed for 1 min, then two tothree loopfuls of the suspension were dilution streaked ontopetri plates containing modified TZC medium (Norman andAlvarez 1989; 17 g/L agar, 10 g/L peptone, 5 g/L glucose and0.001% 2, 3, 5-triphenyl-tetrazolium chloride) and then incu-bated at 26 °C (± 2 °C). To the culturable ooze samples, 100 μlof sterile deionized water containing 25 mg/L cycloheximidewas added before vortexing and streaking. Two types of col-onies were saved for further purifications: i) those resemblingthe raised, white colonies of Klebsiella with red to crimsoncenters and transparent margins and ii) colonies that did notappear to be Klebsiella but were similar enough in morphol-ogy to warrant further purification. Cultures were consideredpure after a minimum of three successive single-colony isola-tions were achieved through dilution streaking onto TZCplates. Purified strains were cultured on peptone glucose me-dium (17 g/L agar, 10 g/L peptone, 5 g/L glucose) for shortterm storage at 4 °C and transferred to 1% w/v tryptone, 0.5%w/v yeast extract containing 15% v/v glycerol for long termstorage at −80 °C.

DNA was extracted from cultures using either Tris-EDTA buffer or a DNA purification kit. The buffer ex-traction consisted of taking a loopful of a single agarcolony and placing it into 1 mL of Tris-EDTA (Tris-HCL C4H12ClNO3, disodium ethylenediaminetetraace-tate: Na2C10H1408N2. 2H20) buffer (10 mM Tris-HCL;pH 7.5, 1 mM EDTA; pH 8.0) and heated for 10 minat 95 °C in a heat block. The DNA purification kitconsisted of growing a culture for approximately 10 hin yeast extract broth (0.8% w/v peptone and 0.01%w/v yeast extract) and then extracted using the WizardGenomic DNA purification kit (Promega, Corp.,Madison, WI) following the manufacturer’s instructions.DNA extracted using both methods was quantified with aNanodrop ND-100 Spectrophotometer (NanoDrop tech-nologies, Inc., Rockland, DE) and adjusted to approxi-mately 20 ng/μL with Tris-EDTA buffer.

Presumptive identification of enteric bacteriaand phylogenetic tree

Purified strains were designated Gram negative or Gram pos-itive based on the potassium hydroxide test (Gregersen 1978).Oxidation-fermentation tests (Hugh and Leifson 1953) wereperformed in 96 well plates containing 200 μL of Difco OFbasal medium (Becton, Dickinson and Co., Sparks, MD) sup-plemented with 1% glucose. Oxidase tests (Gordon andMcLeod 1928) were performed on isolates transferred to glassslides with toothpicks. Single drops of oxidase reagent(Becton, Dickinson and Company, Sparks, MD) were addedto the slides. Since culturing R. solanacearum is difficult inthe presence of wetwood bacteria, tree samples were assessedfor the presence or absence of R. solanacearum by LAMPassessment of the heat-treated samples.

The 16S rDNA PCR was carried out in a 10 μL reactioncontaining 5 μM of the primer pair fd1 (5′-AGA GTT TGATCC TGG CTC AG-3′) and rp2 (5’-ACG GCT ACC TTGTTA CGA CTT-3′) (Weisburg et al. 1991), 1X JumpstartREDtaq Ready Mix (Sigma-Aldrich, St. Louis, MO) and1 μL of purified DNA (~20 ng). PCR amplification reactionconditions were as follows: a denaturing step at 94 0 C for7 min, followed by 30 cycles of 94 0 C for 30 s, 55 0 C for1 min, 72 0 C for 2 min, with a final step at 72 0 C for 2 min toallow for elongation of all PCR products. PCR products wereverified by gel electrophoresis on a 1.5% agarose gel stainedwith ethidium bromide. For sequencing, excess primers andnucleotides were removed using ExoSap-It (Affymetrix Inc.,Santa Clara, CA) according to the manufacturer’s instructions.

The phylogenetic relationships between the putativeKlebsiella strains isolated from ironwood tissues and knownKlebsiella strains as well as related species were determinedby 16S sequence analysis. Comparisons were also made withKlebsiella strains A6125-A6128 previously isolated in Guam(Ayin et al. 2013). Sequencing was performed at the BiotechCore facility, University of Hawaii at Manoa. Trees were root-ed using an orthologous sequence from Vibrio cholerae strainATCC 14035 (NCTC 8021) while reference strain sequenceswere obtained from the National Center of BiotechnologyInformation database. The analysis involved 53 nucleotidesequences. Codon positions included were 1st + 2nd + 3rd +Noncoding. There were a total of 1517 positions in the finaldataset. The evolutionary history was inferred using theMaximum Parsimony method in MEGA X (Kumar et al.,2018), applying the Subtree-Pruning-Regrafting (SPR) algo-rithm with search level 1 in which the initial trees were ob-tained by the random addition of sequences (10 replicates).

Five strains representing different genera, identifiedthrough 16S sequence analysis, were further characterized.BIOLOG identification scores were obtained usingBIOLOG Gen III plates, MicroStation reader and database(BIOLOG, Hayward, CA, and U.S.A). Identification scores

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for the API system required access to the API online identifi-cation website (API web, https://apiweb.biomerieux.com). Animmunoblot assay was performed on the purified isolates(Kaneshiro 2003; Alvarez et al. 2005) with modifications(Yasuhara-Bell et al. 2015). Immunoblots were analyzed withthe aid of a dissecting scope. LAMP reactions were performedusing previously established primers and protocols (Yasuhara-Bell et al. 2015). Primers developed specifically for K.oxytoca and K. variicola were used individually with 5 μLof DNA extracted with Tris-EDTA or purification kit. LAMPreactions were run using an iQ5 Multicolor Real-Time PCRdetection system (Bio-Rad, Hercules, CA) for 40 min at65 °C, with fluorescence readings at 30-s or 1-min intervals.

Results

Tree survey and analyses

The presence of both R. solanacearum and G. australe waswidespread across Guam. At the 15 survey sites used in thisstudy (Fig. 1), 11 were positive for R. solanacearum and 7 forG. australe.At least one of these organisms was found in sitesthat had a decline severity level of 1 or greater in 2009 (Fig. 1).From a summary of the 77-tree data set (Table 1), the frequen-cy of R. solanacearum and G. australe increased both singu-larly and together with increased decline severity. Ralstoniasolanacearum was found in 18% of the symptomless trees,while neither organism was found in 15% of the nearly deadtrees. The trees’ DBH and height, ranged from 15 to 79 cm to4-21 m, respectively.

A proportional odds logistic regression model using 77trees was fit with the covariates DBH, R. solanacearum andG. australe. The DBH (p = 0.33), as well as height (p = 0.25),were found to be non-significant predictors of decline, where-as G. australe was found to be significant (p < 0.008) andR. solanacearum was found to be highly significant (p <0.001). As a further note of goodness-of-fit, the proportionalodds model chi-square yielded a good fit (p > 0.18).Interaction effects were also explored. The R. solanacearumby G. australe interaction was not a strong effect, indicating

an additive rather than a multiplicative behavior of these ef-fects on decline severity (p value>0.087). Further, the resultscan be interpreted such that if a tree exhibits Ganodermabasidiocarps, the odds of that tree being symptomless on theordinal scale (DS 0-4) is reduced by 75%, whenR. solanacearum is held fixed. If G. australe is held fixedand a tree tests positive for R. solanacearum, the odds of thattree being symptomless is reduced by 78% in comparison tothe absence of R. solanacearum. Said differently, the recipro-cal of the odd-ratios states that the odds of a symptomlessclassification is estimated to be approximately 4.5 times great-er when a tree tests negative for R. solanacearum, holdingG. australe fixed and 4.0 times greater when a tree tests neg-ative for G. australe, holding R. solanacearum fixed.

It is also interesting to look more closely at the estimatedresponse category probabilities from a model involving onlyR. solanacearum and G. australe. The following values wereobtained using a model based on AIC 235.6, which only con-siders their main effects to model the ordinal disease responsescale (DS 0-4). The estimated response category probabilityprofiles were (0.41, 0.24, 0.16, 0.11, and 0.08) with both ab-sent; (0.15, 0.17, 0.21, 0.23, and 0.25) when only G. australewas present; (0.14, 0.16, 0.20, 0.23, and 0.27) when onlyR. solanacearum was present; and (0.04, 0.06, 0.10, 0.20,and 0.60) when both were present. These values clearly showhow DS leans towards higher levels, when either or both arepresent and in the opposite direction if neither are present. Ifthe model were to include DBH or Height, the AIC valuewould be slightly larger (unfavorable). Including the interac-tion term between R. solanacearum and G. australe did im-prove AIC slightly (234.1), but as stated the term was notstatistically significant.

Tree cross-sections and analyses

Increases in IWTD severity was linked to increased wetwoodand ooze formation. In sites with high decline incidences, it isnot uncommon to find a heavily damaged tree (DS = 3) with across-section showing severe wetwood and heavy ooze for-mation next to a slightly damaged tree (DS = 1) with a cross-section containing no wetwood or ooze (Fig. 2). In DS = 3-4

Table 1 Survey results of 77ironwood trees (Casuarinaequisetifolia) in Guam, evaluatingthe presence and absence ofRalstonia solanacearum (Rs) andGanoderma australe (Ga) acrossfive levels of decline severity

Number oftrees

Declineseveritya

Rssingularb

Gasingularb

Rs and Gacombined

Rs and Gaabsent

17 0 18% 0% 0% 82%

12 1 33% 0% 8% 58%

13 2 62% 0% 23% 15%

15 3 33% 27% 20% 20%

20 4 50% 5% 30% 15%

aDecline severity, DS = 0 (symptomless) to 4 (nearly dead)b Highly significant predictor of decline with proportional odds logistic regression

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level trees, ooze often appeared on the surface of cross-sections immediately after cutting and, in some cases, thecut sections continued to ooze for another 3 days. Whenwetwood was present in DS = 0 trees, it was limited to theheartwood and had little or no ooze associated with it(Fig. 3). When Ganoderma wood-rot and wetwood occurredin the same tree, the wetwood area was always larger than thatof wood-rot (Fig. 4a).Wetwood often formed at the advancingedge of mycelial growth and cross-sections with G. australeoften produced fresh mycelial growth after three to 4 days in amoisture chamber (Fig. 4b). Three forms of ooze were ob-served: viscous ooze (VO), watery amber substance (WO),

and mixtures of the two (MO) (Fig. 5). Drops of VO oftentested positive for R. solanacearum and were most common inthe sapwood and in unstained tissue. Drops of WO often test-ed negative for R. solanacearum and were most common inthe heartwood-sapwood transition zone and in stained tissue.Drops of VO and MO were not randomly distributed, butappeared to coincide with growth rings (Fig. 2c).

The analysis of the 30-tree data subset showed trends forincreased presence of R. solanacearum, G. australe,wetwood, and ooze initiation with increased severity of iron-wood decline (Table 2). Ralstonia solanacearum, G. australe,and wetwood and ooze initiation were recorded in 87%, 33%,93%, and 83% of the tree cross-sections across all levels ofdecline, respectively, of which symptomless trees contributed7%, 0%, 17%, and 7%, respectively. The percentage of treestesting positive for R. solanacearum was the same using theAgdia immunostrip, LAMP, and culturing methods. Samplesfrom 16 different trees were positive for K. oxytoca and oc-curred across all levels of decline. Klebsiella colony types notidentified as K. oxytoca also occurred across all levels ofdecline.

A proportional odds logistic model was fit with nine covar-iates identified in Table 2. Due to the relatively small samplesize, covariates were fit individually, as it is unrealistic toassume that 30 observations could steer nine parameters forfive ordered categories. Nonetheless, these univariate modelscan be insightful for main effects of regressors that are asso-ciated with decline. With an increased sample size, multipleregression models with and without interaction terms couldfurther be explored. Note that the exploration of theR. solanacearum by G. australe interaction is not possiblefor the reduced data set, since there were no data (zero countsacross all DS levels) for the R. solanacearum absent andG. australe present combination. Three covariates were found

Fig. 2 Ironwood tree decline (Casuarina equisetifolia) symptoms: (a)heavily damaged tree (DS = 3); (b) slightly damaged tree (DS = 1); (c)cross-section of tree ‘a’ 5 h after sectioning, wetwood area = 58%, oozeextensive, positive for Ralstonia solanacearum; (d) cross-section of tree‘b’ 5 h after sectioning, wetwood area = 0%, no ooze, positive forR. solanacearum and Klebsiella colony types other than K. oxytoca

Fig. 3 Cross-section of a symptomless ironwood tree (Casuarinaequisetifolia) (DS = 0), 5 h after sectioning. Wetwood area = 17%, oozeslight, positive for Ralstonia solanacearum,Klebsiella oxytoca, and otherKlebsiella colony types

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to be highly significant: percent wetwood, ooze initiation, andR. solanacearum, whereas the significances of G. australewas borderline (Table 3). It is interesting to note thatR. solanacearum significances in the 77-trees model was up-held, while that of G. australe was reduced. In all cases, theproportional odds model satisfied the chi-square goodness-of-fit (p > 0.60). Table 3 shows that as the percent wetwood in asection increased by one unit, there is an estimated 5.5% de-crease in the odds of falling into the healthy direction (relativeto the unhealthy direction). Additionally, with the presence ofR. solanacearum, G. australe and the early initiation of ooze,there was an estimated 97.8%, 76%, and 94.2% decrease inthe odds of falling into the healthy direction (relative to theunhealthy direction), respectively.

Bacterial isolations and identifications

Numerous bacterial colony types were observed on cul-ture plates containing TZC (Fig. 6). Seventy-five isolatesthat displayed morphologies sufficiently similar toKlebsiella (mucoid, raised) colonies, with smooth margins

with or without crimson centers, were obtained (Fig. 6a).After further colony development and inspection, 30 po-tential Klebsiella isolates were identified. Three tests usedfor initial presumptive identification of enteric bacteriaindicated that the 30 strains were Gram negative, oxidasenegative and fermented glucose. All 30 of these cultureswere negative when tested by LAMP using K. variicolaspecific primers. It was determined that colony growth onTZC is not a reliable means to identify R. solanacearumwhen wetwood is present due to similarities in colonymorphology with Klebiella and other wetwood bacteria(Fig. 6b). In addition, when TZC was used with mixedcultures, Klebiella spp. masked the less prolific andslower growing R. solanacearum.

The phylogenetic relationships among the 30 putativeKlebsiella, S-strains (Schlub strains) from the current studyand four A-strains (Alvarez strains) obtained previously(Ayin et al. 2015) were determined by 16S sequence analysis(Fig. 7). The phylogeny showed that the cultures were closelyrelated to six different genera: Kosakonia, Enterobacter,Pantoea, Erwinia, Citrobacter, and Klebsiella. Of the 34 cul-tures, four were closely related to K. variicola and five to K.oxytoca. Three of the strains (S69-1A1, S98-3A1, S105-1A1)were related to Enterobacter cloacae a wilt pathogen of mul-berry (Morus alba) in China (Wang et al. 2008).

A subset of the above strains was selected for furthertesting (Table 4). The Microlog™ multi-substrate meta-bolic analysis confirmed the 16S results for strains S75-2A1, S34-1A1, and S50-3A1 as Pantoea, Enterobacter,and Citrobacter, respectively. Though S73-1A1 was iden-tified as Klebsiella by16S rDNA sequencing, the API NEtest and Microlog™ indicated that it is more likelyEnterobacter. In addition, the strain also was motile andunable to ferment lac tose on MacConkey agar(MacConkey 1990). Though strain S38-1A1 was identi-fied as Erwinia by 16S rDNA sequencing and by itsupdated genus name Pectobacterium with Microlog, itcould possibly be Klebsiella because the strain wasnon-motile and positive for lactose fermentation(Brenner et al. 2005).

Fig. 4 Cross-section of a severelydeclined (DS = 3) ironwood tree(Casuarina equisetifolia).Wetwood area = 68%, positive forGanoderma australe andRalstonia solanacearum. (a) 5 hafter sectioning, ooze slight; (b)close-up of the mycelial growthpresent after 3 days in moisturechamber, wetwood forms at theadvancing edge of the mycelialgrowth indicated by arrow

Fig. 5 Ooze types found in a declining ironwood trees (DS = 2)(Casuarina equisetifolia) that tested positive for Ralstoniasolanacearum and wetwood bacteria: VO = viscous, white to off-whitesubstance; WO = watery, amber substance; MO = a mixture of theviscous and watery ooze

Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree... 631

Discussion

Ganoderma, a genus of more than 300 species of wood-decaying fungi, has been reported as a wood decay pathogenof C. equisetifolia throughout its natural habitat and in manyareas where ironwood has been introduced including Mexico(Cibrián et al. 2007), India (Bakshi 1951; Mohanan andSharma 1993; Foroutan and Jafary 2007), Pacific islands(Kohler et al. 1997), Indonesia and Malaysia (Glen et al.2009). Unfortunately, reports of pathogenicity are often basedon the presence of fruiting bodies on living trees and notsupported by Koch’s postulates. Though completing Koch’sis beyond the scope of this study, evidence to supportG. australe as a likely causal agent includes the presences offruiting bodies in IWTD tree sites, the significants of fruitbodies as predicitors of decline in the study and previous stud-ies (Schlub 2010; Schlub et al., 2011a). In addition,

examination of cross-sections showed colonization byG. australe is not limited by the non-living heartwood, butextends into live sapwood (Fig. 4), which is indicitive of apathogen.

Ralstonia solanacearum is a widespread bacterial pathogenthat infects more than 200 hosts, comprising 53 botanicalfamilies (Hayward 1991). Genetic diversity in a global collec-tion of R. solanacearum strains has led to the characterizationof R. solanacearum as a species complex (Fegan and Prior2005). Strains ofR. solanacearum have been reported to causebacterial wilt of ironwood in several countries including India(Ali et al. 1991; Mohanan and Sharma 1993), China (Liangand Chen 1982), and Mauritius (Orian 1961). Though isolatesof R. solanacearum were found to be pathogenic on tomatoand ironwood seedlings (Ayin et al. 2015), the ironwood treesfrom which the isolates were collected had symptoms ofIWTD and not those of bacterial wilt reported in India (Ali

Table 2 Stem cross-sectionanalyses of 30 ironwood trees(Casuarina equisetifolia) inGuam, across five levels ofdecline severity

Decline Severity (DS) a

Variable 0

5 trees

1

5 trees

2

6 trees

3

6 trees

4

8 trees

Height m (range) 13-16 12-16 04-15 12-16 10-16

Diameter in cm at 1.3 m (range) 15-20 13-23 16-20 14-19 16-25

Percent wetwood area (range) (%) 3-17 0-66 0-77 58-80 56-87

Ooze initiation within 24 h (%) 40 80 83 100 100

Ooze quantity

None within 72 h (%) 20 40 16 0 0

Slight ooze within 72 h (%) 0 0 16 0 13

Abundant ooze within 72 h (%) 80 60 68 100 87

Ooze type

Viscous (%) 20 60 83 100 100

Watery (%) 80 60 50 67 50

Mixture (%) 20 20 0 0 25

Klebsiella colony types b 80 80 83 67 63

Klebsiella oxytoca c 40 60 83 67 25

Ralstonia solanacearum 40 80 100 100 100

Ganoderma australe 0 20 33 50 50

aDecline severity, DS = 0 (symptomless) to 4 (nearly dead)b Based on colony morphology but negative for the Klebsiella oxytoca-specific LAMP assayc Based on the K. oxytoca LAMP assay

Table 3 Proportional oddsmodels fits of 30 ironwood trees(Casuarina equisetifolia) cross-sectional covariates that aresignificant when fit univarately

Covariate P value Odds-ratio

Odds-ratio confidence interval Goodness-of-fit p value

Percent wetwood area <0.0001 0.945 (0.904; 0.968) 0.64

Ooze initiation 0.007 0.058 (0.007; 0.464) 0.90

Ralstoniasolanacearum

0.005 0.022 (0.001; 0.319) 0.94

Ganoderma australe 0.052 0.240 (0.057; 1.011) 0.72

C. M. Ayin et al.632

84

Vibrio cholerae (ATCC 14035)Pantoea eucalypti (LMG 24197)Pantoea agglomerans (ATCC 27155)

Erwinia mallotivora (LMG 2708)Erwinia amylovora (DSM 30165)

Pantoea dispersa (LMG 2603)

Erwinia carotovora (ATCC 15713)

Klebsiella oxytoca (ATCC 13182)

Klebsiella oxytoca (A6125)

Citrobacter freundi (DSM 30039)Citrobacter braakii (CDC 80-58)

Enterobacter aerogenes (ATCC 13048)

Enterobacter mori (R18-2)

Kosakonia radicincitans (D5/23)

Enterobacter oryzae (Ola 51)Enterobacter sacchari (SP1)

Citrobacter diversus (CDC 3613-63)

Enterobacter cloacae (ATCC 13047)

Klebsiella pneumoniae (ATCC 13883)Klebsiella variicola (F2F9)

Klebsiella variicola (A6128)Klebsiella variicola (A6127)Klebsiella variicola (A6126)

Erwinia mallotivora (S38-1A1; MK968754)Erwinia mallotivora (S49-1A1; MK968739)Erwinia mallotivora (S50-1A1; MK968758)

Citrobacter amalonaticus (S50-3A1; MK968756)Enterobacter asburiae (S44-2A1; MK968757)

Enterobacter cancerogenus (S49-2A1; MK968755)

Enterobacter tabaci (S34-1A1; MK968749)

Phytobacter diazotrophicus (S76-1A1; MK968748)Phytobacter diazotrophicus (S78-2A1; MK968751)

Kosakonia oryzendophytica (S90-1A1; MK968753)Kosakonia oryzendophytica (S48-1A1; MK968738)

Pantoea dispersa (S89-2A1; MK968761)Pantoea dispersa (S75-2A1; MK968760)

Enterobacter cloacae (S69-1A1; MK968744)Enterobacter cloacae (S98-3A1; MK968762)

Enterobacter cloacae (S105-1A1; MK968763) (reversed)

Klebsiella aerogenes (S75-1A1; MK968747)

Klebsiella oxytoca (S55-1A1; MK968740)Klebsiella oxytoca (S96-1A1; MK968746)

Klebsiella pneumoniae (S73-1A1; MK968743)Klebsiella pneumoniae (S32-3A1; MK968735)

Klebsiella variicola (S100-1A1; MK968734)Klebsiella variicola (S59-2A1; MK968741)

Kosakonia radicincitans (S44-1A1; MK968736)Kosakonia radicincitans (S44-3A1; MK968737)Kosakonia radicincitans (S61-1A1; MK968742)Kosakonia radicincitans (S76-2A1; MK968750)Kosakonia radicincitans (S84-2A1; MK968752)

Kosakonia oryzae (S51-2A1; MK968745)

Kosakonia sacchari (S102-1A1; MK968759)

84

8462

100

100

53

75

69

8176

51

9965

97

9784

97

7770

55

63100

100

100

73

89

64

100

0.02

98

Fig. 7 Maximum Parsimony treederived from 16S sequenceanalysis of 34 bacterial isolatesfrom ironwood trees. Those withS numbers were derived from thecurrent study (GenBankaccession numbers MK968734-MK968763), whereas A6125-A6128 were from a previousstudy (Ayin et al. 2015). Thebootstrap consensus tree, inferredfrom 2000 replicates, is taken torepresent the evolutionary historyof the taxa analyzed. Branchescorresponding to partitionsreproduced in less than 50%bootstrap replicates are collapsed.The percentage of replicate treesin which the associated taxaclustered together in the bootstraptest are shown next to thebranches

Fig. 6 Bacterial colony typesstreaked from ironwood tree(Casuarina equisetifolia) tissueon to tetrazolium chloridemedium (TZC) (a) PurifiedKlebsiella species, note therounded, raised, pink colonieswith white borders; (b) coloniesfrom raw bacterial wetwood ooze

Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree... 633

et al. 1991; Mohanan and Sharma 1993) or China (Liang andChen 1982). With IWTD, death is gradual, often taking years.It usually appears at the onset of reproductive maturity, isfrequently associated with wetwood and G. australe heart-wood rot and is most frequently found in windbreaks andlandscape planting (Schlub et al. 2011a; Schlub et al.2012a). The typical reports for bacterial wilt include rapiddeath, often within weeks of onset. It usually first appears atthe young saplings stage and is mainly associated with dense-ly planted plantations or nurseries (Liang and Chen 1982; Aliet al. 1991).

In a previous study using BOX-PCR profiling (Kogan et al.1987; Saitou and Nei 1987), little genetic diversity was re-vealed among R. solanacearum strains isolated from iron-wood in Guam (Ayin et al. 2015). The Guam ironwood strainswere assigned as phylotype I (Ayin et al. 2015), which arethought to have originated in Asia (Fegan and Prior 2005).Interestingly, R. solanacearum strains isolated from Guamironwood displayed RIF marker (dnaA) (Schneider et al.2011) sequence similarities to a R. solanacearum strain isolat-ed from ironwood in China (Ayin et al. 2015). The geneticdiversity of R. solanacearum strains infecting ironwood inGuam merits additional investigation.

In this study, the presence of ooze in cross-sections is at-tributed to infection by the bacterial wilt pathogenR. solanacearum and/or by bacteria from at least six wetwoodgenera within the family Enterobacteriaceae. This group ofclosely related, glucose-fermenting, gram negative rod-shaped bacteria, that are abundant in soil, water, plants, andthe intestines of animals. The wetwood bacteria isolated fromironwood tissues in this study, with and without wetwood,were also isolated from other woody tree species in previousstudies (Hartley et al., 1961; Tiedemann et al. 1977; Schinket al., 1981; Murdoch and Campana 1983; Jeremic et al.2004). Recent studies have shown that bacterial species foundin the genera identified in our study also possess plant growth-promoting properties (Magnani et al. 2010; Quecine et al.2012; Witzel et al. 2012; Zhu et al. 2012; Chen et al. 2013;Weber et al. 2013; Wei et al. 2013) and this may explain theirassociation with trees often found in nutrient deficient soils.These same properties may also explain what appears to be the

promotion by bacterial wetwood of G. australe colonizationbeyond the heartwood tissue in declining ironwood trees.

Though the occurrence of wetwood is useful in predictingIWTD, why it forms, its role in IWTD, and how to detect itwithout destructive sampling needs to be determined.Wetwood bacteria appear to be omnipresent in Guam, asK. oxytoca and bacteria with Klebsiella type colonies wereconsistently isolated across all levels of decline (Table 2).Although they were not significant predictors of decline, man-ifestations of bacterial wetwood colonization in the form ofpercent wetwood and the initiation of ooze were significant(Table 3). Even though wetwood occurred in 93% of the 30cross-sections, it only appeared to impact the health of a tree ifthe percent wetwood area was greater than 55%, which wastrue for all the DS = 3 and 4 trees and none of the DS = 0 trees(Table 2). Two R. solanacearum positive trees are show inFig. 2; however, only the tree with ooze and severe wetwoodshowed heavy decline damage. A possible explanation is thatwetwood bacteria are behaving as opportunistic pathogensand might only be contributing to decline when the barrierbetween heartwood and sapwood is breached as a result ofthe inhibition of water conduction, which has been shown inthe case of Salix (Sakamoto and Sano, 2000).

Due to the slow progression of IWTD and its general spo-radic nature, it is likely that IWTD could be reduced throughincreasing (1) the genetic diversification of its tree population,(2) removal ofR. solanacearum andG. australe infected trees,(3) limiting bacterial movement through equipment and root-grafts, and (4) the application of cultural practices that pro-mote healthy growth. The presence of R. solanacearum andKlebsiella colony types in symptomless trees seems to supportthe concept that a tree could possibly remain symptomlessthough infected with R. solanacearum and wetwood bacteria,provided the percent wetwood area and ooze formation re-mains minimal.

In conclusion, this study provides evidence that the bacte-rium R. solanacearum and, to a lesser degree, the fungusG. australe are significant predictors of IWTD on the islandof Guam and that further research is required to determinecausality. The results of the statistical analyses are alignedwith those expected from two primary pathogens acting

Table 4 Identification of bacterialendophytes associated withsymptomatic wetwood tissue,sequence analysis, and metabolicprofiles

# Sample # 16S rDNA Microlog™ a SIM b MacConkey test

1 S73-1A1 Klebsiella Enterobacter Motile –

2 S75-2A1 Pantoea Pantoea Motile –

3 S34-1A1 Enterobacter Enterobacter Motile –

4 S50-3A1 Citrobacter Citrobacter Motile –

5 S38-1A1 Erwinia Pectobacterium Non-motile + c

a Oxidation of carbon compounds using MicroLog™ (Biolog, Inc., Hayward, CA)b SIM = sulfide indole motility mediumc Fermentation of lactose on MacConkey agar

C. M. Ayin et al.634

independent of each other. This is supported by the fact that33% of DS = 3 trees were positive for R. solanacearum inabsence of G. australe while 27% of the trees withG. australe were negative for R. solanacearum. Future re-search should focus on detection, since the sensitivity of trunkdrill shaving to detect early root infection by R. solanacearumis not known nor is the time lag between infection byG. australe and the appearance of fruiting bodies known.There is evidence that wetwood bacteria are playing a sup-portive role in decline when infection results in a percentwetwood area of 55% or greater and accompanied by earlyooze formation. The appearance of both R. solanacearum andK. oxytoca in symptomless and slightly damaged trees raisesthe possibility that they can exist as asymptomatic infections.The presence of R. solanacearum in asymptomatic trees couldalso be the result of avirulent isolates or strains (Ayin et al.2015). Additional studies and surveys are needed to determinethe extent to which ironwood trees in Guam are infected withR. solanacearum, as well as the genetic profile of ironwoodisolates. This study provides evidence that the loss of trees onGuam is better described as a disease syndrome than a decline.Facts identified in this study that are commonwith the conceptof disease and contrary to decline included the following: (1)symptomless and unhealthy trees are often found adjacent toeach other, (2) specific contributing factors are known, and (3)the condition is not restricted to mature trees. With the estab-lishment that Enterobacter species are capable of causing bac-terial wilt of mulberry in China (Wang et al. 2008; Zhu et al.2011), a thorough phytopathogenic evaluation of Guam’sironwood wetwood bacteria is warranted. Studies of the diver-sity and characteristics of the bacterial flora of ironwood andtheir interactions with R. solanacearum, G. australe, and ter-mites should provide a better understanding of the ecology ofGuam’s ironwood trees and the etiology of IWTD.

Acknowledgements This work was supported in part by the NationalInstitute of Food and Agriculture, U.S. Department of Agriculture,McIntire Stennis project, under accession number 1005476 and theUniversity of Guam Cooperative Extension Service and the Universityof Hawaii Foundation project in Plant Disease Control. The Authorsgratefully acknowledge Joe Afaisen, Roger Brown, and Victoria Santosfor their help on this project and to Dr. Zelalem Mersha and Karl Schlubfor their early contributions to understanding IWTD. Greatly appreciatedare Karl Schlub’s photographic contributions and Dr. Mohammad Arif’sphylogenetic tree contribution.

References

Agresti A (2019) An introduction to categorical data analysis, 3rd edn.Wiley, New York

Ali MIM, Anuratha CS, Sharma JK (1991) Bacteria wilt of Casuarinaequisetifolia in India. Eur J Forest Pathol 21(4):234–238

Alvarez AM, Kaneshiro WS, Vine BG (2005) Diversity of Clavibactermichiganensis subsp. michiganensis populations in tomato seed:what is the significance? Acta Hortic 695:205–213

Athens SJ, Ward JV (2004) Holocene vegetation, savanna origins andhuman settlement of Guam. In a Pacific odyssey: archaeology andanthropology in the Western Pacific. Papers in honour of JimSpecht. Rec Aust Mus 29:15–30

Ayin CM, Schlub RL, Alvarez AM (2013) Identification of bacteria as-sociated with decline of ironwood trees (Casuarina equisetifolia) inGuam. Phytopathology 103:S2.10

Ayin CM, Schlub RL, Yashuhara-Bell J, Alvarez AM (2015)Identification and characterization of bacteria associated with de-cline of ironwood (Casuarina equisetifolia) in Guam. AustralasPlant Pathol 44:225–234. https://doi.org/10.1007/s13313-014-0341-4

Bakshi BK (1951) Mortality of Casuarina equisetifolia Forst. IndianForester 77:269–276

Brenner DJ, Krieg NR, Garrity GM, Staley JT (2005) Bergey's manual ofsystematic bacteriology: volume two. Springer, New York

Cibrián TD, Alvarado DR, García SED (Eds.) (2007) Enfermedadesforestales en México/Forest diseases in Mexico. UniversidadAutónoma Chapíngo; CONAFOR-SEMARNAT, México; ForestService USDA, EUA; NRCAN Forest Service, Canada yComisión Forestal de América del Norte; COFOAM, FAO.Chapingo, México. 221-228

Chen M, Lin L, Zhang Y, Sun L, An Q (2013) Genome sequence ofKlebsiella oxytoca SA2, an endophytic nitrogen-fixing bacteriumisolated from the pioneer grass Psammochloa villosa. GenomeAnnouncements 1:e00601–e00613

Donnegan JA, Butler SL, Grabowiecki W, Hiserote BA, Limtiaco D(2004) Guam’s Forest Resources, 2002. USDA, Forest Service,Pacific Northwest Research Station, Resource Bulletin, PNW-RB-243

Engelbrecht MC (1994) Modification of a semi-selective medium for theisolation and quantification of Pseudomonas solanacearum.ACIAR Bacterial Wilt Newsletter Vol 10:3–5

Fegan M, Prior P (2005) How complex is the "Ralstonia solanacearumspecies complex"? Pages 449-461 in: Bacterial Wilt Disease and theRalstonia solanacearum Species Complex. C. Allen, P. Prior, and A.C. Hayward, eds. American Phytopathological Society, St. Paul,MN

Foroutan A, Jafary N (2007) Diversity of heart and root rot fungi on parkand roadside trees in Maharashtra, India. J Appl Sci EnvironManage December 11(4):55–58

Fosberg FR, Sachet MH, Oliver R (1979) A geographical checklist of theMicronesian Dicotyledonae. Micronesica 15:41–295

Glen M, Bougher NL, Francis AA, Nigg SQ, Lee SS, Irianto R, BarryKM, Beadle CL, Mohammed CL (2009) Ganoderma andAmauroderma species associated with root-rot disease of Acaciamangium plantation trees in Indonesia and Malaysia. AustralasPlant Pathol 38:345. https://doi.org/10.1071/AP09008

Gordon J, McLeod JW (1928) The practical application of the directoxidase reaction in bacteriology. The J Pathol 31:185–190

Gregersen T (1978) Rapid method for distinction of gram-negative fromgram-positive bacteria. World J Microbiol Biotechnol 5:123–127

Hartley C, Davidson RW, Crandall BS (1961) Wetwood, bacteria, andincreased pH in trees. United States Department of AgricultureForest Service in cooperation with the University of Wisconsin,Madison, WI

Hayward AC (1991) Biology and epidemiology of bacterial wilt causedby Pseudomonas solanacearum. Annu Rev Phytopathol 29:65–87

Hugh R, Leifson E (1953) The taxonomic significance of fermentativeversus oxidative metabolism of carbohydrates by various gram-negative rods. J Bacteriol 66:24–26

Jeremic D, Cooper P, Srinivasan U (2004) Comparative analysis of bal-sam fir wetwood, heartwood, and sapwood properties. Can J ForRes 34:1241–1250

Kaneshiro WS (2003) Detection and characterization of virulent,hypovirulent, and nonvirulent Clavibacter michiganensis subsp.

Ralstonia solanacearum, Ganoderma australe, and bacterial wetwood as predictors of ironwood tree... 635

michiganensis. (Master’s thesis) Retrieved from the GraduateDivision of the University of Hawaii at Manoa

Kogan S, Doherty M, Gitschier J (1987) An improved method for prena-tal diagnosis of genetic diseases by analysis of amplified DNA se-quences. N Engl J Med 317:985–990

Kohler F, Pellegrin F, Jackson G, McKenzie E (1997) Diseases of culti-vated crops in Pacific Island countries. pp. 28-29

Kumar S, Stecher G, Li M, Knyaz C, Tamura K (2018) MEGA X: mo-lecular evolutionary genetics analysis across computing platforms.Mol Biol Evol 35:1547–1549

Liang Z, Chen B (1982) Preliminary study on the susceptibility ofCasuarina equisetifolia to Pseudomonas solanacearum and its re-lation to the permeability of the cell membrane and peroxidase iso-enzymes. Journal of South China Agriculture: 1982-02

MacConkey T (1990) A note on a new medium for the growth anddifferentiation of the Bacillus coli communis and the Bacillus typhiabdominalis. Lancet 156(4010):20

Magnani G, Didonet C, Cruz L, Picheth C, Pedrosa F, Souza E (2010)Diversity of endophytic bacteria in Brazilian sugarcane. Genet MolRes 9:250–258

Mersha Z, Schlub RL,Moore A (2009) The state of ironwood (Casuarinaequisetifolia subsp. equisetifolia) decline on the pacific island ofGuam. Phytopathology 99:S85

Mersha Z, Schlub RL, Spaine PO, Smith JA, Nelson SC (2010a) Visualand quantitative characterization of ironwood tree (Casuarinaequisetifolia) decline on Guam. Phytopathology 100:S82

Mersha Z, Schlub RL, Spaine P, Smith J, Nelson S, Moore A,McConnellJ, Pinyopusarerk K, Nandwani D, Badilles A (2010b) In Fungalassociations and factors in Casuarina equisetifolia decline, 9thInternational Mycological Congress, Edinburgh, UK:IMC9:P3.268

Mersha Z, Aime MC, Cannon PG, Nandwani D, Nelson S, Spaine PC,Schlub RL (2011) Decline of Casuarina equisetifolia (ironwood)trees on Guam: Ganoderma and Phellinus. Phytopathology 101:S216

Mohanan C, Sharma JK (1993) Diseases of Casuarina equisetifolia inIndia. Commonwealth Forest Review 72:21

Murdoch CW, Campana RJ (1983) Bacterial species associated withwetwood of elm. Phytopathology 73:1270–1273

Norman D, Alvarez AM (1989) A rapid method for presumptive identi-fication of Xanthomonas campestris pv. diffenbachiae and otherxanthomonads. Plant Dis 73:654–658

Orian G (1961) Diseases of filao (Casuarina equisetifolia) forest inMauritius. Revue agricole et sucriere de I'lle Maurice 40:17–45

Park JS, Husseneder C, Schlub RL (2019) Morphological and molecularspecies identification of termites attacking Ironwood trees,Casuarina equisetifolia (Fagales: Casuarinaceae), in Guam. J EconEntomol 112(4):1902–1911. https://doi.org/10.1093/jee/toz097

Potgieter L, Richardson D, Wilson J (2014) Casuarina: biogeography andecology of an important tree genus in a changing world. BiolInvasions 16:609–633

Quecine MC, Araujo WL, Rossetto PB, Ferreira A, Tsui S, Lacava PT,Mondin M, Azevedo JL, Pizzirani-Kleine AA (2012) Sugarcanegrowth promotion by the endophytic bacterium Pantoeaagglomerans 33.1. Appl Environ Microbiol 78:7511–7518

Safford EW (1905) Useful Plants of Guam. U.S. Government PrintingOffice. Facsimile printing 2009 – Jillette Leon-Guerrero/Guamology Publishing, 416 pp

Saitou N, Nei M (1987) The neighbor-joining method: a new method forreconstructing phylogenetic trees. Mol Biol Evol 4:406–425

Sakamoto Y, Sano Y (2000) Inhibition of water conductivity caused bywatermark disease in Salix sachalinensis. IAWA J 21:49–60

Schink B, Ward JC, Zeikus JG (1981) Microbiology of wetwood: impor-tance of pectin degradation and clostridium species in living trees.Appl Environ Microbiol 42:526–532

Schlub KA (2010) Investigating the ironwood tree (Casuarinaequisetifolia) decline on Guam using applied multinomial modeling

LSU Master's Theses 2676. https://digitalcommons.lsu.edu/gradschool_theses/2676. (etd. 10282012-173615)

Schlub RL, Mersha Z, Aime CM, Badilles A, Cannon PG, Marx BD,McConnell J, Moore A, Nandwani D, Nelson SC, et al. (2011a)Guam ironwood (Casuarina equisetifolia) tree decline conferenceand follow-up, 4th International CasuarinaWorkshop,March 21-25,210 Haikou, China. (Eds Zhong C, Pinyopusarerk K, Kalinganire A,Franche C), pp 239-246

Schlub RL, Moore A, Marx BD, Schlub KA, Kennaway L, QuintanillaM, PutnamM,Mersha Z (2011b) Decline ofCasuarina equisetifolia(ironwood) trees on Guam: symptomatology and explanatory vari-ables. Phytopathology 101:S216

Schlub RL, Mendi RC, Aiseam CC, Davis JK, AimeMC (2012a) Surveyof wood decay fungi or Casuarina equisetifolia (ironwood) on theislands of Guam and Saipan. Phytopathology 102:S4.106

Schlub RL, Schlub KA, Alvarez AM, Aime MC, Cannon PG, Persad A(2012b) Intergrated perspective on Guam ironwood tree decline,60th Annual Western International Forest Disease WorkConference, Lake Tahoe, California, Brown, J., Comp., Ed. LakeTahoe, California, pp 51-60

Schlub RL, Kubota R, Alvarez AM (2013) Casuarina equisetifolia de-cline in Guam linked to colonization of woody tissues by bacteria.Phytopathology 103:S2.128

Schneider KL, Marrero G, Alvarez AM, Presting GG (2011)Classification of plant associated Bacteria using RIF, a computation-ally derived DNA marker. PLoS One 6(4):e18496. https://doi.org/10.1371/journal.pone.0018496

Soil Survey Staff (2015) National Resources Conservation Service,United State Department of Agriculture. Web Soil Survey. https://websoilsurvey.sc.egov.usda.gov/. Accessed 5 Feb 2019

Stone BC (1970) The Flora of Guam. Micronesica 6:1–659Supriadi MK, Sitepu D (2001) Bacterial wilt disease of woody trees

caused by Ralstonia solanacearum: a review. Jurnal Penelitian danPengembangan Pertanian 20:106

Tiedemann G, Bauch J, Bock E (1977) Occurrence and significance ofbacteria in living trees of Populus nigra l. Eur J For Pathol 7:364–374

Wang CF, Praphat K, Xie GL, Zhu B, Li B (2008) Bacterial wilt ofmulberry (Morus alba) caused by Enterobacter cloacae in China.Plant Dis 92:483

Ward JC, PongWY (1980)Wetwood in trees: a timber resource problem.Gen. Tech. Rep. PNW-GTR-112. Portland, OR: U.S. Department ofAgriculture, Forest Service, Pacific Northwest Forest and RangeExperiment Station. 59p

Weber OB, Videira SS, Luiz Simoes de Araujo J (2013) Identification ofculturable endophytes in ‘champaka’pineapple grown in an organicsystem. Afr J Agric Res 8:3422–3430

Wei C-Y, Lin L, Luo L-J, Xing Y-X, Hu C-J, Yang L-T, Li Y-R, An Q(2013) Endophytic nitrogen-fixing Klebsiella variicola straindx120e promotes sugarcane growth. Biol Fertil Soils 50:657–666

Weisburg WG, Barns SM, Pelletier DA, Lane DJ (1991) 16s ribosomalDNA amplification for phylogenetic study. J Bacteriol 173:697–703

Witzel K, Gwinn-Giglio M, Nadendla S, Shefchek K, Ruppel S (2012)Genome sequence of enterobacter radicincitans dsm16656t, a plantgrowth-promoting endophyte. J Bacteriol 194:5469

Yasuhara-Bell J, Ayin CM, Hatada A, Yoo Y, Schlub RL, Alvarez AM(2015) Detection of Klebsiella variicola and K. oxytoca by loop-mediated amplification. J Plant Pathol Microbiol 6:271

Zhu B, Chen M, Lin L, Yang L, Li Y, An Q (2012) Genome sequence ofEnterobacter SP. strain SP1, an endophytic nitrogen-fixing bacteri-um isolated from sugarcane. J Bacteriol 194:6963–6964. https://doi.org/10.1128/JB.01933-12

Zhu B, Lou MM, Xie GL, Wang GF, Zhou Q et al (2011) Enterobactermori sp. nov. associated with bacterial wilt onMorus alba L. J SystEvol Microbiol 61:2769–2774

C. M. Ayin et al.636