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Research Article
Diseases of Cymbopogon citratus (Poaceae) in China: Curvularia nanningensis sp. nov.
expand article infoQian Zhang, Zai-Fu Yang, Wei Cheng, Nalin N. Wijayawardene§, Kevin D. Hyde|, Zhuo Chen, Yong Wang
‡ Guizhou University, Guiyang, China
§ Qujing Normal University, Qujing, China
| Mae Fah Luang University, Chiang Rai, Thailand
Open Access

Abstract

Five Curvularia strains isolated from diseased leaves of lemongrass (Cymbopogon citratus) in Guangxi Province, China, were examined. NCBI-Blast searches of ITS sequences suggested a high degree of similarity (99–100%) to Curvularia akaii, C. akaiiensis, C. bothriochloae, C. heteropogonis and C. sichuanensis. To accurately identify these strains, we further analysed their morphology and phylogenetic relationships based on combinations of ITS, GAPDH, and tef1 gene sequences. Morphological observations indicated that the key character differing from similar species was conidial size, whereas phylogenetic analyses indicated that the five strains represent one species that is also distinct from C. akaii, C. akaiiensis and C. bothriochloae by conidial size and conidiophore length. Thus, the strains examined are found to represent a new species described herein as Curvularia nanningensis. The pathogenicity test on the host and detached leaves confirmed the new species to be pathogenic on Cymbopogon citratus leaves. Standardised requirements for reliable identification of Curvularia pathogens are also proposed.

Keywords

Cymbopogon, phylogeny, plant disease, Pleosporaceae, taxonomy

Introduction

Cymbopogon citratus Stapf (lemongrass), believed to be a native of Malaysia, is now widely distributed in all continents and particularly in America, China, Guatemala and Southeast Asia. Essential oil from lemongrass is often used in aromatherapy (Williamson et al. 1996; Noel et al. 2002; Yang and Lei 2005; Shah et al. 2011). As a traditional Chinese medicine, lemongrass is known to provide relief from a variety of ailments including eczema, cold, headache and stomach-ache (Zhou et al. 2011). Guatemala is known to be the main exporter of lemongrass with about 250 tons per year. China produces 80 to 100 tons of lemongrass annually and the USA and Russia each imports about 70 tons per year (DAFF 2012). Depending on climatic conditions, lemongrass can be severely infected with a rust disease caused by Puccinia nakanishikii Dietel in Hawaii and California (Gardner 1985; Koike and Molinar 1999). In Brazil, a rust on lemongrass caused by another Puccinia species named P. cymbopogonis Massee has been reported (Vida et al. 2006). Joy et al. (2006) summarised the various disease symptoms and their causal agents of lemongrass.

Curvularia spp. infect many herbaceous plants including Cymbopogon Spreng. (Smith et al. 1989). Helminthosporium cymbopogi C.W. Dodge (≡ Curvularia cymbopogonis (C.W. Dodge) J.W.Groves & Skolko) is responsible for a severe disease of lemongrass in the lowlands of Guatemala (Dodge 1942). Barua and Bordoloi (1983) discovered C. verruciformis causing disease on Cymbopogon flexuosus Stapf. Curvularia andropogonis (Zimm.) Boedijn led to foliage blight of Cymbopogon nardus (L.) Rendle in the Philippines (Sato and Ohkubo 1990). Thakur (1994) reported C. lunata (Wakker) Boedijn as the causal agent of a new blight disease of Cymbopogon martini (Roxb.) Wats. var. motia Burk. Chutia et al. (2006) discovered that a leaf blight of Cymbopogon winterianus Jowitt is caused by Curvularia spp., resulting in a dramatic change in oil yield and its constituents. Recently, Santos et al. (2018) characterised the morphological and molecular diversity of the isolates of C. lunata, associated with Andropogon Linn. seeds.

Starting in 2010, there have been outbreak reports of pathogenic Curvularia in Asian countries, especially India and Pakistan (Pandey et al. 2014; Avasthi et al. 2015; Majeed et al. 2015). As China is a neighbouring country, we felt obligated to evaluate the potential threat of Curvularia to our crops. A severe Curvularia leaf blight disease was observed in three farms of Curcuma aromatica Salisb. in Hainan Province during 2010 (Chen et al. 2013). Gao et al. (2012) reported a new rice black sheath spot disease caused by C. fallax Boedijn in Hunan Province. Our research group is also conducting a disease survey on the occurrence of Curvularia diseases in Southwest China since 2017. Two new pathogens (C. asianensis Manamgoda, L. Cai & K.D. Hyde and C. microspora Y. Liang, K.D. Hyde, J. Bhat & Yong Wang bis), which affected Epipremnum pinnatum (L.) Engl. and Hippeastrum rutilum Herb. (Liang et al. 2018; Wang et al. 2018), respectively, were found.

Meanwhile, a severe leaf blast disease on lemongrass was found in Guangxi Province, China, that first appeared on the tips of leaves. As the infection progressed, more than 30% of leaves showed different degrees of abnormalities, while in the later stages more than 50% of the upper leaves appeared diseased and disease incidence reached 80% or above in the lower leaf blades. We provide a detailed morphological description and phylogenetic analyses of the pathogen confirming it as a new Curvularia species. Koch’s postulates (see later text) have been carried out to confirm its pathogenicity. Our study provides a further understanding of Curvularia disease on lemongrass in China.

Materials and methods

Isolation

Leaves of Cymbopogon citratus showing leaf blast symptoms were collected from Guangxi Medicinal Botanical Garden in Nanning, China, during 2017. Diseased leaf pieces were surface disinfected with 70% ethanol for 30 s, 1% NaClO for 1 min and repeatedly rinsed in sterile distilled water for 30 s. For isolation of Curvularia, conidia were removed from the diseased tissue surface using a sterilised needle and placed in a drop of sterilised water followed by microscopic examination. The spore suspension was drawn with a Pasteur pipette and transferred to a Petri dish with 2% water agar (WA) or 2% malt extract agar (MEA) and 100 mg/l streptomycin to inhibit the growth of bacteria. The plates were incubated for 24 h in an incubator (25°C) and examined for single spore germination under a dissecting microscope. Germinating conidia were transferred separately to new 2% MEA plates (Chomnunti et al. 2014).

Morphological studies

Single germinated spores were transferred to PDA or MEA and incubated at 28°C in a light incubator with 12 h light/12 h darkness. Ten days later, the colony and morphological characters were recorded according to Manamgoda et al. (2011, 2012). Colony diameters on PDA and MEA were measured at 1, 3, 5 and 7 days post-inoculation and average growth rates were calculated. Conidia and conidiophores were examined using a compound microscope fitted with a digital camera (Olympus BX53). The holotype specimen is deposited in the Herbarium of the Department of Plant Pathology, Agricultural College, Guizhou University (HGUP). An ex-type culture is deposited in the Culture Collection of the Department of Plant Pathology, Agriculture College, Guizhou University, China (GUCC) and Mae Fah Luang University Culture Collection (MFLUCC) in Thailand (Table 1).

Table 1.

Sequences used for phylogenetic analysis.

Species name Strain number GenBank Accession numbers
ITS GAPDH tef1
Curvularia aeria CBS 294.61T HE861850 HF565450
C. affinis CBS 154.34T KJ909780 KM230401 KM196566
C. ahvazensis CBS 144673T KX139029 MG428693 MG428686
C. akaii CBS 317.86 KJ909782 KM230402 KM196569
C. akaiiensis BRIP 16080T KJ415539 KJ415407 KJ415453
C. alcornii MFLUCC 10-0703T JX256420 JX276433 JX266589
C. americana UTHSC 08-3414T HE861833 HF565488
C. asiatica MFLUCC 10-0711T JX256424 JX276436 JX266593
C. australiensis BRIP 12044T KJ415540 KJ415406 KJ415452
C. australis BRIP 12521T KJ415541 KJ415405 KJ415451
C. bannonii BRIP 16732T KJ415542 KJ415404 KJ415450
C. beasleyi BRIP 10972T MH414892 MH433638 MH433654
C. beerburrumensis BRIP 12942T MH414894 MH433634 MH433657
C. boeremae IMI 164633T MH414911 MH433641
C. borreriae CBS 859.73 HE861848 HF565455
MFLUCC 11-0422 KP400638 KP419987 KM196571
C. bothriochloae BRIP 12522T KJ415543 KJ415403 KJ415449
C. brachyspora CBS 186.50 KJ922372 KM061784 KM230405
C. buchloes CBS 246.49T KJ909765 KM061789 KM196588
C. carica-papayae CBS 135941T HG778984 HG779146
C. chiangmaiensis CPC 28829T MF490814 MF490836 MF490857
C. chlamydospora UTHSC 07-2764T HG779021 HG779151
C. clavata BRIP 61680b KU552205 KU552167 KU552159
C. coatesiae BRIP 24261T MH414897 MH433636 MH433659
C. coicis CBS 192.29T JN192373 JN600962 JN601006
C. colbranii BRIP 13066T MH414898 MH433642 MH433660
C. crustacea BRIP 13524T KJ415544 KJ415402 KJ415448
C. cymbopogonis CBS 419.78 HG778985 HG779129
C. dactyloctenicola CPC 28810T MF490815 MF490837 MF490858
C. dactyloctenii BRIP 12846T KJ415545 KJ415401 KJ415447
C. deightonii CBS 537.70 LT631356 LT715839
C. ellisii CBS 193.62T JN192375 JN600963 JN601007
C. eragrosticola BRIP 12538T MH414899 MH433643 MH433661
C. eragrostidis CBS 189.48 HG778986 HG779154
C. geniculata CBS 187.50T KJ909781 KM083609 KM230410
C. gladioli CBS 210.79 HG778987 HG779123
C. graminicola BRIP 23186T JN192376 JN600964 JN601008
C. gudauskasii DAOM 165085 AF071338
C. harveyi BRIP 57412T KJ415546 KJ415400 KJ415446
C. hawaiiensis BRIP 11987T KJ415547 KJ415399 KJ415445
C. heteropogonicola BRIP 14579T KJ415548 KJ415398 KJ415444
C. heteropogonis CBS 284.91T JN192379 JN600969 JN601013
C. hominis CBS 136985T HG779011 HG779106
C. homomorpha CBS 156.60T JN192380 JN600970 JN601014
C. inaequalis CBS 102.42T KJ922375 KM061787 KM196574
C. intermedia CBS 334.64 HG778991 HG779155
C. ischaemi CBS 630.82T JX256428 JX276440
C. kenpeggii BRIP 14530T MH414900 MH433644 MH433662
C. kusanoi CBS 137.29T JN192381 JN601016
C. lamingtonensis BRIP 12259T MH414901 MH433645 MH433663
C. lunata CBS 730.96T JX256429 JX276441 JX266596
C. malina CBS 131274T JF812154 KP153179 KR493095
C. mebaldsii BRIP 12900T MH414902 MH433647 MH433664
C. micropus CBS 127235T HE792934 LT715859
C. microspora GUCC 6272T MF139088 MF139106 MF139115
C. miyakei CBS 197.29T KJ909770 KM083611 KM196568
C. mosaddeghii IRAN 3131CT MG846737 MH392155 MH392152
C. muehlenbeckiae CBS 144.63T HG779002 HG779108
C. neergaardii BRIP 12919T KJ415550 KJ415397 KJ415443
C. nanningensis sp. nov. GUCC 11000 MH885316 MH980000 MH980006
GUCC 11001 MH885317 MH980001 MH980007
GUCC 11002 MH885318 MH980002 MH980008
GUCC 11003 MH885319 MH980003 MH980009
GUCC 11005T MH885321 MH980005 MH980011
C. neoindica BRIP 17439 AF081449 AF081406
C. nicotiae CBS 655.74T = BRIP 11983 KJ415551 KJ415396 KJ415442
C. nodosa CPC 28800T MF490816 MF490838 MF490859
CPC 28801 MF490817 MF490839 MF490860
CPC 28812 MF490818 MF490840 MF490861
C. nodulosa CBS 160.58 JN601033 JN600975 JN601019
C. oryzae CBS 169.53T KP400650 KP645344 KM196590
C. ovariicola CBS 470.90T JN192384 JN600976 JN601020
C. pallescens CBS 156.35T KJ922380 KM083606 KM196570
C. palmicola MFLUCC 14-0404 MF621582
C. papendorfii CBS 308.67T KJ909774 KM083617 KM196594
C. perotidis CBS 350.90T JN192385 KJ415394 JN601021
C. petersonii BRIP 14642T MH414905 MH433650 MH433668
C. pisi CBS 190.48T KY905678 KY905690 KY905697
C. platzii BRIP 27703bT MH414906 MH433651 MH433669
C. portulacae CBS 239.48T = BRIP 14541 KJ415553 KJ415393 KJ415440
C. prasadii CBS 143.64T KJ922373 KM061785 KM230408
C. protuberata CBS 376.65T KJ922376 KM083605 KM196576
C. pseudobrachyspora CPC 28808T MF490819 MF490841 MF490862
C. pseudolunata UTHSC 09-2092T HE861842 HF565459
C. pseudorobusta UTHSC 08-3458 HE861838 HF565476
C. ravenelii BRIP 13165T JN192386 JN600978 JN601024
C. reesii BRIP 4358T MH414907 MH433637 MH433670
C. richardiae BRIP 4371T KJ415555 KJ415391 KJ415438
C. robusta CBS 624.68T KJ909783 KM083613 KM196577
C. rouhanii CBS 144674T KX139030 MG428694 MG428687
C. ryleyi BRIP 12554T KJ415556 KJ415390 KJ415437
C. senegalensis CBS 149.71 HG779001 HG779128
C. sesuvii Bp-Zj 01T EF175940
C. shahidchamranensis IRAN 3133CT MH550084 MH550083
C. soli CBS 222.96T KY905679 KY905691 KY905698
C. sorghina BRIP 15900T KJ415558 KJ415388 KJ415435
C. spicifera CBS 274.52 JN192387 JN600979 JN601023
C. sporobolicola BRIP 23040bT MH414908 MH433652 MH433671
C. subpapendorfii CBS 656.74T KJ909777 KM061791 KM196585
C. trifolii CBS 173.55 HG779023 HG779124
C. tripogonis BRIP 12375T JN192388 JN600980 JN601025
C. tropicalis BRIP 14834T KJ415559 KJ415387 KJ415434
C. tsudae ATCC 44764T KC424596 KC747745 KC503940
C. tuberculata CBS 146.63T JX256433 JX276445 JX266599
C. uncinata CBS 221.52T HG779024 HG779134
C. variabilis CPC 28813 MF490820 MF490842 MF490863
CPC 28814 MF490821 MF490843 MF490864
CPC 28815T MF490822 MF490844 MF490865
CPC 28816 MF490823 MF490845 MF490866
C. verruciformis CBS 537.75 HG779026 HG779133
C. verruculosa CBS 150.63 KP400652 KP645346 KP735695
CPC 28792 MF490825 MF490847 MF490868
CPC 28809 MF490824 MF490846 MF490867
C. warraberensis BRIP 14817T MH414909 MH433653 MH433672
Bipolaris drechsleri MUS0028 KF500532 KF500535 KM093761
B. maydis CBS 136.29T AF071325 KM034846 KM093794

DNA Extraction and Sequencing

Fungal cultures were grown on PDA at 28°C until the entire Petri dish (90 mm) was colonised. Fresh fungal mycelia were scraped off the surface of the PDA using a sterilised scalpel. A BIOMIGA Fungus Genomic DNA Extraction Kit (GD2416, BIOMIGA, Inc., San Diego, California, USA) was used to extract the genomic DNA. DNA amplification was performed in a 25 μl reaction volume which contained 2.5 μl 10 × PCR buffer, 1 μl of each primer (10 μM), 1 μl template DNA, 0.25 μl Taq DNA polymerase (Promega, Madison, WI, USA) and 18.5 μl ddH2O. Primers used and thermal cycling programme for PCR amplification of the ITS (ITS4/ITS5), GAPDH (gpd1/gpd2) and tef1 (EF-526F/1567R) genes were followed as described previously (White et al. 1990; Berbee et al. 1999; Schoch et al. 2009; Liang et al. 2018).

Phylogenetic analyses

DNA sequences originated from five strains (GUCC 11000, GUCC 11001, GUCC 11002, GUCC 11003 and GUCC 11005) and reference sequences of ex-type or representative sequences of Curvularia species were downloaded from GenBank database (Table 1) with strains of Bipolaris maydis (Y. Nisik. & C. Miyake) Shoemaker (CBS 136.29) and B. drechsleri Manamgoda & Minnis (MUS0028) as outgroup taxa. Alignments for each locus were performed in MAFFT v7.307 online version (Katoh and Standley 2016) and manually verified in MEGA 6.06 (Tamura et al. 2013). Phylogenetic analyses were performed by Maximum Parsimony (MP), Maximum Likelihood (ML) and Bayesian methods. Sequences were optimised manually to allow maximum alignment and maximum sequence similarity as detailed in Manamgoda et al. (2012). MP analyses were performed in PAUP v. 4.0b10 (Swofford 2003) using the heuristic search option with 1,000 random taxa additions and tree bisection and reconnection (TBR) as the branch-swapping algorithm. Five thousand maxtrees were set to build up the phylogenetic tree. The characters in the alignment matrix were ordered according to ITS+GAPDH+tef1 with equal weight, and gaps were treated as missing data. The MP phylogenetic analysis of Curvularia ITS sequences included pathogens from China, India and Pakistan and the wrong sequence (KN879930), actually belonging to Alternaria alternata (taxon:5599), was selected as the outgroup. The Tree Length (TL), Consistency Indices (CI), Retention Indices (RI), Rescaled Consistency Indices (RC) and Homoplasy Index (HI) were calculated for each tree generated. The resulting PHYLIP file was used to generate the ML tree on the CIPRES Science Gateway (https://www.phylo.org/portal2/login.action) using the RAxML-HPC2 black box with 1000 bootstrap replicates and GTRGAMMA as the nucleotide substitution model. For Bayesian inference analysis, the best model of evolution (GTR+I+G) was determined using MrModeltest v2 (Nylander 2004). Bayesian inference analysis was done using MrBayes v 3.2.6 (Ronquist et al. 2012). Bayesian analyses were launched with random starting trees for 2 000 000 generations and Markov chains were sampled every 1000 generations. The first 25% resulting trees were discarded as burn-in. Alignment matrices are available in TreeBASE under the study ID 25080.

Koch’s Postulate test

To confirm the pathogenicity of the fungus, five healthy plants of Cymbopogon citratus were inoculated with 5 mm diameter mycelial plugs of the five isolates (GUCC 11000, GUCC 11001, GUCC 11002, GUCC 11003 and GUCC 11005) cut from the margins of 10-day-old actively growing cultures; the control was treated with sterile agar plugs. The plants were kept for two days in an illuminating incubator at 28° ± 3°C. Additionally, two plants were sprayed with distilled water and kept as control under the same conditions. Both inoculated (host and detached leaves) and control plants were kept for two days in an illuminating incubator at 28 ± 3°C. After four days of incubation, the inoculated plants and leaves were observed for the development of symptoms (Zhang et al. 2018). Infected leaves were collected and the fungus was re-isolated using PDA medium and the ITS sequence was compared with original strains.

Results

Phylogenetic analyses

First, we compared the DNA sequence identity of ITS, GAPDH and tef1 gene regions (Table 2). Among our five strains, there was only one base difference. In the ITS gene region, for C. akaiiensis, the base sequence was identical to our strains; only 1 difference for C. bothriochloae; base differences were 8 for C. akaii, 9 for C. deightonii and 5 for C. sichuanensis. Only C. heteropogonis had noticeable (25) base differences with our strains. In the GAPDH and tef1 gene regions, the mutation rate of DNA bases was apparently faster than the ITS region. There were between 9 to 19 base differences in GAPDH and 3 to 8 in tef1. This means that in Curvularia, GAPDH has a faster evolutionary rate than ITS and tef1 and therefore some mycologists have suggested the use of ITS+GAPDH for phylogenetic analysis and GAPDH as a secondary barcode marker for accurate identification.

Table 2.

DNA sequence differences between Curvularia nanningensis and related species in three gene regions.

Species Strain number ITS (1–547 bp) GAPDH (550–1031bp) tef1 (1034–1899 bp)
C. nanningensis GUCC11000 0 1 0
GUCC11001 0 0 0
GUCC11002 0 1 0
GUCC11003 0 1 0
GUCC11005T 0 0 0
C. akaii CBS 317.86 8 9 4
C. akaiiensis BRIP 16080 T 0 10 5
C. bothriochloae BRIP 12522 T 1 19 8
C. deightonii CBS 537.70 9 13
C. heteropogonis CBS 284.91 T 25 12 3
C. sichuanensis HSAUP II.2650-1 T 5

The alignment of Curvularia combining three gene fragments (ITS, GAPDH and tef1) comprised 116 strains belonging to 104 taxa. In order to accurately identify our strains, phylogenetic analysis included all ex-type and published strains of all Curvularia spp. described recently (Hyde et al. 2017; Marin-Felix et al. 2017; Dehdari et al. 2018; Heidari et al. 2018; Hernández-Restrepo et al. 2018; Mehrabi-Koushki et al. 2018; Tan et al. 2018; Jayawardena et al. 2019) which are listed in Table 1. The final alignment comprised 2032 characters (each gene fragment was separated with 2 “N”) including gaps (ITS: 1−600, GAPDH: 603−1162 and tef1: 1165−2032). Among these characters, 2032 are constant, 125 variable characters are parsimony-uninformative and 503 are parsimony-informative. The parameters of the phylogenetic trees are TL = 2590, CI = 0.38, RI = 0.72 and HI = 0.62. In the Curvularia phylogenetic tree (Figure 1), all isolates grouped together with 100% (MP and ML) bootstrap support. Our strains (GUCC 11000, 11001, 11002, 11003 and 11005) formed a strongly supported group (MP: 100%; ML: 100%; BPP: 1.00) with a close relationship to C. akaii, C. akaiiensis, C. bothriochloae, C. deightonii and C. heteropogonis with high bootstrap support (MP: 94%; ML: 97%; BPP: 1.00). In this group, the five examined strains were closer to C. akaii, C. akaiiensis and C. bothriochloae and also showed high bootstrap support (MP: 82% and ML: 94%; BPP: 0.98).

Figure 1. 

Maximum Parsimony (MP) topology of Curvularia generated from a combination of ITS, GAPDH and tef1 sequences. Bipolaris maydis (CBS 136.29) and B. drechsleri (MUS0028) were used as outgroup taxa. MP and ML above 50% and BPP values above 0.90 were placed close to topological nodes and separated by “/”. The bootstrap values below 50% and BPP values below 0.90 were labelled with “-”. Our main research clade was labelled with green colour.

The phylogenetic analysis of the ITS gene region evaluated all new Curvularia pathogens recently described from China, India and Pakistan. The aligned matrix consisted of fifty-four ITS sequences and included ex-type sequences of 13 Curvularia species (Supplementary Table 1). The phylogenetic tree (Figure 2) indicated that ITS BLAST searches only provided limited value for pathogenic identification. In Curvularia lunata, only one sequence WCCL (MG063428) showed a very close relationship with the ex-type strain sequence of C. lunata CBS 730.96 (MG722981). The other eight sequences were grouped into two branches, e.g. taxon:5503 (LN879926) which might belong to C. aeria, while the other seven formed an independent lineage. ITS sequences did not separate Curvularia affinis, C. asianensis and C. fallax and some of their sequences even clustered with C. australiensis HNWB9-1 (KT719300). After multi-gene analysis, the phylogenetic distance was shown to be unreliable and may suggest whether they belong perhaps to different species.

Figure 2. 

Maximum Parsimony (MP) analysis of Curvularia pathogens in China, India and Pakistan based on ITS sequences. Alternaria alternata (taxon:5599) was used as outgroup taxon. Bootstrap values (≥ 50%) of the MP method are shown near the nodes.

Taxonomy

Curvularia nanningensis Qian Zhang, K.D. Hyde & Yong Wang bis, sp. nov.

MycoBank No: 829056
Facesoffungi number: FoF 05596
Figure 3A–I

Diagnosis

Characterised by the size of conidia.

Type

China, Guangxi Province, Nanning City, Guangxi Medicinal Botanical Garden, 22°51’N, 108°19’E, on blighted leaves of Cymbopogon citratus, 30 September 2017, Q. Zhang, ZQ0091 (HGUP 11005, holotype, MFLU19-1227, isotype), GUCC 11005 and MFLUCC 19-0092, ex-type.

Description

Pathogenic on Cymbopogon citratus. Fungus initially producing white to grey lesions with dark borders on all parts of the shoot, later enlarging and coalescing over entire leaf.

Colonies on PDA irregularly circular, with mycelial growth rate = 1.0 cm/day, vegetative hyphae septate, branched, subhyaline to brown, smooth to verruculose, 2–3 µm, anastomosing. Aerial mycelium dense, felted, initially pale grey, becoming darkened and greyish-green at maturity, producing black extracellular pigments. On MEA, the colony morphology similar to PDA, with growth rate = 1.35 cm/day. Sexual morph: Undetermined. Asexual morph: Hyphomycetous. Conidiophores macronematous, arising singly, simple or branched, flexuous, 8–10 septate, geniculate, pale brown to dark brown, paler towards apex, 120–200 × 2–3 µm (av. = 170 × 2.5 µm, n = 30). Conidiogenous cells polytretic, sympodial, terminal, sometimes intercalary, cicatrised, with thickened and darkened conidiogenous loci up to 1.0–1.2 µm diam., smooth. Mature conidia 3 to rarely 4 septa, acropleurogenous, obovoid, usually straight to curved at the slightly wider, smooth-walled, larger third cell from the base, 24.5–36.0 × 14.0–20.5 µm (av. = 29.5 × 17.5 µm, n = 50), sub-hyaline to pale brown end cells, pale brown to dark brown at intermediate cells, with conspicuous or sometimes slightly protuberant hilum. Germination of conidia bipolar.

Distribution

China, Guangxi Province, Nanning City.

Other material examined

China, Guangxi Province, Nanning city, Guangxi Medicinal Botanical Garden, on blight leaves of C. citratus, 30 September 2017, Q. Zhang, ZQ0087 (HGUP 11000); ZQ0088 (HGUP 11001); ZQ0089 (HGUP 11002); ZQ0090, (HGUP 11003).

Etymology

With reference to the location, Nanning City where the fungus was isolated.

Figure 3. 

Curvularia nanningensis (GUCC11005, holotype) A, B diseased symptom C colony on PDA from above D colony on PDA from below EG conidia and conidiophores HI conidia. Scale bars: 50 μm (E), 20 μm (F), 10 μm (GI).

Pathogenicity test

Four days after inoculation, blast symptoms appeared on all inoculated plants, which were similar to symptoms of plants in the field (Figures 3A, B, 4A, B). Non-treated control plants remained healthy without any symptoms (Figure 4C). Curvularia nanningensis was re-isolated from the lesions of inoculated plants and the identity of the fungus was confirmed by sequencing the ITS region. Meanwhile, a detached leaf-experiment was also conducted in an illuminated incubator at 28 ± 3°C, where similar symptoms appeared on healthy inoculated leaves of Cymbopogon citratus after four days (Figure 4 D right), while the control leaf (Figure 4 D left) did not show symptoms.

Figure 4. 

Pathogen inoculation and symptom (4 days). A Cymbopogon citratus inoculated and disease symptom B inoculation point and disease symptom C control D detached experiment. Left. Control. Right. Inoculation point and disease symptoms.

Discussion

Phylogenetic analysis based on combined DNA sequences of ITS, GAPDH and tef1 showed that our strains were related to three Curvularia species named C. akaii (Tsuda & Ueyama) Sivan., C. akaiiensis Sivan. and C. bothriochloae Sivan., Alcorn & R.G. Shivas. The main morphological characters that discriminate our strains from related species are the size-range of conidia and length of conidiophores. Curvularia bothriochloae produced conidia measuring 30–47 × 15–25 µm (Sivanesan et al. 2003) while C. akaiiensis produced the smallest conidia (22.5–27.5 × 7.5–15.5 µm). Conidial length of C. nanningensis was very close to C. akaii (24–34 µm) (Tsuda and Ueyama 1985) but the conidia of our species were broader than those of C. akaii (8.7–13.8 µm). Conidiophores of C. nanningensis were shorter than those of C. bothriochloae (360–425 µm) (Alcorn 1990). In the case of C. sichuanensis Meng Zhang & T.Y. Zhang, only one ITS sequence AB453881 was available in GenBank for analysis. While examining our sequences, only 4–5 bp differences were revealed in 499 bp characters between C. nanningensis and C. sichuanensis, thus indicating a close relationship between the two strains based on ITS sequence data and likely between the two species. However, according to Zhang et al. (2007), the conidial width of C. sichuanensis (10–15 µm) is smaller than C. nanningensis (14–20.5 µm) on PDA. For C. sichuanensis, the conidial wall of the median cell is deepened and thickened while C. nanningensis obviously does not have these characters. Meanwhile, the hilum of conidia in C. sichuanensis is obviously protuberant while C. nanningensis lacked this character.

The pathogenicity test based on natural inoculation and detached leaves (Figure 3) confirmed that Curvularia nanningensis is a pathogen of Cymbopogon citratus blast disease. We previously named our strains as C. cymbopogonis following a previous report of the species by Groves and Skolko (1945) as a seed-borne pathogen of Cymbopogon nardus. Curvularia cymbopogonis is a common pathogen which also causes diseases of sugar-cane, rice, seedlings of itchgrass, Agrostis palustris Huds. and Dactylis glomerata L. (Santamaria et al. 1971; Walker and White 1979; Olufolaji 1996; Yi et al. 2002). A single strain named C. cymbopogonis (CBS 419.78) included in our analyses grouped distant from C. nanningensis but its reliability seems questionable and apparently belongs to a different species (Fig. 1). We further checked the original description of this species (Groves and Skolko 1945) and found that differences in conidial shape mainly resulted from conidial width (C. cymbopogonis: 11–13 µm vs C. nanningensis: 14–20.5 µm). Additionally, Groves and Skolko (1945), Hall and Sivanesan (1972) and Yi et al. (2002) reported that C. cymbopogonis produced 4 to 5-septate conidia, whereas conidia of C. nanningensis only had 3-septa. Curvularia spp. are important pathogens of lemongrass. Morphological studies together with phylogenetic analyses provided evidence that C. nanningensis is a new pathogen distinct from all hitherto reported diseases on lemongrass. Our findings expanded the documented diversity of Cymbopogon pathogens within the genus Curvularia and further clarified the taxonomy of this novel pathogen, Curvularia nanningensis.

Moreover, 29 first reports of Curvularia diseases on different plants in China, India and Pakistan were found in the literature from 2010 to the present. It is evident that in this vast geographical area, Curvularia spp. have maintained a close association with plant diversity and thereby possess a rich fungal diversity that is affected by crops distribution. Among them, six reports only provided morphological data and more than half (16) only referred to ITS sequence data and morphological description (Suppl. Table 1). For unknown reasons, Iftikhar et al. (2016) misidentified the Curvularia pathogen with an Alternaria sequence (LN879930.1). Our phylogenetic tree, based on 54 reported ITS sequence data of Curvularia diseases in these countries (Figure 2), also indicated that this approach is not effective for identifying these pathogens, especially in the case of C. lunata as a prevalent species. However, identification of Curvularia isolates by multi-gene phylogenetic analyses has withstood scrutiny (Liang et al. 2018; Wang et al. 2018; Zhang et al. 2018). Additionally, nearly all reports, even for severe diseases, are based on a single isolate, which preclude an objective evaluation. We, therefore, propose the following standardised steps as required for the reliable identification of Curvularia diseases: 1) collect several isolates from diseased samples, 2) obtain sequences of the ITS, GAPDH and tef1 or at least ITS+GAPDH for phylogenetic analysis, 3) perform BLAST searches with sequences originated from ex-type or representative strains in GenBank, and 4) combine morphological comparison and phylogenetic analysis for accurate identification.

Acknowledgments

This research is supported by the following projects: National Natural Science Foundation of China (No. 31972222, 31560489), Program of Introducing Talents of Discipline to Universities of China (111 Program, D20023), Science and Technology basic work of MOST [2014FY120100], National Key Technology Research and Development Program of the Ministry of Science and Technology of China (2014BAD23B03/03), Talent project of Guizhou Science and Technology Cooperation Platform ([2017]5788-5 and [2019]5641) and Guizhou Science, Technology Department International Cooperation Base project ([2018]5806). Nalin Wijayawardene thanks National Natural Science Foundation of China (No. NSFC 31950410558). We thank Mr Mike Skinner for linguistic editing.

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Supplementary material

Supplementary material 1 

Table S1

Qian Zhang, Zai-Fu Yang, Wei Cheng, Nalin N. Wijayawardene, Kevin D. Hyde, Zhuo Chen, Yong Wang

Data type: (measurement/occurrence/multimedia/etc.)

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