A destructive new disease of Syzygium samarangense in Thailand caused by the new species Pestalotiopsis samarangensis

^IInstitute of Excellence in Fungal Research, Mae Fah Luang University, Chiang Rai 57100, Thailand

^IISchool of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand

^IIIKey Laboratory of Systematic Mycology & Lichenology, Institute of Microbiology, Chinese Academy of Sciences, Beijing 100190, P.R. China

^IVLandcare Research, Private Bag 92170, Auckland, New Zealand

ABSTRACT

A new fungal species, Pestalotiopsis samarangensis, was isolated from fruit rot in Syzygium samarangense from markets in Chiang Mai and Chiang Rai provinces, Thailand. Initially small, circular, black, slightly sunken spots developed on fruits. Later, the spots enlarged rapidly, became sunken, and resulted in a soft decay of the fruit flesh. Molecular analysis of the ITS, β-tubulin, and tef1 combined gene sequences distinguish P. samarangensis from other species in the genus. Pathogenicity testing proved that wounding triggers the disease symptoms and thus careful handling of fruits during transport and storage helps to prevent the disease.

Key words: β-tubulin, fruit rot, ITS, neighbor-joining, phylogenetic, tef1.

INTRODUCTION

Syzygium samarangense Merr. (Myrtaceae) is widely grown for its fruits throughout Cambodia, Laos, India, Philippines, Samoa, Sri Lanka, Taiwan, Thailand, and Vietnam (Srisaard, 2003; Vara-Ubol et al., 2006). In Thailand, the wax apple fruit is commonly known as chomphu and harvested the year round with a peak during January to March from the cultivars 'Dang Indo', 'Phet Ban Plew', 'Phet Jin Da', 'Phet Nam Pueng', 'Phet Sai Rung', 'Phet Sam Phran', 'Thub Thim Chan', and 'Thun Klao'. In 2004, Thailand earned US$ 26.5 million from 69,608 tons of wax apple planted over 10,240 ha (Shü et al., 2008). The fruits have a thin, delicate skin and are thus easily susceptible to pest and disease attack. Diseases of wax apple include Pestalotiopsis fruit rot (Pestalotiopsis eugeniae (Thüm.) S. Kaneko), shoot dieback (Fusarium sp.), anthracnose (Colletotrichum gloeosporioides (Penz.) Penz. & Sacc.), Phytophthora fruit rot (Phytophthora palmivora (E.J. Butler) E.J. Butler), Dothiorella fruit rot (Dothiorella sp.), and Pseudocercospora fruit rot (Pseudocercospora sp.) (Janick & Paull, 2008; Lan, 2001), although these species need to be confirmed by molecular data (Phoulivong et al., 2010; Ko Ko et al., 2011).

Pestalotiopsis spp. are important plant-pathogenic species known mostly from the tropics, where they cause leaf blights (Guba, 1961) in many plant species (Hyde & Fröhlich, 1995; Xu et al., 1999; Das et al., 2010; Maharachchikumbura et al., 2011). Species may also cause rots of fruit and other post-harvest diseases (Ullasa & Rawal, 1989; Korsten et al., 1995; Xu et al., 1999). Several post-harvest diseases are caused by species of Pestalotiopsis, e.g., postharvest decay of mangos by P. glandicola (Castagne) Steyaert (Ullasa & Rawal, 1989), fruit rot of grapevine by P. menezesiana (Bres. & Torrend) Bissett as well as P. uvicola (Speg.) Bissett (Xu et al., 1999), and fruit rot of rambutan by Pestalotiopsis sp. (Sangchote et al., 1998). Scabby fruit canker of guava is caused by P. psidii (Pat.) Mordue (Kaushik et al., 1972).

We surveyed market fruit disease of wax apple in Chiang Mai and Chiang Rai provinces in Thailand in 2010 and 2011, and constantly observed a distinctive fruit rot disease. In this study, we introduce this new Pestalotiopsis fruit rot disease of wax apple, with a description of morphological and molecular characteristics of the fruit rot agent.

MATERIALS AND METHODS

Symptoms and sample collection

Surveys of post-harvest disease of wax apples were conducted in markets of Chiang Mai and Chiang Rai provinces from January to August in 2010 and from March to December in 2011. Wax apple fruits with disease symptoms were carried to the laboratory and photographed. Samples were sorted by symptoms and the presence of wounds and incubated in a clean polythene bag to promote fungal development. The type herbarium specimen (BPI 406804) of P. eugeniae, a pathogen previously recorded from wax apple was borrowed and studied.

Isolation and identification of the causal agent of the disease

A single-conidium culture technique was applied to obtain pure colonies of the fungi following a modified method as outlined by Chomnunti et al. (2011). In short, a small piece of diseased fruit bearing acervuli was immersed in 300 µL of sterile distilled water and left a few minutes so that conidia liberate from the conidiomata. A suspension was made and small drops of this suspension were dispersed on water agar (WA) in Petri dishes and kept at room temperature in the laboratory. Conidia were left for 8-12 hours to germinate and single germinating conidia were transferred to fresh PDA plates. The plates were incubated at 25ºC for 7 to 10 days. Colonies were grown on PDA plates, later transferred to PDA slants, and stored at 4 ºC for further study. Sporulation was induced using sterilized (autoclaved) carnation leaves, which were aseptically placed on the surface of the PDA medium with growing mycelium. The morphology of fungal colonies was recorded following the method of Hu et al. (2007). Fungal mycelium and spores were observed under the light microscope and photographed. All microscopic measurements were made with Tarosoft image framework (v. 0.9.0.7).

Pathogenicity tests

Ten-days-old sporulating cultures of Pestalotiopsis strains on PDA (25ºC, day light) were flooded with Czapek-Dox nutrient solution (Lingappa & Lingappa, 1965), and the mycelia were scraped using a spatula. The conidiomata were crushed using a glass rod. The mycelial suspension was passed through glass wool to remove hyphal fragments and the filtrate containing conidia was collected. The filtrate was centrifuged at 3000 rpm for 3 min and a suspension of 10³ mL^-1 conidia was prepared. In vitro pathogenicity testing was carried out on 15 healthy fruits of wax apple bought from the market. The inoculations were carried out in April 2010 as well as September and November 2011. The surface of the fruits was wiped with 70% ethanol and allowed to air-dry. Fruits were separated into three groups of five fruits each. One group was wounded by pin-pricking. A conidial suspension (20 µL) was placed on the surface of the five unwounded fruits and on five wounded fruits. The remaining five fruits were maintained as controls and inoculated with drops of sterile deionized water. The fruits were placed in polythene bags, labeled, and the moisture was maintained for 10 days by the addition of wet sterile cotton wool. Daily observations were made on the development of disease symptoms. Fruits showing symptoms were removed and the pathogen in the diseased tissue was re-isolated on PDA as described above.

DNA extraction and PCR conditions

Total genomic DNA was extracted by the modified protocol of Guo et al. (2000). Fresh fungal mycelium (500 mg) was scraped from the margin of a PDA plate incubated at 25ºC for 7 to 10 days and transferred into a 1.5 mL centrifuge tube with 100 µL of preheated (60ºC) 2× CTAB extraction buffer (2% (w/v) CTAB, 100 mM Tris-HCl, 1.4 M NaCl, 20 mM EDTA, pH 8.0) and 200 mg sterilized quartz sand. Mycelium was ground using a glass pestle for 5 min, and an extra 500 µL 2× CTAB preheated (60ºC) was added and incubated in a 65ºC water bath for 30 min with occasional shaking. 500 µL of phenol:chloroform (1:1) was added to each tube and shaken thoroughly to form an emulsion. The mixture was spun at 11900 g for 15 min at 25ºC in a microcentrifuge and the supernatant phase decanted into a fresh 1.5 mL tube. Supernatant containing DNA was re-extracted with phenol:chloroform (1:1) at 4ºC until no interface was visible. 50 µL of 5 M KOAc (KO(C₂H₃O)) was added into the supernatant followed by 400 µL of isopropanol and inverted gently to mix. The genomic DNA was precipitated at 9200 g for 2 min at 4ºC in a microcentrifuge. The DNA pellet was washed twice with 70% ethanol and dried using SpeedVac^® (AES 1010; Savant, Holbrook, NY, USA). The DNA pellet was then resuspended in 100 µL TE buffer (10 mM Tris-HCl, 1 mM EDTA).

PCR amplification

The ITS and 5.8S region of the rDNA molecule was amplified using primer pairs ITS4 (5'-TCC TCC GCT TAT TGA TAT GC-3') and ITS5 (5'-GGA AGT AAA AGT CGT AAC AAG G-3') (White et al., 1990), the β-tubulin gene region was amplified with primer pairs BT2A (5'-GGT AAC CAA ATC GGT GCT GCT TTC-3') and BT2B (5'-ACC CTC AGT GTA GTG ACC CTT GGC-3') (Glass & Donaldson, 1995; O'Donnell & Cigelnik, 1997), and tef1 was amplified using the primer pairs EF1-526F (5'-GTC GTY GTY ATY GGH CAY GT-3') and EF1-1567R (5'-ACH GTR CCR ATA CCA CCR ATC TT-3') (Rehner, 2001). PCR was performed with the 25 µL reaction system consisting of 19.75 µL of double-distilled water, 2.5 µL of 10× Taq buffer with MgCl₂, 0.5 µL of dNTP (10 mM each), 0.5 µL of each primer (10 µM), 0.25 µL Taq DNA polymerase (5 U/µL), 1.0 µL of DNA template. The thermal cycling program followed the methods used by Maharachchikumbura et al. (2012). The PCR products were verified by staining with Goldview (Guangzhou Geneshun Biotech, China) on 1% agarose electrophoresis gels.

Phylogenetic analysis

Sequences were optimized manually to allow maximum alignment and maximum sequence similarity as detailed in Maharachchikumbura et al. (2012). Phylogenetic trees were constructed using the neighbor-joining method (Saitou & Nei, 1987) using Mega 4 (Tamura et al., 2007). Evolutionary distances are in the units of the number of base pair substitutions per site and were computed using the maximum composite likelihood method (Tamura et al., 2004) and are in the units of the number of base substitutions per site. All positions containing alignment gaps and missing data were eliminated only in pairwise sequence comparisons (Pairwise deletion option in Saitou & Nei, 1987). Branching pattern confidence levels were estimated by the bootstrap resamplings of data based on 1000 random replicates and 64238 random seeds. Gaps were treated as a pairwise deletion and trees were viewed with Mega 4.

RESULTS

Symptoms

Initially small, circular, black, slightly sunken spots appear on fruits of Syzygium samarangense, which later expand outwards on the surface of the fruit (Figure 1). Following storage these spots enlarge rapidly, become sunken, and result in a soft decay of the fruit flesh. Raised masses of black conidia arranged in concentric bands (up to 1 - 1.5 cm diam.) develop on the surface of the lesions and are surrounded by white mycelium. In most cases, spots coalesce and form large irregular rotting areas with lesions extending into the pulp. The disease appears the year round.

FIGURE 1 - A. Pestalotiopsis samarangensis (holotype). B. Fruit rot of wax apple C.D. Acervular conidiomata, epidermal to superficial in origin. E. Conidiogenous cells and conidia. F-H. Versicoloured conidia. I-J. Colony on PDA top (I) and reverse (J). Scale bars: C, D = 50 µm, E-H = 20 µm.

Pathogenicity testing

To comply with Koch's postulates, lesions resembling initial symptoms were observed after 8 days on pin-pricked fruits inoculated with a conidial suspension. No symptoms were observed on fruits inoculated with a conidial suspension without pin-pricking. No symptoms were observed in the control fruits. The taxon re-isolated from diseased fruits was identical with the original isolates. The experiment was carried out using five replicates and was repeated three times with the same results.

Phylogeny

Sequences of nineteen stains of Pestalotiopsis are included in the ITS phylogram (Figure 2) including one new sequence generated in this study, six type or epitype strains, and 12 strains used in our previous study. In the ITS phylogram, seven strains (SD004, SD047, SD072, SD091, SG064, SG100 and SS010) cluster in Clade A with strong bootstrap support (100%). However, the strain isolated from wax apple (SS010) is separated from the other six strains. In the combined phylogram (Figure 3), the seven strains again clustered with strong bootstrap support (100%). Three strains of Pestalotiopsis cf. versicolor form a strongly supported subclade (99%), while the wax apple strain is separated from this subclade. Two strains of P. cf. meneziana isolated from tea also cluster in Clade A with 100% bootstrap support. Both, ITS and multigene combined phylograms (Figures 2,3) show that the sequences obtained from Pestalotiopsis sp. on wax apple (SS010) are different from sequences obtained from strains SD004, SD047, SD072, SD091, SG064, and SG100. Therefore, the species represented by the SS010 strain from wax apple apparently is different from the Pestalotiopsis species included in the analysis and might correspond to a species new to science.

FIGURE 2 -
Neighbor-joining phylogram generated from ITS gene sequence analysis of type and putatively named Pestalotiopsis species in GenBank. Data were analyzed with random addition sequence, unweighted parsimony and treating gaps as missing data. Seiridium sp. is placed as the outgroup taxon, ex-type sequences are written in bold.

FIGURE 3 -
Neighbor-joining phylogram of Pestalotiopsis spp. generated from combined sequences of ITS, β-tubulin, and tef1. Seiridium sp. is the outgroup taxon; ex-type sequences are written in bold.

Taxonomy

Pestalotiopsis samarangensis Maharachchikumbura & K.D. Hyde, sp. nov.

MycoBank: MB 800178

FIG 1 A - J

Etymology: The specific epithet is based on the host species, from which the fungus was isolated.

Conidiomata acervuli, in concentric bands, confluent, erumpent when mature, rounded to oval in outline, epidermal to superficial in origin, basal stroma and lateral wall 2 - 4 cells thick; cells hyaline to pale brown, textura angularis, 100 - 350 µm wide, 80 - 150 deep (Figure 1 B - C). Conidiophores correspond to conidiogenous cells arising within the acervuli. Conidiogenous cells discrete, simple, short, filiform (Figure 1 E). Conidia 18 - 21 × 6.5 - 7.5 µm ( = 20 × 7 µm), fusiform to ellipsoid, broadly clavate, straight to slightly curved, 4-septate, versicoloured; basal cell conical, hyaline, thin and smooth-walled, 3.5 - 4.8 µm long (= 4 µm); apical cell 2.5 - 4.6 µm long (= 3.4 µm), conical, hyaline, thin- and smooth-walled; three median cells together 12.8 - 13.8 µm long (= 13.5 µm), with thick verruculose walls, dark brown, the second cell from base pale brown, 4.3 - 5.3 µm long (= 4.8 µm); third cell darker brown, 3.7 - 5 µm long (= 4.1 µm; the fourth cell darkest, 4.5 - 5.3 µm (= 4.9 µm); three apical appendages 12 - 18 µm long (= 15 µm), tubular, without terminal inflation, arising from the upper portion of the apical cell; single basal appendage, 3.5 - 5.2 µm long, filiform (Figure 1 F - H).

Colonies on PDA reaching 7 cm diam after 6 days at 25ºC, edge entire, whitish aerial mycelium, fruiting-bodies black, gregarious; reverse of culture white (Figure 1 I - J).

Habitat/Distribution: Known to cause fruit rot on Syzygium samarangense in Thailand.

Material examined: THAILAND, Chiang Mai Province, Chiang Mai, on fruits of Syzygium samarangense, 20 January 2010, S.S.N. Maharachchikumbura S200110b (MFLU 12-0133; holotype) - ex-type culture MFLUCC 12-0233; ibid., 15 May 2011, S.S.N. Maharachchikumbura S200511 (MFLU 12-0134); Chiang Rai Province, Chiang Rai, 15 September 2011, S.S.N. Maharachchikumbura S150911 (MFLU 12-0135) (Table 1).

TABLE 1 - Isolates and sequences used in the phylogenetic study

*Acronyms: CGMCC= China General Microbiological Culture Collection, MFLUCC= Mae Fah Luang University Culture Collection; **SD = sample number 2, SG = sample number 1, SS = sample number 3

As the names of Pestalotiopsis taxa downloaded from GenBank have been shown to be unreliable (Maharachchikumbura et al., 2011) we prefer to label the downloaded taxa as Pestalotiopsis cf. menezesiana, P. cf. versicolor, and P. cf. virgatula to prevent future confusion. The task of epitypification will however take some time and therefore we use cf. (similar to the available species, but not certainly identified as this species = compare with) until more species have been epitypified.

DISCUSSION

Previously, Pestalotiopsis eugeniae (Thüm.) S. Kaneko has been described from wax apple fruit (Lan, 2001). Pestalotiopsis eugeniae differs from P. samarangensis by concolourous conidia. Pestalotiopsis samarangensis also has overlapping morphology with P. versicolor and P. virgatula (Table 2). However, P. samarangensis differs from P. versicolor and P. virgatula by molecular data and by shorter apical appendages (Guba, 1961; Nag Raj, 1993). Pestalotiopsis samarangensis is somewhat similar in morphology to P. palmarum (Cooke) Steyaert), which was isolated in India from coconut. In the original description, Cooke (1876) did not indicate the range of conidial dimensions for P. palmarum, but only the average conidial size as 15 × 5 µm, whereas in P. samarangensis conidia measure on average 20 × 7 µm, thus definitely different. According to Guba (1961), the conidia of P. palmarum measure 16 - 22 × 5 - 7 µm, overlapping with our fungus (Table 2). In pathological consideration, Pestalotiopsis species attacking members of the family Myrtaceae may be specific since the essential oils of the family are likely to exert a selection on pathogens capable to attack these plants (Lee et al., 2008). In this family, scab disease of Psidium guajava caused by P. clavispora (G.F. Atk.) Steyaert, P. microspora (Speg.) G.C. Zhao & N. Li, P. psidii (Pat.) Mordue, and P. disseminata (Thum.) Steyaert (Keith et al., 2006); bark lesions of Eucalyptus globulus stems by P. guepinii (Desm.) Steyert (Alonso et al., 2009); a leaf spot of Anogeissus latifolia caused by P. versicolor (Speg.) Steyaert (Agarwal and Ganguli 1959), and leaf spot of Eucalyptus camaldulensis by P. mangiferae (Henn.) Steyaert (El-Sayed et al., 1985) are known. Pestalotiopsis disseminata, P. guepinii, P. microspora, and P. psidii have concolorous median cells. P. samarangensis is distinct from all these species by versicoloured median cells. It is distinct from P. clavispora (conidia 18 - 26 × 6.5 - 8.5 µm; apical appendages 17 - 31 µm) and P. mangiferae (conidia 22 - 26 × 8 - 11 µm; apical appendages 17 - 31 µm) by smaller conidia (18 - 21 × 6.5 - 7.5 µm) and shorter apical appendages (12 - 18 µm) (Guba, 1961; Nag Raj, 1993). We therefore introduce a new species to accommodate this taxon which causes a disease of wax apple.

TABLE 2 - Comparison of conidia of Pestalotiopsis samarangensis and other similar species

^a BPI 406804

^b Guba (1961)

Species of Pestalotiopsis are not well resolved in the ITS tree, however, ITS, β-tubulin, and tef1 combined gene analysis successfully resolved the species. The combined phylogenetic tree comprises three ex-type strains and a further twelve putatively named species (due to their economically importance) from our studies. Only for six species of Pestalotiopsis, ex-type strains are available; P. furcata, P. hainanensis, P. jesteri, P. kunmingensis, P. pallidotheae and P. theae. Thus there is a need to epitypify other Pestalotiopsis species (Maharachchikumbura et al., 2011). Epitypification in Pestalotiopsis spp. is, however, somehow problematic as species are not thought to be host-specific, and material of morphologically similar species taken from the same host and geographical location may actually belong to a different species to what was originally described. Maharachchikumbura et al. (2011) showed that many Pestalotiopsis species had been previously named according to host association, and that only a small number of characteristics are available to differentiate species. Therefore, identification of Pestalotiopsis to species level is presently difficult and many sequences for Pestalotiopsis spp. deposited in GenBank are likely to be wrongly named. Because of this unreliability of sequences in GenBank we are referring these taxa as Pestalotiopsis cf. menezesiana, P. cf. versicolor, and P. cf. virgatula for the time being. Correct species identification is essential in plant-pathogenic genera (Rossman & Palm-Hernández, 2008). Plant pathologists need to be able to confidently name causal agents in Pestalotiopsis, so that effective quarantine measures can be put in place to prevent entry of unwanted diseases into a country (Maharachchikumbura et al., 2011). Only wounded or damaged fruits have developed disease symptoms indicating that careful handling of the fruit during transportation and storage can help prevent P. samarangensis fruit rot.

ACKNOWLEDGMENTS

We thank King Saud University, the Mushroom Research Foundation, Chiang Mai, Thailand and the National Research Council of Thailand (grant for Pestalotiopsis No: 55201020008), and Mae Fah Luang University (grant for Pestalotiopsis No: 55101020004) for supporting this research. Appreciation is extended to the U.S. National Fungus Collections for providing type herbarium material.

Received 11 May 2012

Accepted 21 December 2012

Author for correspondence: Sajeewa S.N. Maharachchikumbura, e-mail: [e-mail]; Kevin D. Hyde, e-mail: [e-mail]

TPP-2012-0013

Section Editor: Meike Piepenbring

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A destructive new disease of Syzygium samarangense in Thailand caused by the new species Pestalotiopsis samarangensis

Sajeewa S.N. Maharachchikumbura^{I, II, III}; Liang-Dong Guo^III; Ekachai Chukeatirote^{I, II}; Eric H.C. McKenzie^IV; Kevin D. Hyde^{I, II}

Publication Dates

• Publication in this collection
  14 May 2013
• Date of issue
  June 2013

History

• Received
  11 May 2012
• Accepted
  21 Dec 2012