Introduction

Deserts provide a harsh habitat for lifeforms due to the high salinity and acidity of the soils, fluctuating temperatures, low precipitation, and high UV irradiation (Makhalanyane et al. 2015; Whitford and Wade 2002). One of the oldest and driest deserts on earth is the Namib, where a phenomenon known as Fairy Circles occurs.

Fairy circles are barren, circular to almost circular patches surrounded at their edges by healthy grass (Poaceae) species (Albrecht et al. 2001). These unusual circles in Namibia were first documented by Tinley (1971) and have puzzled scientists for over 50 years, with many hypotheses as to their origin and maintenance. These hypotheses include factors such as termite activity and plant self-organization due to water stress (Albrecht et al. 2001; Getzin et al. 2021, 2022). Ramond et al. (2014) suggested that a soil-borne microbial plant pathogen could be the cause of Namibian fairy circles. Later, van der Walt et al. (2016) further pursued the microbial phytopathogen hypothesis by analysing the fungal composition of fairy circles in the dunes and gravel plains of the Namib Desert and adjacent soils using a metabarcoding high-throughput sequencing (HTS) approach. They discovered 57 fairy circle-specific fungal operational taxonomic units (OTUs), which they hypothesized might play a role in the formation and maintenance of the fairy circles.

The genus Curvularia [MB#7847] was described by Boedjin (1933) and is currently classified in the family Pleosporaceae (order Pleosporales, class Dothidiomycetes) together with Alternaria [MB#7106], Bipolaris [MB#7375], Exserohilum [MB#8241], Stemphylium [MB#10081] and others (Ferdinandez et al. 2021). Curvularia currently contains 232 described species (https://www.mycobank.org), of which Marin-Felix et al. (2020) recognised 105 based on DNA sequence data. Curvularia species can be saprophytes, endophytes, or human pathogens, and are found in a variety of habitats including air, indoor environments, soil, water, or plant material (Almaguer et al. 2012; Dransfield 1966; Manamgoda et al. 2015; Marin-Felix et al. 2017; Verma et al. 2013). For example, C. hominis [MB#806054], C. lunata [MB#269889] and C. spicifera [MB#278597] can cause infections in humans and animals (Barde and Singh 1983; Carter and Boudreaux 2004; Manamgoda et al. 2012; Rai et al. 2021; Rinaldi et al. 1987), while C. lunata causes a leaf spot disease on maize (Zea mays) (Manamgoda et al. 2012).

Curvularia species are dematiaceous and characterised by their curved conidia which arise from the unevenly enlarged intermediate cells of these distoseptate spores (Marin-Felix et al. 2020). However, some species also produce straight conidia. These features are similar to those observed in the closely related Bipolaris (Manamgoda et al. 2012). The sexual morphs of these genera were previously classified as Cochliobolus [MB#1158], but this state is rarely observed in nature and is difficult to induce in culture, and its species were thus incorporated into both Bipolaris and Curvularia (Manamgoda et al. 2014). Due to the difficulty in distinguishing these fungi using morphological characters, species recognition in these genera relies on multi-locus sequence analyses of the internal transcribed spacer rDNA region (ITS), and the partial gene regions of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and translation elongation factor 1 alpha (TEF1) (Manamgoda et al. 2012; Marin-Felix et al. 2017, 2020; Tan et al. 2018).

During a survey aimed at characterising the cultural fungal communities associated with Stipagrostis ciliata and its rhizosphere soils collected from fairy circles in the Namib Desert, Curvularia was among the most frequently isolated fungi. Here, we report on Curvularia species identified, introduce five new species, and compare them with existing species using morphological characters and phylogenetic analyses based on ITS, GAPDH, and TEF1.

Materials and methods

Sampling, isolations, and preservations

The strains included in this study were isolated from the tissue of Stipagrostis ciliata and associated rhizosphere soils collected in the fairy circles or so-called reverse circles of the Namib Desert. Three sites were sampled in Namibia, namely “Mirabib” (-23.479167, 15.335000), “Far East” (-23.732500, 15.775000) and an area known as the “Reverse” region (-23.545167, 15.234333). In the “Reverse” region, there were patches of vegetation surrounded by bare ground. Grass and associated rhizosphere soils were sampled at the edges of the fairy circles and in the areas between the circles. Samples were also taken from an area without fairy circles in the Mirabib region.

Stipagrostis ciliata tissue was surface disinfested with 2% sodium hypochlorite (bleach) for 3 min, with 70% ethanol for 30 s and then rinsed in distilled water for 10 s and air-dried on sterile paper towel. The surface disinfested material and soil samples were plated directly onto Malt Extract Agar (MEA) (20 g/L malt extract, 20 g/L Difco agar) supplemented with 0.4 mg 50 ppm Streptomycin. The plates were incubated for 1–3 wks at 19 − 21 °C. Isolates were purified on quarter strength Potato Dextrose Agar (12 g/L Difo Agar, 10 g/L Potato Dextrose Agar) supplemented with 2 mL 100 ppm Chloramphenicol. These were incubated for a further 1 − 3 wks for culture preservation and DNA extraction. The isolates obtained were accessioned in the CN collection (working culture collection of the Applied Mycology group) housed at the Forestry Agriculture and Biotechnology Institute (FABI) at the University of Pretoria (South Africa). Representative strains were accessioned in the CMW and CMW-IA culture collections also housed at FABI, and the CBS-KNAW culture collection of Westerdijk Institute in Utrecht (the Netherlands) (see Table 1).

Table 1 Strains included in this study including their location and GenBank accession numbers
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DNA extractions, polymerase chain reaction (PCR) amplification, sequencing, identification, and phylogenetic analyses

Genomic DNA was extracted using the PrepMan™ Ultra Sample Preparation Reagent (Applied Biosystems, Foster City, CA, USA) according to the manufacturer’s instructions and stored at -20 °C.

PCR amplification of the ITS, GAPDH, and TEF1 loci was conducted using primer pairs and thermal cycle conditions as described in Table 2. Reactions were set up in 25 µL volumes as follows: 17.3 µL Milli-Q® water (Millipore Corporation), 2.5 µL 10 × FastStart™ Taq PCR reaction buffer with 20 mM MgCl2 (Sigma-Aldrich, Roche Diagnostics), 2.5 µL of 100 mM of each deoxynucleotide (New England Biolabs®, Inc), 0.5 µL forward primer (10 µM), 0.5 µL reverse primer (10 µM), 0.5 µL 25 mM MgCl2 (Sigma-Aldrich, Roche Diagnostics), 0.2 µL of 5 U/µL FastStart™ Taq DNA Polymerase (Sigma-Aldrich, Roche Diagnostics), and 1 µL template DNA. PCR products were prepared for sequencing using 2 µL ExoSAP-IT™ PCR clean-up reagent [1 U/µL FastAP™ Thermosensitive Alkaline Phosphatase (Thermo Fisher Scientific)], 20 U/µL Exonuclease I (Thermo Fisher Scientific)) and 5 µL PCR product. Reactions were incubated at 37 °C for 15 min, followed by 85 °C for 15 min, and stored at 4 °C until used.

Table 2 PCR reactions and primer details for loci
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Bidirectional sequencing was conducted in 96-well plates with each reaction having a total volume of 13 µL [7.4 µL Milli-Q® water, 2.1 µL 5 × BigDye™ Terminator v3.1 Sequencing buffer (Applied Biosystems, Foster City, CA, USA), 0.5 µL BigDye™ Terminator v3.1 Cycle Sequencing Ready Reaction Mix (Applied Biosystems, Foster City, CA, USA), 1 µL forward or reverse primer, and 2 µL ExoSap product]. Initial denaturation at 94 °C for 5 min was conducted, followed by 40 cycles of denaturation at 96 °C for 30 s, annealing at 50 °C for 10 s, and elongation at 60 °C for 4 min. Reactions were precipitated for Sanger sequencing using sodium acetate and ethanol. Sanger sequencing was conducted on an ABI PRISM™ 3500xl Auto-sequencer (Applied Biosystems, Foster City, CA, USA) at the Sanger Sequencing Facility of the University of Pretoria (Bioinformatics and Computational Biology Unit, v 19.8.22).

Forward and reverse sequences were obtained from SeqServe v 19.8.22 (Bioinformatics and Computational Biology Unit), hosted by the DNA Sanger Sequencing Facility. Contigs were assembled and manually reviewed in Geneious Prime® v 2023.2.1 (Biomatters Ltd., Auckland, New Zealand). BLASTn analysis was performed to compare obtained sequences against the NCBI (National Centre for Biotechnology Information, USA) GenBank nucleotide database to obtain preliminary identifications. The sequences for Curvularia were used in subsequent phylogenetic analyses. The newly generated sequences were deposited in GenBank with accession numbers listed in Table 1.

For phylogenetic analyses, a reference sequence database (Table 1) was compiled based on recent literature (Crous et al. 2020; Ferdinandez et al. 2021; Iturrieta-González et al. 2020; Kiss et al. 2020; Manamgoda et al. 2012; Marin-Felix et al. 2020). Exserohilum turcicum (CBS 690.71 T) and Bipolaris zeae (BRIP 11512T) were included as outgroups. Sequences were aligned in Geneious Prime® 2023.2.1 using the MAFFT v 7.450 plugin, selecting the L-INS-i algorithm (Katoh and Standley 2013), and then manually trimmed where appropriate. The datasets were partitioned to take into account gene regions, as well as introns and exons. For multi-gene phylogenies, alignments were concatenated in Geneious Prime. Maximum likelihood trees were calculated in IQtree v 2.1.3 (Nguyen et al. 2015), and support in nodes was calculated using a bootstrap analysis with 1000 replicates. Phylogenetic trees were visualised using TreeViewer v 2.2.0 and visually edited using Affinity Designer v 2.3.1 (Serif (Europe) Ltd, Nottingham, UK). The reference datasets, alignments, and tree files were uploaded to the University of Pretoria research data repository hosted on Figshare (https://doi.org/10.25403/UPresearchdata.25817800).

Morphology

Strains of novel species were inoculated onto 90 mm Petri dishes containing potato dextrose agar [PDA; 39% (w/v) BD Difco™ Potato Dextrose Agar], 2% malt extract agar [MEA; 20% (w/v) Malt Extract, 20% (w/v) Difco Agar], synthetic nutrient agar (SNA), oatmeal agar [OA; 30% (v/v) oatmeal extract, 20% (w/v) Difco Agar] and water agar [WA; 20% (w/v) Difco Agar] as described by Marin-Felix et al. (2020). One set of plates was incubated at 25 °C in complete darkness, and a second set for 7 d in a 12 h UV light and dark diurnal cycle (Marin-Felix et al. 2020). Colony diameters were measured in triplicate from colonies incubated in complete darkness. Colour names used in descriptions follow the colour charts in the Methuen Handbook of Colour (Kornerup and Wanscher 1967). Images of colonies were captured using a Sony Alpha a7 III camera equipped with a Sony FE 90 mm f/2.8 macro G OSS lens (Tokyo, Japan). Micromorphology was studied using a Zeiss AXIO Imager.A2 compound microscope equipped with an AxioCaM 512 color camera driven by Zen Blue v. 3.2 software (Carl Zeiss CMP GmbH, Göttingen, Germany). Microscopic specimens were prepared from colonies on WA using water as mounting fluid. At least 25 measurements of each character were made for representative strains of each species with NIS-Elements Basic Research software v 4.30.00 (Nikon Europe B.V.). The mean (x̄) and standard deviation values for each structural element were calculated and the ranges expressed as follow: (minimum value) general range (maximum value). Photo plates were assembled using Affinity Photo v 2.3.1 (Serif (Europe) Ltd, Nottingham, UK).

Results

Identifications and phylogenetic analyses

Fungal isolations resulted in 80 Curvularia strains from which 173 new DNA sequences were generated for ITS (n = 75), GAPDH (n = 70), and TEF1 (n = 28) (Table 1).

A total of 142 strains (62 reference and 80 from the current study) were included in the multi-locus sequence analyses. Alignments of the ITS, GAPDH, and TEF1 datasets were 618, 552 and 828 bp long, respectively. The GTR + I + G nucleotide substitution model was applied to ITS, GAPDH (including introns and codons 1, 2 and 3), and TEF1 (including codons 1, 2 and 3). Tree topologies for individual gene trees were not in conflict (Supplementary Figs. 1, 2 and 3), and thus, a concatenated phylogeny was used to display results (Fig. 1). The most suitable nucleotide substitution models for the concatenated phylogeny including GAPDH and TEF1 was GTR + I + G.

Fig. 1
figure 1figure 1

Phylogenetic tree based on a maximum-likelihood approach of the concatenated data from GAPDH and TEF1 loci from phylogenetically related Curvularia species. The tree was rooted to Exserohilum turcicum and Bipolaris zeae. The taxonomic novelties proposed in this study are represented in bold and highlighted, and additional strains included in this study are shown in bold. Bootstrap values above 80% are shown on the branch nodes.T Type

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Our strains were identified to 13 species, including: C. bannonii [MB#135463] (n = 2), C. mebaldsii [MB#825457] (n = 1), C. moringae [MB#837854] (n = 17), C. papendorfii [MB#329447] (n = 1), C. prasadii [MB#296253] (n = 5), C. pseudolunata [MB#806056] (n = 1), C. rouhanii [MB#823474] (n = 7), C. tribuli [MB#830062] (n = 8), and five that were phylogenetically distinct from other Curvularia species. These are described below in the Taxonomy section (Fig. 1).

Even though ITS performed relatively well as an identification marker for some clades, it was less informative in others and did not allow robust identifications (e.g. clade containing C. buchloes, C. manamgodae, C. rouhanii and C. spicifera). GAPDH and TEF1 were useful in distinguishing species, but since GAPDH had a larger dataset available, it was more useful as an identification marker. The TEF1 locus was less informative for some clades and did not allow for robust identification (e.g. clade containing C. annellidiconidiophora, C. austriaca [MB#830045], C. coatesiae [MB#825452], C. determinata, C. desertus, C. eleusinicola, C. flexuosa, C. graminicola, C. guangxiensis, C. homomorpha, C. maraisii, C. microspora [MB#822544], C. namibensis, C. pallescens [MB#273299], C. pandanicola, C. pseudointermedia, C. saccharicola, C. sacchari-officinarum, C. vidyodayana, and C. warraberensis). However, our analyses showed that some species share very similar sequences making their identification difficult. For example, C. bannonii (BRIP 16732 T) and C. guangxiensis (CGMCC 3.19330 T) which have 2 bp differences in ITS, a single bp difference in GAPDH, and 4 bp difference in TEF1. Similarly, distinguishing between C. pseudolunata (UTHSC 09-2092 T) and C. chlamydospora [MB#806053] (UTHSC 07-2794 T) is difficult, as is differentiating C. aeria (CBS 294.61 T) and C. homomorpha (CBS 156.60 T) using ITS and GAPDH. Further work is needed to establish the inter- and intra-species relationships between these strains/species.

Taxonomy

Curvularia deserticola van Vuuren, M.J. Wingf., Yilmaz & Visagie, sp. nov. Figure 2

Fig. 2
figure 2

Curvularia deserticola; a CMW 64023 Colony after incubation for 7 d on, from left to right, PDA in complete darkness, PDA exposed to a 12-h UV light diurnal cycle, MEA exposed to a 12-h UV light diurnal cycle, and OA exposed to a 12-h UV light diurnal cycle; b CMW 64023 Colony texture on PDA; c CMW 58190, d, h CMW 64023, f, g CMW 64025 Conidiophores and conidia; e CMW 64023 Chlamydospore; i CN025B5, CMW 64025 Conidia. Scale bars 10 μm

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MycoBank MB#853988

Etymology. Latin, deserticola, refers to the arid desert environment in which this species occurs.

Typus. NAMIBIA, Far East region of the Namib desert, from Stipagrostis ciliata tissues from fairy circles, November 2019, coll. Neriman Yilmaz (holotype PRU(M) 4583, dried specimen in metabolically inactive state, ex-type strain CMW-IA 6932 = CMW 58190 = CBS 151410 = CN025G1).

Aditional material examined. NAMIBIA, Erongo region, from Stipagrotis ciliata tissues and surrounding rhizosphere, CMW-IA 6933 = CMW 64024 = CBS 151411 = CN027A5, CMW-IA 6929 = CMW 64022 = CN022I3, CMW 64023 = CN025B7, CN025B5, CMW-IA 6946 = CMW 64029 = CN037D9, CMW-IA 6937 = CMW 64025 = CBS 151412 = CN034A5.

DNA barcodes. ITS = ON074985, GAPDH = ON355399 & TEF1 = ON355360.

Conidiophores on WA. Hyphae subhyaline to pale brown, branched, septate, smooth, (1)3 − 6(7) μm wide. Conidiophores single or in small groups, semi-macronematous, septate, straight to flexuous, geniculate, branched, cell walls thicker than those of vegetative hyphae, mononematous, pale brown to brown, not swollen at the base, (24)90 − 115(180) × (3)4–5(7) μm. Conidiogenous cells smooth-walled, terminal or intercalary, proliferating sympodially, pale brown to brown with dark scars, size (4)4–6(13) × (3)4–6(19) μm. Conidia smooth-walled, ellipsoidal, straight, rarely curved, brown to dark brown, rounded at the apex, 3-distoseptate, sometimes 1 to 2-distoseptate, (11)15–19(24) × (6)9–12(15) μm, (x̄ = 17.09 ± 2.53 × 10.03 ± 1.51 μm); hila not protuberant to slightly protuberant to flat, darkened and thickened, 1–4 μm wide. Chlamydospores borne intercalary, cylindrical, distoseptate, smooth, 6–7 × 10 μm.

Culture characteristics on PDA (25 ºC, 7 d). Cultures incubated in the dark: Colonies 84–85 mm, olive to silver, margins white, few aerial mycelia giving the colony a slightly cottony appearance, reverse olive brown to greenish grey at margin. Cultures incubated in a 12-h diurnal UV light cycle: Colonies 84–85 mm, olive, margin subhyaline to olive, few aerial mycelium, reverse olive brown.

Notes. Curvularia deserticola resides in a clade containing C. bannonii, C. clavata, C. eleucinicola, C. elliptiformis, C. eragrostidis, C. guangxiensis and C. sacchari-officinarum (Fig. 1). Curvularia deserticola has smaller conidia ((11)15–19(24) × (6)9–12(15) μm) than the closely related C. banonnii (24–34 × 13–17 μm) (Morgan-Jones 1988). Curvularia deserticola has 3 septa and non-protuberant hila as found in the closely related clade of species mentioned above. Similar to various species of Curvularia, C. deserticola occurs on a member of the Poaceae (Ferdinandez et al. 2021; Jain 1962; Raza et al. 2019). Examples include C. clavata on Tripogon jacquemontii, C. eleusinicola on Eleusine coracana, C. elliptiformis and C. sacchari-officinarum on Saccharum officinarum, and C. eragrostidis Eragrostis chapelieri (Ferdinandez et al. 2021; Jain 1962; Raza et al. 2019). Curvularia deserticola differs in at least 1 bp for ITS, 6 bp for GAPDH, and 1 bp for TEF1 from other Curvularia species.


Curvularia gobabebensis van Vuuren, M.J. Wingf., Yilmaz & Visagie, sp. nov. Fig. 3.

Fig. 3
figure 3

Curvularia gobabebensis; a CMW 58192 Colony after incubation for 7 d on, from left to right, PDA in complete darkness, PDA exposed to a 12-h UV light diurnal cycle, MEA exposed to a 12-h UV light diurnal cycle, and OA exposed to a 12-h UV light diurnal cycle; b CMW 58192 Colony texture on PDA; c–e CMW 58192, f CMW 58191 Conidiophores and conidia; g CMW 58192 Conidia. Scale bars 10 μm

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MycoBank MB#853989

Etymology. Latin, gobabebensis, named for the Gobabeb research and training Centre in Namibia, in recognition of its contribution to research in the Namib desert.

Typus. NAMIBIA, Mirabib, from the roots of Stipagrostis ciliata in an area where no fairy circles occurred (-23.479230, 15.335000), November 2019, coll. Neriman Yilmaz (holotype PRU(M) 4565, dried specimen in metabolically inactive state, ex-type strain CMW-IA 6925 = CMW 58192 = CBS 149140 = CN013C4).

Aditional material examined. NAMIBIA, Erongo region, from Stipagrotis ciliata tissues and surrounding rhizosphere, CMW-IA 6921 = CMW 58191 = CBS 149139 = CN010F9, CMW-IA 6926 = CMW 58193 = CBS 149141 = CN013F6.

DNA barcodes. ITS = ON074797, GAPDH = ON355381 & TEF1 = ON355347.

Conidiophores on WA. Hyphae subhyaline to pale brown, branched, septate, smooth, (3)4 − 5(8) μm wide. Conidiophores single or in small groups, semi- to macronematous, septate, straight to flexuous, geniculate towards the upper part, branched, cell walls thicker than those of vegetative hyphae, mononematous, pale brown to brown, not swollen at the base, (28)40 − 108(316) × (3)4–6(9) μm. Conidiogenous cells smooth-walled, terminal or intercalary, proliferating sympodially, pale brown to brown with dark scars, (5)8–12(18) × (3)4–6(8) μm. Conidia smooth-walled, ellipsoidal, straight, rarely curved, brown to dark brown, rounded at the apex, 5 distoseptate, sometimes 1–6 distoseptate, (13)34–36(45) × (8)10–11(14) μm (x̄ = 32 ± 6.23 × 10.6 ± 0.95 μm); hila slightly protuberant, darkened and thickened, 1–3 μm wide. Chlamydospores not observed.

Culture characteristics on PDA (25 ºC, 7 d). Cultures incubated in the dark: Colonies 49–66 mm, greenish grey to dark green, margin greenish grey, aerial mycelium moderate giving the colony a slightly cottony appearance, reverse greenish grey to black. Cultures incubated in a 12-h diurnal UV light cycle: Colonies 55–70 mm, colony greenish grey to dark green, margin greenish grey, aerial mycelium moderate giving the colony a slightly cottony appearance, reverse greenish grey to black.

Notes. Curvularia gobabebensis is closely related to C. tribuli (Fig. 1). Curvularia gobabebensis differs from that species as it has mostly 5-distoseptate conidia (vs 1–4(6)-distoseptate) and longer conidia (34–36 vs 17–30 μm). Curvularia gobabebensis also grows more rapidly on PDA (49–66 vs 41–53 mm) and has greenish grey to dark green colonies compared to the olivaceous grey to olivaceous black colonies of C. tribuli (Marin-Felix et al. 2020). This new species has at least 1 bp difference for ITS, 9 bp for GAPDH, and 4 bp for TEF1 from other Curvularia species.


Curvularia maraisii van Vuuren, M.J. Wingf., Yilmaz & Visagie, sp. nov. Fig. 4.

Fig. 4
figure 4

Curvularia maraisii; a CMW 58195 Colony after incubation for 7 d on, from left to right, PDA in complete darkness, PDA exposed to a 12-h UV light diurnal cycle, MEA exposed to a 12-h UV light diurnal cycle, and OA exposed to a 12-h UV light diurnal cycle; b CMW 58194 Colony texture on PDA; c Chlamydospores CMW 58194; d–g Conidiophores and conidia CMW 58194; h CMW 58195 Conidia. Scale bars 10 μm

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MycoBank MB#853991

Etymology. Latin, maraisii, named for Dr Eugene Marais, an exceptional scientist and research manager based at the Gobabeb Reserch Centre.

Typus. NAMIBIA, Far East region of the Namib desert, from soil surrounding fairy circles, November 2019, coll. Neriman Yilmaz (holotype PRU(M) 4563, dried specimen in metabolically inactive state, ex-type strain CMW-IA 6951 = CMW 58195 = CBS 149143 = CN037F7).

Aditional material examined. NAMIBIA, Erongo region, from Stipagrotis ciliata tissues and surrounding rhizosphere, CMW-IA 6927 = CMW 58194 = CBS 149142 = CN021G3.

DNA barcodes. ITS = OR471647, GAPDH = ON355385 & TEF1 = OR486044.

Conidiophores on WA. Hyphae hyaline to pale brown, branched, septate, smooth, (2)3–5(7) μm. Conidiophores single or in small groups, macronematous, septate, straight to flexuous, geniculate towards the apex, sometimes branched, cell walls thicker than those of vegetative hyphae, mononematous, pale brown to brown, tapers towards the base, apex often darker than base, (15)71–88(254) × (3)4–5(7) μm. Conidiogenous cells smooth-walled, terminal or intercalary, proliferating sympodially, pale brown to brown with dark scars (3)7–9(16) × (4)5–6(18) μm. Conidia ellipsoidal to curved, sometimes atypical and bifurcate (forking at the apex), the third cell from the base is often swollen unequally, asymmetrical, pale brown to dark brown, base and apex often paler, rounded at the apex, 3-distoseptate, sometimes 2- to 4-distoseptate, (12)24–29(33) × (6)10–12(16) μm (x̄ = 26.99 ± 5.90 × 11.82 ± 2.51 μm); hila slightly protuberant, darkened and thickened, (2)3–4 μm. Chlamydospores borne intercalary in chains, spherical to ovoid, rough, (9)10–13(16) × (8)9–12(15) μm.

Culture characteristics on PDA (25 ºC, 7 d). Cultures incubated in the dark: Colonies 63–70 mm, dark green, margin hyaline and fimbriate, sporulation abundant, reverse greenish grey to black. Cultures incubated in a 12-h diurnal UV light cycle: Colonies 54–71 mm, colour dark green, margin hyaline and fimbriate, sporulation abundant, reverse greenish grey to black.

Notes. Curvularia maraisii strains resolve in a clade that has a poorly supported backbone, with C. austriaca, C. borreriae [MB#283049], C. coatesiae, C. gladioli [MB#125511], C. gudauskasii [MB#312389], C. harveyi [MB#329444], C. indica [MB#296247], C. microspora, C. pallescens, C. richardiae [MB#312391], C. tanzanica [MB#838305], and C. trifolii [MB#280637]. C. maraisii differs from C. austriaca in its slower growth on PDA (54–71 vs 75–90 mm) and in having dark green colonies compared to those of C. austriaca that are luteous to orange (Marin-Felix et al. 2020). The new species differs from C. gudauskasii in having mostly 3-distoseptate vs 4-septate conidia, and its conidiophores have a tyically tapered base and darker apex compared to those of C. gudauskasii that has a bulbous base and a paler apex (Morgan-Jones and Karr 1976). Curvularia maraisii differs from C. harveyi by growing faster on PDA (63–70 vs 57 mm), and in having colonies that are dark green vs grey-black to brownish black (Shipton 1966). The new species differs from C. microspora in having larger conidia ((12)24–29(45) × (6)10–12(19) vs (4.5)8.2–(11.5) × (2)3.8–(6) μm) (Liang et al. 2018). Curvularia maraisii differs from C. richardiae by its darker conidiophore apices and tapering basal cells compared to the more swollen basal cell of C. richardiae. C. maraisii also has dark green colonies compared to the grey to dark greyish brown to almost black colonies of C. richardiae (Alcorn 1971). Curvularia maraisii differs from C. tanzanica based on its curved rather than straight conidia, and its faster growth on PDA (54–71 vs 40 mm). The colonies of C. maraisii are dark green compared to the dark brown to black colonies of C. tanzanica colonies (Crous et al. 2021). Curvularia maraisii has at least 2 bp differences for ITS, 5 bp for GAPDH, and 4 bp for TEF1 from other Curvularia species.


Curvularia namibensis van Vuuren, M.J. Wingf., Yilmaz & Visagie, sp. nov. Fig. 5.

Fig. 5
figure 5

Curvularia namibensis; a CMW 58197 Colony after incubation for 7 d on, from left to right, PDA in complete darkness, PDA exposed to a 12-h UV light diurnal cycle, MEA exposed to a 12-h UV light diurnal cycle, and OA exposed to a 12-h UV light diurnal cycle; b CMW 58197 Colony texture on PDA; c CMW 58211, e CMW 58205 Chlamydospores; d CMW 58211, f, g CMW 58197, h CMW 58211 Conidiophores and conidia; i CMW 58198 Conidia. Scale bars 10 μm

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MycoBank MB# 853992

Etymology. Latin, namibensis, name referring to the Namib desert.

Typus. NAMIBIA, Mirabib, from the roots of Stipagrostis species in an area with no fairy circles present, November 2019, coll. Neriman Yilmaz (holotype PRU(M) 4562, dried specimen in metabolically inactive state, ex-type strain CMW-IA 6973 = CMW 58196 = CBS 149144 = CN015H8).

Aditional material examined. NAMIBIA, Erongo region, from Stipagrotis ciliata tissues and surrounding rhizosphere, CMW-IA 6930 = CMW 58197 = CN023D3, CMW-IA 6931 = CMW 58198 = CBS 149145 = CN024D2, CMW-IA 6934 = CMW 58199 = CN027A9, CMW-IA 6935 = CMW 58200 = CN027C4, CN034A7, CMW 58202 = CN036F1, CMW-IA 6942 = CMW 58203 = CN036F6, CMW-IA 6943 = CMW 58204 = CN036G9, CMW-IA 6944 = CMW 58205 = CBS 149146 = CN036I5, CMW-IA 6945 = CMW 58206 = CN037D8, CMW-IA 6947 = CMW 58207 = CN037F2, CMW-IA 6948 = CMW 58208 = CN037F3, CMW-IA 6949 = CMW 58209 = CN037F5, CMW-IA 6950 = CMW 58210 = CN037F6, CMW-IA 6952 = CMW 58211 = CBS 149147 = CN044C8, CMW 58212 = CN060F9.

DNA barcodes. ITS = ON074819, GAPDH = ON355384 & TEF1 = ON355350.

Conidiophores on WA. Hyphae hyaline to pale brown, branched, septate, smooth walled, (1)4–5(8) μm. Conidiophores single or in small groups, macronematous, septate, straight to flexuous, geniculate at the upper part, sometimes branched, cell walls thicker than those of vegetative hyphae, mononematous, pale brown to brown, (13)51–88(284) × (2)4–5(7) μm. Conidiogenous cells smooth-walled, terminal or intercalary, proliferating sympodially, sometimes swollen, (5)7–10(25) × (3)4–5(9) μm. Conidia ellipsoidal to curved, the third cell from the base is often swollen unequally, asymmetrical, pale brown to dark brown, base and apex often paler, rounded at the apex, 3-distoseptate, sometimes 1-distoseptate, (12)20–24(29) × (8)10–12(17) μm (x̄ = 21.65 ± 3.23 × 11.18 ± 1.53 µm); hila flat, darkened and thickened, 2–3 μm. Chlamydospores borne intercalary, spherical to cylindrical, smooth, (3)6–12(24) × (3)8–11(28) μm.

Culture characteristics on PDA (25 ºC, 7 d). Cultures incubated in the dark: Colonies 57–84 mm, colour nickel green to dull green, moderate aerial mycelia giving the colony a cottony appearance in the center, margin fimbriate and hyaline to white, reverse greenish grey to black. Cultures incubated in a 12-h diurnal UV light cycle: Colonies 38–84 mm, olive green to ivy green, moderate aerial mycelia giving the colony a cottony appearance in the center, margin fimbriate and hyaline to brown reverse greenish grey, grey or black.

Notes. Curvularia namibensis strains resolve in a clade with C. caricae-papayae [MB#329436], C. chlamydospora, C. ovoidea [MB#296250], C. prasadii, C. pseudolunata and C. warraberensis [MB#825462] (Fig. 1). Curvularia namibensis differs from C. prasadii in having shorter conidiophores (51–88 vs 80–320 μm) (Mathur and Mathur 1959). Curvularia warraberensis has a more restricted growth than C. namibensis on PDA (6–7 vs 77 mm) and was described from Dactyloctenium aegyptium, a member of the Poaceae (Tan et al. 2018) Curvularia namibensis has at least 3 bp for GAPDH and 4 for TEF1 from other Curvularia species, but no bp differences for ITS.


Curvularia stipagrostidicola van Vuuren, M.J. Wingf., Yilmaz & Visagie, sp. nov. Fig. 6.

Fig. 6
figure 6

Curvularia stipagrostidicola; a CMW 58217 Colony after incubation for 7 d on, from left to right, PDA in complete darkness, PDA exposed to a 12-h UV light diurnal cycle, MEA exposed to a 12-h UV light diurnal cycle, and OA exposed to a 12-h UV light diurnal cycle; b CMW 58217 Colony texture on PDA; c–f CMW 58217, g CMW 58214 Conidiophores and conidia; h CMW 58217 Conidia. Scale bars 10 μm

Full size image

MycoBank MB#853993

Etymology. Latin, stipagrostidicola, name refers to Stipagrostis, the genus of grass from which the holotype was isolated.

Typus. NAMIBIA, Far East, Mirabib and Reverse region, from shoots of Stipagrostis species on the margin of a vegetation patch, November 2019, coll. Neriman Yilmaz (holotype PRU(M) 4602, dried specimen in metabolically inactive state, ex-type strain (CMW-IA 6968 = CMW 58218 = CBS 149150 = CN060H5).

Aditional material examined. NAMIBIA, Erongo region, from Stipagrotis ciliata tissues and surrounding rhizosphere, CMW-IA 6922 = CMW 58213 = CN011D7, CMW-IA 6923 = CMW 58219 = CN011D8, CMW-IA 6939 = CMW 64028 = CN034B7, CMW-IA 6940 = CMW 58214 = CBS 149148 = CN034B8, CMW-IA 6941 = CMW 58215 = CN034H8, CMW-IA 6953 = CMW 58216 = CN044D1, CMW-IA 6954 = CMW 58217 = CBS 149149 = CN060G3, CMW-IA 6955 = CMW 64030 = CN060G4.

DNA barcodes. ITS = ON332838, GAPDH = ON355415 & TEF1 = ON355368.

Conidiophores on WA. Hyphae hyaline to pale brown, branched, septate, smooth walled (3)5–7(8) μm. Conidiophores single or in small groups, semi-maronematous, septate, straight to flexous, geniculate towards upper part, sometimes branched, cell walls thicker than those of vegetative hyphae, mononematous, uniformly brown (30)62–97(226) × (4)5–7(9) μm. Conidiogenous cells smooth-walled, terminal or intercalary, proliferating sympodially, 4–11(31) × (4)6–8(13) μm. Conidia curved, uniformly pale brown to dark brown, 3-distoseptate, sometimes aseptate to 4-distoseptate (27)30–35(37) × (12)13–16(18) μm (x̄ = 31.92 ± 2.51 × 14.77 ± 1.11 µm); hila flat, thickened and darkened 2–4 μm. Chlamydospores not observed.

Culture characteristics on PDA (25 ºC, 7 d). Cultures incubated in the dark: Colonies 26–38 mm, colour greenish grey to olive, little to moderate aerial mycelia giving the colony a cottony appearance, margin hyaline to white and lobate, sulcation present reverse coal to black. Cultures incubated in a 12-h diurnal UV light cycle: Colonies 32–61 mm, greenish grey to olive, aerial mycelia sparse to moderate giving the colony a cottony appearance, margin hyaline to white and lobate reverse coal to black.

Notes. Curvularia stipagrostidicola is closely related to C. eragrostidicola [MB#827458] (Fig. 1). Compared to the new species, C. eragrostidicola has a more restricted growth on PDA (20 mm compared with 26–38 mm) after 7 d, while its conidia is paler towards the apex (Tan et al. 2018). Curvularia stipagrostidicola has at least 7 bp differences for ITS, 15 bp for GAPDH and 8 for TEF1 from other Curvularia species.

Discussion

Our study represents the most extensive effort to document Curvularia species from Africa and specifically from Namibia. A total of 80 Curvularia strains were isolated from S. ciliata and its associated rhizosphere soils. Strains were identified as 13 species using gene sequences from the ITS, GAPDH and/or TEF1. Notably five of those species including C. deserticola, C. gobabebensis, C. maraisii, C. namibensis and C. stipagrostidicola were found to be new taxa.

Curvularia species have previously been reported from the Namib Desert. Eicker et al. (1982) surveyed rhizosphere soils associated with fairy circles in the Giribes Plain and reported isolating Curvularia, but they did not identify the species. Crous et al. (2020) described Curvularia moringae from Moringa ovalifolia (Moringaceae) collected from Namibia. In addition, C. eragrostidis [MB#296246] and C. carica-papayae were identified from Stipagrostis sabulicola plant litter in the Namib Sand Sea using a culture-dependent and culture-independent approach (Wenndt et al. 2021).

Members of the genus Curvularia cannot be reliably distinguished from the genus Bipolaris based on morphological characteristics alone (Marin-Felix et al. 2017, 2020; Tan et al. 2018). This is due to the many overlapping morphological characters; therefore, phylogenetic inference based on DNA sequence data is essential (Manamgoda et al. 2014). An ITS sequence is useful to assign strains to one of the two genera, but it is generally poor in distinguishing closely related species. This is evident from our ITS phylogeny (Supplementary Fig. 1), with a few to no base pair differences between, for example, C. buchloes [MB#622507], C. manamgodae [MB#556662], C. rouhanii, and C. spicifera strains. ITS, GAPDH and TEF1 were recommended as useful identification and phylogenetic markers in Curvularia (Manamgoda et al. 2015). For our new species, GAPDH typically showed at least 3 bp differences, and TEF1 showed 1 bp difference from close relatives. However, some clades appear problematic. In our opinion, species concepts for the genus have been applied narrowly, with undersampling further complicating this problem. In our study we isolated several species previously known only from a single isolate. The DNA sequences generated for these strains are thus important to capture infraspecies variation. In future, additional gene regions, as well as genomes, should be incorporated to achieve more robust species delineation. Additionally, a taxonomic revision will be necessary to evaluate species boundaries once deeper sampling has been achieved across various geographic locations and substrates.

Curvularia moringae and C. namibensis were the most frequently isolated species in this study, each including 17 strains isolated from grass and rhizosphere samples in the Far East, Mirabib and Reverse locations, respectively. This is the first report of C. moringae occurring in grasses and soil; previously only recovered from Moringa ovalifolia in the Namib Desert (Crous et al. 2020). Eight strains of C. tribuli were isolated from samples collected from the Far East, Mirabib and Reverse locations. This species was described by Marin-Felix et al. (2020) from puncturevine (Tribulus terrestris) leaves. We also isolated seven strains of C. rouhanii from samples collected from all three sampling sites. It was described from leaves of both the American Evergreen (Syngonium vellozianum) and Eucalyptus trees (Mehrabi-Koushki et al. 2018). To the best of our knowledge, this is the first report of these species from a member of Poaceae.

Of the new species described in this study, C. gobabebensis was found only in the tissues of S. ciliata, while others were also isolated from rhizosphere soils. In addition, C. gobabebensis was found exclusively in the Mirabib region, and not associated with fairy circles but rather isolated from the rhizosphere from an area not having fairy circles. Curvularia maraisii was only found in the Far East region. While these strains showed some association with specific substrates and locations in this study, the biology of these Curvularia species and their substrate associations, and distribution would be better understood through more extensive sampling.

It is becoming increasingly clear that fungi are well-adapted to living in extreme environments like deserts, which have high UV radiation, radically fluctuating temperatures, low rainfall, and often highly saline and/or acidic soils, by adopting a variety of lifestyles (Coleine et al. 2022; Makhalanyane et al. 2015; Porras-Alfaro et al. 2008; Whitford and Wade 2002). To inhabit these harsh environments, microorganisms usually have resistance mechanisms (Porras-Alfaro et al. 2008; Selbmann et al. 2021). The production of melanin, that protects microorganisms from harmful UV radiation, is one of these (Eisenman and Casadevall 2012; Gessler et al. 2014; Newsham 2011). Fungi can also have vegetative survival structures such as chlamydospores and/or ascomata (sexual states) that are often resistant to extreme temperatures (Manamgoda et al. 2012). In this regard, Curvularia species have cell walls that are melanised and/or produce chlamydospores (Bengyella et al. 2019; Kiss et al. 2020).

Identification of thirteen Curvularia species, including five new species, contributes to an expanding knowledge regarding species in this genus, including their distribution. It also provides a substantially increased database of reference sequence data available for the genus. Furthermore, the results of this study contribute to a better understanding of the diversity of fungi in the Namib Desert.