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Research Article
Morpho-molecular characterization of Discosia ravennica sp. nov. and a new host record for Sporocadus rosigena
expand article infoDigvijayini Bundhun§|, Rajesh Jeewon, Indunil C. Senanayake#¤, Erio Camporesi«, Janith V. S. Aluthmuhandiram|», Alvin M. C. Tang˄, Ji-Chuan Kang§, Vishwakalyan Bhoyroo, Kevin D. Hyde|¤
‡ Chiang Mai University, Chiang Mai, Thailand
§ Guizhou University, Guiyang, China
| Mae Fah Luang University, Chiang Rai, Thailand
¶ University of Mauritius, Reduit, Mauritius
# Shenzhen University, Shenzhen, China
¤ Zhongkai University of Agriculture and Engineering, Guangzhou, China
« A.M.B. Gruppo Micologico Forlivese “Antonio Cicognani”, Forli, Italy
» Institute of Plant and Environment Protection, Beijing Academy of Agriculture and Forestry Sciences, Beijing, China
˄ Hong Kong Baptist University, Hong Kong, Hong Kong
Open Access

Abstract

Collections of fungal samples from two dead leaf specimens from Italy were subjected to morphological examination and phylogenetic analyses. Two coelomycetous taxa belonging to two different genera in Xylariomycetidae, Sordariomycetes, namely Discosia and Sporocadus, were identified. The Discosia taxon is revealed as a new species and is herein introduced as Discosia ravennica sp. nov. while the Sporocadus taxon is identified as Sporocadus rosigena. Multi-locus phylogeny based on DNA sequence data of the large subunit (LSU) and internal transcribed spacer (ITS) of nuclear ribosomal genes, β-tubulin (β-tub) and RNA polymerase II second largest subunit (rpb2) showed that D. ravennica is related to D. neofraxinea but it forms an independent lineage that supports its new species status. The new taxon also differs from other Discosia species by its unilocular to bilocular, superficial and applanate conidiomata with basal stroma composed of cells of textura angularis, elongate-ampulliform conidiogenous cells and conidia smaller in size. Sporocadus rosigena is here reported as a new host record from Quercus ilex from Italy. Descriptions, illustrations and molecular data for both species are provided in this paper.

Keywords

Amphisphaeriales, asexual morphs, new species, saprobes, taxonomy

Introduction

Members of the Sporocadaceae (Amphisphaeriales, Sordariomycetes) are generally appendage-bearing coelomycetes equally known as “pestalotioid fungi” (Tanaka et al. 2011; Liu et al. 2019). Discosia Lib. ex Durieu & Mont. and Sporocadus Corda are two genera in this family and they were shown to be phylogenetically linked as sister taxa (Jeewon et al. 2002; Maharachchikumbura et al. 2016).

After Libert (1837) established Discosia, it was re-studied by Subramanian and Reddy (1974) who designated D. strobilina Lib. ex Sacc. as lectotype for the genus (Nag Raj 1993; Tanaka et al. 2011). Later, when Sphaeria artocreas Tode was transferred to the genus and combined under D. artocreas (Tode) Fr., the latter was chosen as lectotype of the genus (Fries 1849; Vanev 1991). Morgan-Jones (1964) investigated both D. artocreas [same material examined by Fries (1849)] and D. strobilina and reported them as two different species. Subramanian and Reddy (1974) did not examine the type of D. artocreas, but the features of D. strobilina they observed did not match the same reported by Morgan-Jones (1964). The status of D. artocreas as type species of Discosia, therefore, has not been confirmed (Sutton 1980). Nevertheless, it is currently accepted as the type species of the genus (Crous et al. 2013; Index Fungorum, http://www.indexfungorum.org/Names/Names.asp). Recently, an epitype for D. artocreas was designated (Liu et al. 2019).

Delineation of Discosia taxa was earlier, primarily focused on morphological characteristics such as septation of the conidia, varying proportional lengths of the conidial cells and the conidium size (Subramanian and Reddy 1974; Sutton 1980; Vanev 1991, 1992, 1996; Nag Raj 1993). However, these similar morphological characters have been found to be overlapping for most Discosia species (Sutton 1977, 1980; Nag Raj 1993; Jeewon et al. 2002; Barber et al. 2011; Tanaka et al. 2011). Species of Discosia were earlier also divided into four sections based on the size, septation and pigmentation of the conidia (Subramanian and Reddy 1974). Later, six sections for the species were proposed based on the same conidial morphology (Vanev 1991). Acquisition of DNA sequence data for Discosia species followed by phylogenetic analyses have, however, shown that the concept of subdivision based on morphology alone has been inaccurate and that proper delineation of species must rely on both morphology and molecular phylogeny (Tanaka et al. 2011).

Sporocadus is a recently resurrected genus, characterized by integrated or discrete conidiogenous cells and generally 3-septate, ellipsoid, cylindrical or obovoid conidia which lack appendages (Liu et al. 2019). The genus was originally introduced to accommodate four species, including S. herbarum Corda, S. georginae Corda, S. lichenicola Corda and S. maculans Corda (Corda 1839). No type species for the genus was designated when these species were introduced. However, S. lichenicola was chosen as the lectotype by Hughes (1958). Although Wijayawardene et al. (2016) followed the synonymy of Sporocadus under Seimatosporium by Sutton (1975), Brockman (1976) and Nag Raj (1993) did not accept this. Recently, multi-loci phylogenetic analyses showed that Sporocadus and Seimatosporium are two separate genera, with the former genus usually accommodating taxa without appendages and epitypified by S. lichenicola (Liu et al. 2019).

Documenting fungal species, whether they are novel species or new records, is an important contribution to diversity, taxonomy and plant pathology. It is also imperative that these fungal taxa are studied as a number of them are recognized to be potential emerging plant pathogens and they can impact on disease management strategies (Dugan et al. 2009; Giraud et al. 2010; Ghelardini et al. 2016; Rodeva et al. 2016; Jayasiri et al. 2019; Jayawardena et al. 2020). The aim of this paper is to introduce a new Discosia species collected from Italy based on morphology supported by phylogenetic analyses of combined LSU, ITS, β-tub and rpb2 sequence data. In addition, we report a new host record for a sporocadus-like taxon, identified as Sporocadus rosigena, isolated from Quercus ilex (Fagaceae) in Italy.

Materials and methods

Sample collection and isolation

Samples of plant materials bearing discosia-like and sporocadus-like fungi were collected from dead land leaves of Pyrus sp. and Quercus ilex in the provinces of Ravenna, Oriolo dei Fichi– Faenza and Forlì-Cesena, Fiumana di Predappio, Italy, respectively. They were brought to the laboratory in paper bags and labelled initially as IT 3632 and IT 3569. The specimens were then examined using a dissecting microscope (Motic SMZ-168).

Single-spore isolation was carried out as described in Senanayake et al. (2020). Conidia of the sporocadus-like taxon successfully germinated and were transferred aseptically to malt extract agar (MEA) plates. The cultures were incubated at 18 °C for 2–3 weeks with frequent observations to assess the colony color and other characters.

Morphological studies

Free-hand sections of conidiomata of the Discosia taxon were prepared to examine their morphological characters. The following structures were observed and measured: height, diameter, and shape of conidiomata, conidiomatal wall cell structure, shape and dimensions of conidiophores and conidiogenous cells, length and width of conidia. Morphology of the representatives of the Sporocadus species was obtained from the culture and the morphological characters examined included conidiomata, conidiophores, conidiogenous cells and conidia. All the fungal characters were examined with a fluorescence microscope (Nikon Eclipse E600) and digital images were captured with a Nikon DS-U2 and Cannon 750D camera. All measurements were made using the Tarosoft (R) Image Frame Work software v.0.9.0.7. Images used for photo plates were processed with Adobe Photoshop CS6 v. 12.0 (Adobe Systems, USA).

Material deposition

The holotype of the newly described taxon herein was deposited in the Mae Fah Luang University Herbarium (MFLU), Chiang Rai, Thailand while the isotype at the Cryptogamic Herbarium, Kunming Institute of Botany Academia Sinica (HKAS), Chinese Academy of Sciences, Kunming, China. Herbarium specimen for S. rosigena was also deposited in MFLU while its living culture in Mae Fah Luang University Culture Collection (MFLUCC). Facesoffungi and MycoBank numbers are provided as described in Jayasiri et al. (2015) and MycoBank (http://www.MycoBank.org) respectively. Species concepts are discussed following Jeewon and Hyde (2016).

DNA extraction, PCR amplification and sequencing

Fresh mycelium from the culture of S. rosigena (MFLUCC 18-0387) scraped from the margin of colonies on MEA plates (incubated at room temperature for 4 weeks), and conidiomata of the new taxon (MFLU 18-0131) from natural substrate were used for DNA extraction. Around 20 conidiomata of the new taxon (MFLU 18-0131) were carefully picked from the sterilized material using a fine sterile needle, observed through a stereomicroscope and collected in a 1.5 ml micro-centrifuge tube for subsequent DNA extraction. Genomic DNA was extracted using Forensic DNA Kit (D3591-01, OMEGA bio-tek), following the manufacturer’s instructions. The loci LSU, ITS, β-tub and rpb2 were amplified using primers LR0R/LR5 (Vilgalys and Hester 1990; Rehner and Samuels 1994), ITS5/ITS4 (White et al. 1990; Ward and Adams 1998), BT-2a/BT-2b (Glass and Donaldson 1995) and fRPB2-5F/fRPB2-7cR (Liu et al. 1999; Sung et al. 2007) respectively. Polymerase Chain Reactions (PCR) were conducted in an Applied Biosystems C1000 TouchTM Thermal Cycler with the following PCR conditions for LSU, ITS, β-tub and rpb2 regions: initial denaturation at 95 °C for 3 min followed by 34 cycles of denaturation at 95 °C for 30 s and 30 s of annealing and elongation at 72 °C for 1 min, and a final extension at 72 °C for 10 min. The annealing temperatures were 52 °C for LSU and 58 °C for ITS, β-tub and rpb2. The PCR reaction mixture, 25 µL in final volume, was composed of 0.3 µL of TaKaRa Ex-Taq DNA polymerase (TaKaRa, China), 2.5 µL of 10x Ex-Taq buffer (TaKaRa, China), 3.0 µL (2.5 μM) of dNTPs (TaKaRa, China), 1 µL of genomic DNA, 1 µL (0.4 μM) of each primer, and 16.2 µL of double-distilled H2O. Sequencing of PCR products was carried out with the same primers as mentioned above at the Beijing Biomed Gene Technology Co., Ltd, and Sangon Biotech, Shanghai China. The newly generated sequences were deposited in GenBank (Table 1).

Phylogenetic analyses

Newly generated sequences from LSU, ITS, β-tub and rpb2 during this study (Table 1) were analyzed with other sequences obtained from GenBank along with recently published relevant phylogenies (Wanasinghe et al. 2018; Liu et al. 2019). Sequences for each locus (LSU, ITS, β-tub and rpb2) were aligned using MAFFT V.7.036 (http://mafft.cbrc.jp/alignment/server/; Katoh et al. 2019), with L-INS-i Iterative refinement methods and manually improved when necessary in BioEdit v. 7.0 (Hall 2004). Phylogenetic analyses of the aligned data were based on maximum likelihood (ML) and Bayesian inference (BI) analyses with details as outlined by Tang et al. (2007, 2009).

Table 1.

Taxa used in the phylogenetic analyses and corresponding GenBank accession numbers.

Taxa Strain number GenBank accession numbers
LSU ITS β-tub rpb2
Discosia artocreas CBS 124848T MH554213 MH553994 MH554662 MH554903
Discosia aff. brasiliensis NRBC 104198 AB593706 AB594774 N/A N/A
Discosia brasiliensis MFLUCC 12-0429 = NTCL094-2 KF827436 KF827432 KF827469 KF827473
MFLUCC 12-0431 = NTCL095 KF827437 KF827433 KF827470 KF827474
MFLUCC 12-0435 = NTCL097-2 KF827438 KF827434 KF827471 KF827475
Discosia fagi MFLU 14-0299A =IT-722AT KM678048 KM678040 N/A N/A
MFLU14-0299B = IT-722B KM678047 KM678039 N/A N/A
Discosia italica MFLU 14-0298A = IT-712AT KM678045 KM678042 N/A N/A
MFLU 14-0298B = IT-712B KM678046 KM678043 N/A N/A
MFLU14-0298C = IT-712C KM678044 KM678041 N/A N/A
Discosia macrozamiae CPC 32109 MH327856 MH327820 MH327895 N/A
Discosia neofraxinea MFLUCC 12-0670 = NTIT469 KF827439 KF827435 KF827472 KF827476
MFLU 15-0375T KR072672 KR072673 N/A N/A
Discosia pini MAFF 410149 AB593708 AB594776 AB594174 N/A
Discosia aff. pleurochaeta KT2192 = MAFF 242782 AB593714 AB594782 AB594180 N/A
KT2179 = MAFF 242778 AB593709 AB594777 AB594175 N/A
KT2188 = MAFF 242779 AB593713 AB594781 AB594179 N/A
Discosia pseudoartocreas CBS 136438T KF777214 KF777161 MH554672 MH554913
Discosia querci MFLUCC 16-0642T MG815830 MG815829 N/A N/A
Discosia ravennica MFLU 18-0131T MT376617 MT376615 MT393594 MW468059
Discosia rubi CBS 143893T MH554334 MH554131 MH554804 MH555038
Discosia tricellularis MAFF 237478 AB593730 AB594798 AB594189 N/A
Discosia tricellularis NBRC 32705T AB593728 AB594796 AB594188 N/A
Discosia yakushimensis MAFF 242774 = NBRC 104194T AB594796 AB594789 AB594187 N/A
Pestalotiopsis hollandica CBS 265.33T AB594188 KM199328 KM199388 MH554936
Pseudopestalotiopsis cocos CBS 272.29T NR_145246 KM199467 MH554938
Sporocadus biseptatus CBS 110324 = MYC 754T MH554179 MH553956 MH554615 MH554853
Sporocadus cornicola CBS 143889 = CPC 23235 MH554326 MH554121 MH554794 MH555029
MFLUCC 14-0448T N/A KU974967 N/A N/A
Sporocadus cotini CBS 139966 = MFLUCC 14-0623T MH554222 MH554003 MH554675 MH554916
Sporocadus incanus CBS 123003T MH554210 MH553991 MH554659 MH554900
Sporocadus lichenicola CBS 354.90 = NBRC 32677 MH554252 MH554035 MH554711 MH554948
CPC 24528 MH554332 MH554127 MH554800 MH555036
NBRC 32625 = IMI 079706T MH883646 MH883643 MH883645 MH883647
Sporocadus mali CBS 446.70T MH554261 MH554049 MH554725 MH554960
Sporocadus microcyclus CBS 424.95T MH554258 MH554045 MH554721 MH554956
CBS 887.68 = NBRC 32680 MH554280 MH554068 MH554744 MH554981
Sporocadus multiseptatus CBS 143899 = CPC 26606T MH554343 MH554141 MH554814 MH555047
Sporocadus rosarum CBS 113832 = UPSC 2172 MH554189 MH553970 MH554629 MH554864
Sporocadus rosigena CBS 116498 MH554200 MH553983 MH554642 MH554883
CBS 129166 = MSCL 860 MH554215 MH553996 MH554665 MH554905
CBS 182.50 MH554233 MH554013 MH554689 MH554926
CBS 250.49 MH554245 MH554023 MH554699 MH554934
CBS 466.96 MH554265 MH554052 MH554728 MH554965
MFLU 16-0239T MG829069 MG828958 N/A N/A
Sporocadus rosigena MFLUCC 18-0387 MT376616 MT376614 MT393595 N/A
Sporocadus rotundatus CBS 616.83T MH554273 MH554060 MH554737 MH554974
Sporocadus sorbi MFLUCC 14-0469T KT281911 KT284774 N/A N/A
CBS 160.25 MH554229 MH554008 MH554684 MH554924
Sporocadus sp. CBS 506.71 MH554268 MH554055 MH554731 MH554968
Sporocadus trimorphus CBS 114203 = UPSC 2430T MH554196 MH553977 MH554636 MH554876

RAxML-HPC2 on XSEDE (v. 8.2.8) (Stamatakis et al. 2008; Stamatakis 2014) in the CIPRES Science Gateway platform (Miller et al. 2010) was used to generate the ML trees. Optimal ML tree search was conducted with 1000 separate runs, using the default algorithm of the program from a random starting tree for each run. The ultimate tree was selected among suboptimal trees from each run by comparing likelihood scores under the GTRGAMMA substitution model.

Bayesian analysis was executed in MrBayes v. 3. 1. 2 (Huelsenbeck and Ronquist 2001) through Markov Chain Monte Carlo (MCMC) sampling to calculate the posterior probabilities (PP) (Rannala and Yang 1996; Zhaxybayeva and Gogarten 2002). Partitioning of data was initially done by locus and then the parameters of the nucleotide substitution models for every partition were selected independently using MrModeltest v. 2.3 (Nylander 2004). Six Markov chains were run in parallel for 5M generations with trees being sampled every 1000th generation. The distribution of log-likelihood scores was examined to determine the stationary phase for each search and to decide whether additional runs were required to reach convergence, using the program Tracer 1.5 (Rambaut and Drummond 2007). Convergence was declared when the average standard deviation of split frequencies at the end of the total MCMC generations was at 0.01. First 20% of generated trees was discarded as burn-in and the remaining 80% was used to calculate PP of the majority rule consensus tree (Dissanayake et al. 2020). The resulting trees were viewed in FigTree v. 1.4.0 (Rambaut 2012) and annotated in Microsoft PowerPoint (2013). The final alignment was registered in TreeBASE under the submission ID: 27601.

Results

Phylogenetic analyses

The combined gene dataset (LSU, ITS, β-tub and rpb2) used to generate ML tree in Fig. 1 comprised 51 taxa including the newly generated sequences. Pestalotiopsis hollandica (CBS 265.33) and Pseudopestalotiopsis cocos (CBS 272.29) were selected as outgroup. The ML tree topology was similar to the one of the BI consensus tree. The best scoring RAxML tree with final optimization had a likelihood value of -15179.071239. The matrix had 1020 distinct alignment patterns, with 24.74% of gaps and completely undetermined characters. Estimated base frequencies were as follows: A= 0.247496, C= 0.245307, G= 0.252993, T= 0.254204, with substitution rates AC= 1.621276, AG= 6.173475, AT= 1.526832, CG= 1.406021, CT= 9.022198, GT= 1.000000; gamma distribution shape parameter α= 0.158554 and Tree-length = 1.305620.

Figure 1. 

Phylogram generated from maximum likelihood (RAxML) based on analysis of a combined dataset of LSU, ITS, β-tub and rpb2 sequence data. Bootstrap support values for ML equal to or greater than 70% (black) and Bayesian posterior probabilities (PP) equal to or greater than 0.90 (blue) are defined as ML/PP above or below the nodes. Type collections are in bold while the newly generated sequences are in blue bold type. The tree is rooted to Pestalotiopsis hollandica (CBS 265.33) and Pseudopestalotiopsis cocos (CBS 272.29). The scale bar represents the expected number of nucleotide substitutions per site.

Discosia taxa were divided into two separate clades (A and B). Clade A, consisting of 3 strains of Discosia, grouped with and was sister to Sporocadus with strong statistical support (100% ML, 1.00 PP). Clade B, comprising 21 strains of Discosia, was basal to both Sporocadus and clade A with strong statistical support (100% ML, 1.00 PP). Our strain MFLU 18-0131 was positioned in clade A, basal to both strains of D. neofraxinea (MFLU 15-0375 and MFLUCC 12-0670 = NTIT469), forming an independent lineage with good statistical support (96% ML/ 1.00 PP).

All the Sporocadus species formed a monophyletic clade with strong statistical support (100% ML, 1.00 PP). The strain MFLUCC 18-0387 from this study clustered with the other existing S. rosigena strains with a bootstrap support of 91% ML and 0.98 PP (Fig. 1).

Taxonomy

Discosia ravennica Bundhun, Jeewon, Camporesi, J.C. Kang & K.D. Hyde,, sp. nov.

MycoBank No: 837963
Figure 2

Etymology

The specific epithet ravennica refers to the province of Ravenna, where the fungus was collected.

Holotype

MFLU 18-0131

Description

Saprobic on leaves of Pyrus sp. Sexual morph: Undetermined. Asexual morph: Conidiomata 45–70 µm high, 410–800 µm diam., stromatic, scattered to gregarious, superficial, rounded to unevenly outlined with complete margins, applanate, unilocular to bilocular, rugose, not glabrous, dull black, ostiolate. Ostiole 50–90 µm diam., circular to oval, opening to the exterior, central. Conidiomatal wall 10–20 µm thick at the base, dark brown in the outermost layer, comprising thick-walled cells of textura angularis, gradually becoming pale towards the inner layer; 10–20 µm thick near the apex, dark brown to black, made up of thick-walled cells of textura epidermoidea; interlocular wall composed of dark brown thick-walled cells of textura prismatica, becoming thin-walled and paler towards the outer layers. Conidiophores up to 40 µm high, originating from the innermost layer cells of the basal stroma, unbranched or at times branched, mostly 0–1-septate, rarely 2-septate or reduced to conidiogenous cells, cylindrical, hyaline, smooth. Conidiogenous cells 8–30 × 0.7–1.5 µm (x– = 14.3 × 1.1 µm, n = 15), subcylindrical to elongate-ampuliform, hyaline, smooth-walled, holoblastic. Conidia 12–16 × 1.5–3 µm (x– = 13.8 × 2.3 µm, n = 40) naviculate, to subcylindrical, narrow towards the base, straight or faintly curved, euseptate, mostly 3-septate, occasionally 2-septate, with septa thicker and darker than the periclinal wall, with cells unequal, hyaline to sub-hyaline, smooth-walled, without constriction at septa, bearing appendages on both apical and basal cells; basal cell 3–6 µm (x– = 3.8 µm) long, narrowly obconic, with truncate base bearing a conspicuous dehiscence scar; 2 median cells, together 6–10 µm (x– = 7.4 µm) long [second cell 4–6 µm (x– = 5.0 µm) long, close to apical cell, almost twice the size of the third cell 2–4 µm (x– = 3.0 µm) long, close to basal cell]; apical cell 3–5 µm (x– = 3.6 µm) long, subconical with acute apex, hyaline at apex and sub-hyaline below; appendages tubular, faintly broad at the base, unbranched, flexuous; appendage on apical cell 5–17 µm (x– = 10.1 µm) long, single, polar; appendage on basal cell 4–17 µm (x– = 9.4 µm) long, single, inserted slightly above conidium base.

Figure 2. 

Discosia ravennica (MFLU 18-0131, holotype) a Herbarium specimen b Conidiomata on the host c, d Vertical sections of conidiomata e Conidioma wall at the base f–h Conidiogenous cells and developing conidia i, j Conidia. Scale bars: 500 μm (b); 200 μm (c, d); 10 μm (e, g–j); 20 μm (f).

Material examined

ITALY. Province of Ravenna [RA], Oriolo dei Fichi– Faenza; on dead land leaves of Pyrus sp.; 24 Dec. 2017; Erio Camporesi; IT 3632 (MFLU 18-0131, holotype; HKAS 104973, isotype).

Notes

In the present study, no culture could be obtained for D. ravennica despite several trials on various media including MEA, potato dextrose agar, corn meal agar or water agar at different incubation conditions, the reason for which the species was subjected to direct DNA extraction from conidiomata. Discosia ravennica is morphologically similar to D. neofraxinea in terms of superficial conidiomata, which are not glabrous and 3-septate conidia with cells of unequal length. It also closely resembles D. fraxinea (Schwein.) Nag Raj (1993) in having uni-to bi-locular applanate conidiomata and naviculate to subcylindrical 3-septate conidia with cells of unequal length. The new species, however, also differs from the latter two species as mentioned in Table 2.

Sporocadus rosigena F. Liu, L. Cai & Crous, in Liu, Bonthond, Groenewald, Cai & Crous, Stud. Mycol. 92: 402 (2018)

Figure 3

Seimatosporium rosicola Wanas., Goonas., Camporesi, & K.D. Hyde, in Wanasinghe et al., Fungal Diversity 193 (2018)

Description

Saprobic on Quercus ilex L. Sexual morph: Illustrated in Wanasinghe et al. (2018). Asexual morph: Conidiomata (on host) 115–145 µm diam., 70–130 µm high, acervular, solitary to aggregated, semi-immersed, black; (on MEA) 50–70 µm diam., acervular, solitary to aggregated, erumpent, black. Conidiophores (on MEA) cylindrical, branched, hyaline, smooth, up to 30 µm long. Conidiogenous cells (on MEA) 7–18 × 2–3 µm (x– = 10.1 × 2.1 µm, n = 20) cylindrical, enteroblastic, annellidic, integrated or discrete, hyaline, determinate, smooth. Conidia (on MEA) 12–15 × (3–) 5–7 µm (x– = 13.5 × 5.4 µm, n = 47), obovoid, ellipsoid, broad fusiform or subcylindrical, straight or curved, hyaline when immature, pale to moderate brown at maturity, with 3 transverse, thick, darker septa, rarely constricted at the septa, often obtuse at both ends, or well rounded, smooth-walled, no appendage or sheath; basal cell obconic with a truncate base, pale brown or hyaline, thin-walled, 1–2.5 µm long (x– = 2 µm); two median cells doliiform, hyaline or pale brown, turning brown at maturity, together 5–7 µm long (x– = 6.1 µm), second cell from the base 1–3 µm long (x– = 2.5 µm), third cell from the base 1.4–4 µm long (x– = 2.6 µm); apical cell conical with obtuse or rounded apex, concolorous with the median cells, 1.8–3.5 µm long (x– = 2.5 µm).

Figure 3. 

Sporocadus rosigena (MFLU 17-2803) a Leaf of Quercus ilex L b Close-up of conidiomata on host c Upper view of colony on MEA d Conidiomata in culture (MFLUCC 18-0387) e, f Different stages of conidiogenesis (MFLUCC 18-0387) g–j Conidia (MFLUCC 18-0387). Scale bars: 10 μm (e, f); 5 μm (g–j).

Culture characteristics

Colonies on MEA reaching 2–3 cm diam. after 11 days at 18 °C in darkness, filamentous, circular, flat with entire margin, white from above, reverse pale yellow.

Material examined

ITALY. Province of Forlì-Cesena, Fiumana di Predappio; on dead land leaf of Quercus ilex L. (Fagaceae); 20 Nov. 2017; Erio Camporesi; IT 3569 (MFLU 17-2803); living culture MFLUCC 18-0387.

Notes

Sporocadus rosigena from the present study shares similar morphology with the other S. rosigena strains in having almost obovoid, ellipsoid or fusiform to subcylindrical conidia (Wanasinghe et al. 2018; Liu et al. 2019). Pairwise comparison of DNA sequence data of the isolate MFLUCC 18-0387 with the other strains of S. rosigena revealed very minor differences and thus, the strain MFLUCC 18-0387 is considered as S. rosigena.

Discussion

Discosia ravennica sp. nov. forms an independent lineage, basal to the two strains of D. neofraxinea (96% ML/ 1.00 PP) (Fig. 1). It is different from D. neofraxinea in its unilocular to bilocular, applanate conidiomata along with elongate-ampulliform conidiogenous cells and conidia smaller in size (Table 2). With regard to DNA sequence data comparison, D. ravennica differs from both strains of D. neofraxinea (MFLU 15-0375 and MFLUCC 12-0670 = NTIT469) in having 14 out of 531 (2.6 %) and 8 out of 512 (1.6%) different base pairs (bp) in the ITS alignments respectively. Moreover, 13 bp out of 229 (5.7%) and 82 bp out of 832 (9.9%) differences in the ß-tub and rpb2 alignments respectively can be observed between D. ravennica and D. neofraxinea (MFLUCC 12-0670 = NTIT469). Sequence data of ß-tub and rpb2 are not available for the strain of D. neofraxinea (MFLU 15-0375) in GenBank and hence could not be compared. Similarly, no molecular data for D. fraxinea are accessible in GenBank, following which the new species, D. ravennica, has been delineated based on morphology (Table 2). The 5.7% and 9.9% differences in nucleotides in ß-tub and rpb2 respectively may acceptably support the establishment of a new species (Jeewon and Hyde 2016). Following this assumption along with the above-mentioned morphological differences and high statistical support, D. ravennica is herein established as a new species.

Table 2.

Features distinguishing Discosia ravennica, D. fraxinea and D. neofraxinea.

Features Discosia ravennica (this study) Discosia fraxinea (Nag Raj 1993) Discosia neofraxinea (Senanayake et al. 2015)
Host occurrence Leaves of Pyrus sp. Amelanchier vulgaris, Crataegus sp., Fraxinus americana, Populus sp., Sorbus americana and undetermined leaves Leaves of Fagus sylvatica
Known distribution Italy Austria, France, Germany, U.S.A. Italy
Conidiomata Superficial Erumpent Superficial
Basal stroma Composed of cells of textura angularis Composed of cells of textura prismatica Composed of cells of textura prismatica
Conidiogenous cells 8–30 × 0.7–1.5 µm
Subcylindrical to elongate-ampuliform
7–40 × 1.5–2.5 µm
Subcylindrical to langeniform or ampuliform
6–40 × 1–2 μm
Cylindrical
Conidia 12–16 × 1.5–3 µm
(x– = 13.8 × 2.3 µm)
12.5–19 × 2.5–3.5 µm
(x– = 16.2 × 3 µm)
15–18 × 2.5–3.5 μm
(x– = 16 × 3 μm)

A peculiar finding from our DNA sequence analyses is the placement of D. neofraxinea and D. ravennica. Both of them constitute a strongly supported independent clade (clade A) basal to species of Sporocadus. One might argue that given their distinct phylogenetic nature, a new genus accommodating these two species might be a possibility. However, in this particular scenario, we would rather take a more conservative and lumping taxonomic approach and maintain the latter two species in Discosia. The reasons we would advocate are that there is a lot of morphological resemblance between members of clades A and B. For instance, when we compare D. neofraxinea and D. ravennica (clade A) with the type species, D. artocreas (clade B), they all have stromatic conidiomata, conidiophores which arise from the upper cell layer of the basal stroma, and hyaline to sub-hyaline, usually 3-septate conidia bearing two appendages (Nag Raj 1993; Senanayake et al. 2015; Liu et al. 2019). The main difference is that D. neofraxinea and D. ravennica have the third cell of their conidia from the base longer than the second cell while D. artocreas has the second cell of its conidia from the base longer than the third cell (Nag Raj 1993) or both median cells of almost equal length (Liu et al. 2019). However, this distinctive characteristic is not sufficient enough for the establishment of a new genus. It might be that the genus is paraphyletic, but until more species are recovered and analyzed to provide further taxonomic insights, we refrain from making any taxonomic amendments. It might also be possible that there is a need to establish species complexes given the wide intraspecies variation as we have seen in other genera such as Phyllosticta (Norphanphoun et al. 2020).

The second recovered species from this study, Sporocadus rosigena, clusters with other S. rosigena strains in a well-supported clade (91% ML / 0.98 PP) in our 4-gene phylogeny (Fig. 1). The latter shows similar topology to the 5-gene phylogeny reported by Liu et al. (2019). Sporocadus rosigena has earlier been reported as saprobic or endophytic on species of Rosa, Rubus, Pyrus (Rosaceae), Rhododendron (Ericaceae) and Vitis (Vitaceae) (Wanasinghe et al. 2018; Liu et al. 2019). In this study, the species was found from Quercus ilex (Fagaceae) and is therefore introduced as a new host record. Different fungi have equally been reported from Quercus ilex in Italy; for instance, the genera Alternaria (Lunghini et al. 2013), Beltrania (Pirozynski 1963), Endothia (Spaulding 1961), Monochaetia (Nag Raj 1993), Neognomoniopsis (Crous et al. 2019), Pestalotia (Nag Raj 1993), Xylaria and Zygosporium (Lunghini et al. 2013), indicating a broad diversity of fungi on the same host. All Sporocadus species in their asexual stage possess 3-septate, obovoid, fusoid to cylindrical conidia, which do not have any appendage. The only exceptions are S. trimorphus and S. rosarum, which are known to produce conidia both with and without appendages (Liu et al. 2019).

Fungal diversity and classification are always ever-changing and require an ongoing assessment (Hyde and Soytong 2008; Jeewon et al. 2017). This becomes especially essential in cases where taxa are described from genera which usually accommodate pathogens. Discosia, for instance, is known to comprise the plant pathogen D. yakushimensis which causes leaf spots on plants such as Symplocos prunifolia (Tanaka et al. 2011). Identifying novel species in a genus may also potentially imply the discovery of emerging pathogens which can cause damage to crops of economic importance (Jayawardena et al. 2019a, 2019b). Evolutionary relationships and ecological roles of fungi have been reported to be intricately linked to the emergence of new species (Zhang et al. 2008; Hyde et al. 2020). However, such phenomena also extend to the recognition of existing species from new hosts, as is the case for S. rosigena in the present study. Documenting records from new hosts has become useful repertoires for mycologists who aim to understand evolution of fungi, host jumping, expanding host diversity and adaptations to different environmental conditions (Hyde et al. 2020). These are equally important for proper quarantine measures, whereby potential pathogens or species known to have a wide host diversity are to be closely monitored with a view to avoid unintentional disturbance to a specific environment (Cai et al. 2011).

Acknowledgements

This work was funded by grants of the National Natural Science Foundation of China (NSFC Grants Nos. 31670027 & 31460011) and the Open Research Foundation of the Key Laboratory (Engineering Center) of National Education Ministry at Guizhou University (Grant no. GZUKEY 20160705). Mae Fah Luang University, the Center of Excellence in Fungal Research and the Mushroom Research Foundation, Thailand, are acknowledged for the support provided for research. Rajesh Jeewon is grateful to the University of Mauritius and Mae Fah Luang University for enabling research collaboration. Digvijayini Bundhun thanks Yuanpin Xiao, RS Jayawardena and CS Bhunjun for their help and suggestions. All authors thank Shaun Pennycook for checking the nomenclature of the new taxon.

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