Hymenoscyphus fraxineus vs. Hymenoscyphus albidus – A comparative light microscopic study on the causal agent of European ash dieback and related foliicolous, stroma-forming species

Hans-Otto Baral; Martin Bemmann

doi:10.1080/21501203.2014.963720

. 2014 Oct 14;5(4):228–290. doi: 10.1080/21501203.2014.963720

Hymenoscyphus fraxineus vs. Hymenoscyphus albidus – A comparative light microscopic study on the causal agent of European ash dieback and related foliicolous, stroma-forming species

Hans-Otto Baral ^a,^*, Martin Bemmann ^b

PMCID: PMC4270420 PMID: 25544935

Abstract

Five species of Hymenoscyphus that fruit on black stromatized parts of dead leaves of deciduous trees are presented, giving details on their morphological and ecological characteristics. Several of these species have previously been misplaced in rutstroemiaceous genera because of the presence of a substratal stroma. However, the heteropolar, scutuloid ascospores with an often hook-like lateral protrusion at the rounded apex and the ascus apical ring of the Hymenoscyphus-type represent two reliable morphological characteristics that, together with molecular data, provide clear evidence for their placement in the genus Hymenoscyphus (Helotiaceae). Among the species treated is Hymenoscyphus fraxineus (=Hymenoscyphus pseudoalbidus), the causal agent of the European ash dieback disease. Since 1992 this species started within Europe to replace the rather uncommon Hymenoscyphus albidus, which is likewise confined to leaves of Fraxinus. Hy. fraxineus has been recorded already since 1990 in Eastern Asia (Japan, Korea, northeast of China), where it had been initially misidentified as Lambertella albida (≡Hy. albidus). In these regions, it occurs as a harmless saprotroph on Fraxinus mandshurica and Fraxinus rhynchophylla, suggesting that those populations are native while the European ash dieback disease has a recent Eastern Asiatic origin. The distinctly higher genetic diversity found in Japanese Hy. fraxineus in contrast to European Hy. fraxineus supports this view. Genetic similarities between Japanese Hy. fraxineus and European Hy. albidus suggest that also Hy. albidus might be a descendant of Asian Hy. fraxineus, though having invaded Europe much earlier. However, consistent genetic deviation between European and Asian Hy. fraxineus at two nucleotide positions of the ITS region indicates that the European ash disease originates from a region different from the presently known areas in Eastern Asia.

Our results underline the importance of detailed morphological studies in combination with molecular work. Hy. fraxineus was described from Europe as a cryptic species that differed from Hy. albidus by molecular data alone. However, the Hy. albidus/Hy. fraxineus species complex represents one of many examples within the ascomycetes in which subtle microscopic differences between closely related species, in this case the presence or absence of croziers at the ascus base, are strictly correlated with molecular characteristics. Two species that closely resemble Hy. albidus and Hy. fraxineus form pseudosclerotia in Aesculus leaves and again differ from each other mainly in the ascus base: Hymenoscyphus aesculi on Aesculus hippocastanum from Europe lacks croziers, whereas Hymenoscyphus honshuanus from Japan on Aesculus turbinata possesses croziers. Other taxa treated here include Hymenoscyphus vacini, a European species growing on stromatized net veins of skeletonized leaves of Acer, and Hymenoscyphus torquatus, a Chinese species on unidentified herbaceous stems. An equivalent stroma-forming North American species on leaves of Fraxinus, Rutstroemia longipes (Rutstroemiaceae), is discussed and compared. A key to the Hymenoscyphus species that form a dark stroma on leaves of Acer, Aesculus, Fraxinus, and Picrasma is provided.

Nomenclatural novelties: Hymenoscyphus aesculi comb. nov. (for Helotium/Lanzia aesculi), Hy. honshuanus nom. nov. (for Lambertellinia scutuloides), Hy. torquatus comb. nov. (for Lambertella torquata)

Keywords: croziers, simple septa, homothallism, pseudosclerotium, Fraxinus, Aesculus, Acer, Helotiaceae, Rutstroemiaceae, invasive species, morphology, molecular markers

Introduction

Ash dieback is a serious disease in Europe causing death of European ash. The disease is caused by Hymenoscyphus fraxineus (T. Kowalski) Baral et al. (≡Chalara fraxinea T. Kowalski, =Hymenoscyphus pseudoalbidus Queloz et al.), which was recently introduced from Asia and has rapidly replaced the non-pathogenic European species Hymenoscyphus albidus. The present study outlines the introduction and cause of the disease, and compares closely related harmless species on ash and other hosts.

Hymenoscyphus albidus, a saprotroph on leaves of European ash

In late summer and early autumn, Hy. albidus (Roberge ex Gillet) W. Phillips forms white stipitate apothecia on fallen, previous year’s rachises (here used for the entire main axis including the basal petiole) and leaflet veins of the pinnate leaves of ash (Fraxinus excelsior, Oleaceae, Lamiales). During the past 150 years, the species has infrequently been recorded in temperate and montane Europe. It can be assumed that the ascospores infect living leaves, and the mycelium grows endophytically inside them, similar as it is known in Hy. fraxineus. Quite a long time after the leaves have fallen, and after the leaf blades have disappeared, the fungus forms a very thin black stromatic layer on the surface of the remaining rachises and veins, which is referred to as ‘pseudosclerotial plate’. This layer becomes slowly apparent during the winter months, and might serve as a protection of the underlying hyphae against ultraviolet light, or to avoid invasion into the pseudosclerotium by hyphae of other species. The terms pseudosclerotium and pseudosclerotial plate were adopted by Kowalski and Holdenrieder (2009) and Queloz et al. (2011) in the Hy. albidus complex, although they are somewhat ambiguously defined (see Gross & Holdenrieder 2013).

Due to the black pseudosclerotium, the fungus was placed by some authors in the genus Lanzia Sacc. or Lambertella Höhn. (Sclerotiniaceae, now Rutstroemiaceae). However, the type of ascus apical ring and the heteropolar (scutuloid) ascospores refer it to the genus Hymenoscyphus Gray (Baral & Krieglsteiner 1985, p. 121). This placement was meanwhile confirmed by molecular data (Queloz et al. 2011).

Appearance of a second species of Hymenoscyphus

Recent research on the current epidemic ash dieback in Europe revealed the asexual state Ch. fraxinea as the causal agent of ash disease (Kowalski 2006), which, by molecular comparison, proved to be connected to a sexual morph (Kowalski & Holdenrieder 2009), though at first misidentified as Hymenoscyphus albidus. A molecular study by Queloz et al. (2011) on a variety of teleomorph collections, mainly from Switzerland, revealed that in fact two different ascocarp-forming species fruit on the blackened rachises of ash, which could not be distinguished morphologically. One of them (Hy. albidus) proved to be rather harmless, whereas the other represented the serious pathogen. The latter was described as a new species, Hy. pseudoalbidus Queloz et al., based mainly on the deviating rDNA sequence. It formed slightly longer ascospores and larger fruit bodies, but was otherwise thought to concur completely with Hy. albidus and was, therefore, referred to as a ‘cryptic species’ (Queloz et al. 2011).

Apothecia of Hy. fraxineus, as the holomorph of Ch. fraxinea/Hy. pseudoalbidus is now called, have been collected in Europe at least since 2006 (two collections dating from 1978 and 1987 as listed in the specimens examined by Queloz et al. 2011 were based on confused DNA sequences and concern Hy. albidus, Queloz et al. 2012). The symptoms of leaf wilting were first observed in the northeastern part of Poland in 1992 (Kirisits et al. 2009), and the first isolate of its asexual state dates from December 2000 and represents the type culture of Ch. fraxinea (Kowalski 2006).

Between 2000 and 2010, the invasive fungus spread to the west and south of Central Europe, to southern parts of Northern Europe, and to Eastern and Southeastern Europe (Kirisits 2010), and during 2008–2013 it invaded Great Britain (Hendry 2013). In the last years, Hy. albidus could still be found in high-montane areas of Central Europe (see Queloz et al. 2011), Southern and Southeastern Europe (N. Matočec & I. Kušan personal communication), in Atlantic lowland regions of Western Europe and in boreal to continental regions of Northern and Eastern Europe, where Hy. fraxineus did so far only as an exception expand or now occurs sympatric. Hy. albidus appears to be extinct all over Central Europe north of the Alps.

Besides the here treated Hymenoscyphus honshuanus Baral, Hymenoscyphus aesculi (Velen.) Baral & E. Rubio, and Hymenoscyphus vacini (Velen.) Baral & E. Weber, a further species was recently described, Hy. albidoides (Zheng & Zhuang 2014). It was found fruiting in Eastern China on blackened veins of leaves of Picrasma quassioides (Simaroubaceae, Sapindales). In its morphological characters, this species can hardly be distinguished from Hy. fraxineus, except for the shape of the crystals in the stipe base, but its molecular data significantly deviate. Another species was recently detected in Japan growing on rachises of Fraxinus platypodia, which will be described in a separate paper as Hymenoscyphus linearis Hosoya, A. Gross & Baral. It concurs microscopically quite well with Hy. albidus but forms very narrow, linear stromata of considerable length. We have included both species in our dichotomous key.

Purpose of study

The observation by the first author of striking and unequivocal deviations in the ascus base within European populations collected in 1988–1991 (simple septate) and 2006–2010 (croziers) raised the question whether this character is correlated with the observed molecular markers and, as a consequence, Hy. albidus can be distinguished from Hy. fraxineus by morphological methods. In course of this study, a large number of fresh as well as herbarium specimens were examined in order to verify the presence or absence of croziers and to compare the result with the available molecular data. A detailed documentation of teleomorph morphology was made in order to clarify whether further distinguishing features exist. Japanese herbarium material of H fraxineus [originally identified as Lambertella albida (Roberge ex Gillet) Korf] was included in this study in order to find out whether differences exist to European Hy. fraxineus.

Materials and methods

Microscopy

Collections were examined preferably in the living state, but also from rehydrated herbarium material, using a Zeiss Standard 14 and a Zeiss Standard KF microscope equipped with achromatic and plan-apochromatic objectives. Tap water (H₂O) was used as a standard medium, and viability of cells was evaluated in that medium according to Baral (1992). The iodine reaction was tested with Lugol’s solution (IKI = ~1% I₂, 2% KI, in H₂O), without (rarely with) pre-treatment by potassium hydroxide (KOH 3–5%). Brilliant Cresyl Blue (CRB, ~1% in H₂O) added to a water mount was used for testing the presence of gel, also for vital staining of the refractive vacuoles (VBs). For testing the presence of croziers, fresh apothecia were sectioned free-hand, and sections transferred to the slide in a drop of H₂O; any pressure on the cover slip was avoided during preparation. In the case of herbarium specimens, hymenial fragments (mainly sections) were placed in a drop of H₂O, to which a small drop of KOH and often also one of aqueous Congo Red (CR) was added. Alternatively, CR_SDS (SDS = sodium dodecyl sulphate) was applied to a water mount. Dissolution of the oil drops (lipid bodies, LBs) by ethanol was conducted as follows: a dry apothecium was rehydrated and a fragment placed in 90% ethanol for ½ min; then the ethanol was removed and a drop of 3% KOH and one of 1% phloxine was added. Waterman blue-black ink was applied for a better visibility of ascospore sheaths. Photographic images (macro- and microphotos) were obtained using a Nikon Coolpix E4500 and a Nikon Coolpix 5000 (Nikon Corporation, Tokyo, Japan). All drawings are free-hand.

Abbreviations

* = living state, † = dead state, n.v. = non visus (specimen or documentation not seen by us), d.v. = documentum visus (photos/drawings/descriptions seen by us), sq. = sequence of ITS1-5.8S-ITS2 region, vid. = examined, ø = unpreserved, # = not tested for the ascus base. Values in curled parenthesis {} refer to the number of collections that were examined or, after the host plant and the associated taxa, the number of certain/uncertain records. Countries are abbreviated according to the ISO 3166 two-letter standard code.

Herbaria

Type and other herbarium material was studied from CUP (Cornell University, Ithaca, New York), HMIPC (Mycological Herbarium of the Department of Forest Pathology of Kraków), LUX (Musée national d’histoire naturelle Luxembourg), NMLU (Natur-Museum Luzern), PRM (National Museum, Praha), TNS (National Museum of Nature and Science, Tsukuba), and ZT (Eidgenössische Technische Hochschule Zürich), and two specimens were deposited in STU (Staatliches Museum für Naturkunde Stuttgart) and KR (Staatliches Museum für Naturkunde Karlsruhe). Mentioned herbaria from which material was not examined are the following: FH (Harvard University, Cambridge, Massachusetts), GENT (Ghent University, Belgium), K (Royal Botanic Gardens, Kew, London), KUS (Korea University), PC (Muséum National d’Histoire Naturelle, Paris). Additional collections are held in the private herbaria of A.G. = Alain Gardiennet (Véronnes, Côte d’Or), B.D. = Bernard Declercq (Stekene, Eastern Flanders), C.B. = Céline Besch (†, Mertert, Luxembourg, in LUX), C.Y. = Chris Yeates (Huddersfield, West Yorkshire), E.R.D. = Enrique Rubio (Salinas near Avilés, Asturias), G.K. = Gerhard Koller (Mattersburg Burgenland), G.M. = Guy Marson (Hesperange, Luxembourg), H.B. = H.-O. Baral, H.E. = Heinz Engel (Weidhausen near Coburg), H.H. = Hans Haas (†, Stuttgart, in STU), M.B. = Martin Bemmann, M.H. = Michel Hairaud (Poivendre de Marigny, Deux-Sèvres), M.T. = Marie-Thérèse Tholl (Doncols, Luxembourg), R.D. = René Dougoud (Fribourg, Switzerland), S.Å.H. = Sven-Åke Hanson, S.H. = Stip Helleman (Boxmeer, Noord-Brabant), W.D. = Wolfgang Dämon (St. Georgen, Salzburg), W.W. = Wulfard Winterhoff (Sandhausen near Heidelberg).

Taxonomy

Hymenoscyphus albidus (Roberge ex Gillet) W. Phillips, Man. Brit. Discomyc. (London): 138 (1887) (as Hymenoscypha albida) – Figures 1 – 4

Figure 1. — *Hymenoscyphus albidus*: a. paraphysis containing vacuolar bodies (=VBs); b. freshly ejected ascospores, containing oil drops (=LBs); c. simple-septate ascogenous hyphae without crozier formation; d. ascus apex with euamyloid apical ring (*Hymenoscyphus*-type); e. apothecium. – living state (except for d).

Figure 4. — *Hymenoscyphus albidus*: a, d. median section of receptacle (ee = ectal excipulum, me = medullary excipulum); b. surface view on bright yellowish-ochraceous exudate covering the stipe base; c. paraphyses containing guttular VBs, immature and mature asci; e. median section of ectal excipulum at transition of stipe and receptacle; f–h. paraphyses (containing ± elongate VBs; f: under vital staining, g–h: VBs distorted in dead state); i–m. ascospores (containing large and small LBs; j–k: dead shrunken spores showing confluent LBs); n. ascus apices with euamyloid apical ring; p–u. ascus bases arising from simple septa. – Living state: a–d, f (in CRB), i, l–m; dead state: g (in H₂O), h (in IKI), n (in IKI), j–k, p–u (in KOH or KOH+CR). – a–b, d–j, l. H.B. 9699 (FR-PC, Granzay-Gript); c, m. 16.VI.2007 (ibid.); k. H.B. 9611 (BE-LUX, Florenville); n. LUX 047699 (LU, Kehlen); o. ZT 3295 (CH-TI, Aquarossa); p. 21.VII.2007 (CH-ZG, Unterägeri); q. H.B. 9454 (CH-LU, Aesch); r. H.B. 1950 (DE-BW, Stockach); s. ZT 3299 (CH-BE, Eymatt); t. ZT 3294 (CH-TI, Lavorgo); u. ZT 3293 (FR-BN, Bellême). – Phot. c, l–m: M. Hairaud.

Figure 2. — *Hymenoscyphus albidus*: a–j. apothecia emerging from black pseudosclerotia in rachises of *Fraxinus excelsior* (e–f: in median section). – a–i: fresh state, j: dry state. – a–f. H.B. 9699 (FR-PC, Granzay-Gript); g–h. 21.VII.2007 (CH-ZG, Unterägeri); i. 24.VI.2007 (FR-PC, Poitevin); j. H.B. 9454 (CH-LU, Aesch). – Phot. g–h: U. Graf, i: M. Hairaud.

≡ Peziza albida Roberge in Desm., Pl. Crypt. N. France, #2004 (1850), Annls. Sci. Nat., Bot., sér. 3, 16: 323 (1851), nom. illegit. [ICBN Art. 53.1, non Peziza albida With. 1792, nec P. albida Sowerby 1799, nec P. albida (All.) Lam. 1804]
≡ Helotium scutula var. albidum (Roberge in Desm.) P. Karst. Bidr. Känn. Finl. Nat. Folk 19: 112 (1871) [nom. nov., ICBN Art. 58, as He. scutula var. η albidum (Desm.) Karst.]
≡ Phialea albida Roberge ex Gillet, Champignons de France, Discomyc., livr. 4, p. 105 (1881) [1879] [nom. nov., ICBN Art. 58, as Phialea albida Desm.]
≡ Helotium albidum (Roberge ex Gillet) Pat., Tabl. analyt. Fung. France (Paris) 4: 173 (1885) nom. illegit. [ICBN Art. 53.1, non Helotium albidum P. & H. Crouan 1867, nec Helotium albidum (With.) P. Karst. 1884 (as ‘albellum’)]
≡ Helotium robergei Dennis, Mycol. Pap. 62: 93 (1956) [nom. nov., ICBN Art. 53.1]
≡ Lanzia albida (Roberge ex Gillet) S.E. Carp., Mem. N. Y. Bot. Gdn 33: 187 (1981) [as (Roberge & Desm.) S.E. Carp.]
≡ Lambertella albida (Roberge ex Gillet) Korf, Mycotaxon 14(1): 2 (1982)

Etymology

Albidus: named after the white colour of the apothecia, robergei: after M.R. Roberge.

Lectoype (selected by White (1944))

France, Calvados, [(?) NE of Caen, Bois de Lébisey,] rachises of F. excelsior, summer ca. 1850, M.R. Roberge (Desm., Pl. Crypt. N. France, Ed. 1, n° 2004, FH; not seen).

Apothecia

Moist (0.8–)1–2.5(–4) mm diam., receptacle 0.24–0.4 mm thick (at margin 0.15–0.2 mm), ±round, scattered; disc whitish to very pale cream, turning reddish-brown when bruised, slightly concave to flat; margin distinct, up to 70 µm protruding, finely pubescent, exterior concolorous, stipe 0.2–1.5 mm long, 0.25–0.5 mm wide at the base, 0.4–7(–1) mm below receptacle, white, pubescent, whole exterior finely striate, turning reddish-brown with age, at base light reddish-brown, with a black basal collar, ±erumpent from sclerenchyma-phloem layer; dry with cream-ochraceous to orange disc. Asci *(75–)80–110(–125) {5} × 9–11.5 µm {4}, †(67–)75–100(–116) {7} × 7.5–8.5 {2} or (8.5–)9–9.5(–10) {5} µm, 8-spored, spores *obliquely biseriate, pars sporifera *44–53 µm long; apex (†) conical, dome †2–3 → 1–1.3 µm thick, apical ring occupying lower 1/3–3/4 of dome, medium to strongly blue (bb) in IKI, ring apically gradually or abruptly fading, Hymenoscyphus-type {8}; base arising from simple septa {27}, never with a subbasal protuberance. Ascospores *(13.5–)14.5–18.5(–20.5) × (3.5–)4–4.8(–5) µm {9}, †(13–)14–19(–21.3) × (3–)3.5–4.2(–4.5) µm {6}, slightly to strongly scutuloid, with ± cylindrical middle part, apex rounded to obtuse, with a slight to strong lateral protrusion, base medium attenuated, with ± blunt, never acute end, straight to inequilateral or medium curved, especially at the apex, containing 1–3(–4) large LBs in each half [(1–)2–3(–3.5) µm diam.], associated with many small LBs 0.3–1 µm diam. {6}, lipid content 5, rarely a very delicate sheath was seen that slips off the spore, without setulae; wall surface CRB–; overmature spores not observed. Paraphyses apically uninflated, terminal cell *40–61 × 3–4 µm {1}, †40–62 × 2–2.7 µm {1}, lower cells *9–23(–27) × (2–)3–4 µm {1}, †12.5–22 × 2–3 µm {1}; branched only in lower part, anastomoses not seen. Medullary excipulum hyaline, of a loose textura intricata, individual cells *(20–)35–73(–80) × (3–)5–10(–16) µm {2}, delimited from ectal excipulum by an 50–100 µm thick layer of textura porrecta, cells *35–85 × 3–8 µm, all hyphae covered by thin gel staining deep lilac in CRB. Ectal excipulum hyaline, at flanks of (*) slightly († medium) gelatinized textura prismatica(–porrecta), 30–40 µm thick, cells *15–35(–50) × (6–)8–13(–17) µm {1}, †7–11(–14) µm wide {1}, oriented at a (0–)10–30(–40)° angle to the surface, walls †1–1.5(–2) µm thick, common walls 2–4 µm, CRB–; 20–25 µm thick at margin, oriented at 0°; in stipe of 25–50 µm thick, of (†) slightly gelatinized t. angularis(-globulosa) oriented irregularly or t. prismatica oriented at 0°, cells *7.5–15(–20) × 5–7 or 7–10(–17 µm) {1}; whole exterior of receptacle and stipe covered by a 10–15 µm thick layer of cortical hyphae forming a loose network in surface view, individual cells *11–20 × 3–5 µm, partly forming projecting or appressed hair-like protrusions *10–45 × 3–5 µm, 1–3-celled, straight to irregularly flexuous. Crystals absent in complete tissue, except for medullary tissue in lower region of stipe at a length of ~200 µm, here forming abundant druses 10–20 µm diam., made up of rhomboid, KOH-inert individual crystals 7–15 µm diam. VBs in paraphyses medium to strongly refractive {6}, hyaline, turning light yellowish-ochre in dead state, young multiguttulate then angular to shortly elongate, finally very elongate, occupying upper 30–50(–70) µm {3} (terminal and partly first lower cell), staining bright turquoise in CRB and light reddish-brown in IKI. Exudate pale yellowish-ochraceous, smooth to rough, covering cortical hyphae of receptacle and stipe, more bright-coloured towards base of stipe. Anamorph: unknown.

Habitat

various subassociations of Alnetum, Fagetum, Fraxinetum, Fraxino-Aceretum, Pruno-Fraxinetum, Salicetum, (Querco-)Ulmetum, on fallen previous year’s blackened rachises {34} or rarely primary veins {4}, exceptionally on a corticated, blackened, 1 mm thick twig {1}, of F. excelsior {36}, lying on ground. Assoc.: None. Desiccation tolerance: not tested. Geology: basalt, limestone, sandstone, Knollenmergel. Phenology: (VI–)VII–IX(–X). Altitude: 3–1690 m.

Specimens included

All on rachises of F. excelsior, two records on Acer possibly concern in fact Fraxinus; list incomplete concerning records not examined by us.

Sweden: Skåne, 7.5 km ENE of Helsingborg, 1.2 km SSE of Kropp, Ljungberga, 42 m, 24.VIII.1994, S.Å. Hanson (S.Å.H. 15290). – The Netherlands: Gelderland, 6 km SW of Nijmegen, W of Meijhorst, Staddijkpark, 8 m, 7.VII.2007, S. Helleman (S.H. 444, H.B. 9854). – Belgium: West Vlaanderen, 5 km ENE of Kortrijk, 1.8 km SE of Harelbeke, De Gavers, 14 m, 30.IX.1989, B. Declercq (B.D. 89/118, n.v.). – 17 km NNE of Brugge, ESE of Knokke-Zoute, Koningsbos, 8 m, 24.VIII.1994, B. Declercq (B.D. 94/108, as Pyrus communis, H.B. 9847). – Oost Vlaanderen, 24 km NE of Ghent, 3.2 km NW of Sinaai, Heirnisse, 5 m, 16.VII.1994, B. Declercq (B.D. 94/092, d.v.). – Luxembourg, 21 km SW of Arlon, 3 km NNE of Virton, Bois de Virton, 250 m, 13.VIII.1989, B. Declercq (B.D. 89/076, d.v.). – ~7 km SSE of Florenville, Orval, ~200 m, partly on veins, ~5.IX.1990, C. Besch 1091 (C.B. 1091, LUX 047702, H.B. 9611ø). – Luxembourg: Gutland, 7 km W of Echternach, Schnellert, ~200 m, partly on a twig, 14.VII.1985, C. Besch (C.B. 183, LUX 047701, H.B. 9612ø). – 3 km NE of Grevenmacher, 1 km WNW of Mertert, N of Schënnerkaul, S of Fausermillen, 155 m, on petiole of ‘Acer pseudoplatanus’, 24.IX.1990, H.O. Baral, C. Besch & C.M. Swart-Velthuyzen (ø). – 8 km NW of Luxembourg, 1.3 km E of Kehlen, Maasselter, 370 m, 17.VII.1988, C. Besch (C.B. 181, LUX 047699, H.B. 9619ø). – 3 km N of Grevenmacher, E of Manternach, Syre, ~200 m, 15.VIII.1985, C. Besch (C.B. 182, LUX 047700, H.B. 9620ø). – Germany: Rheinland-Pfalz, 9 km NE of Pirmasens, 3 km SE of Waldfischbach-Burgalben, near Ruine Heidelsburg, 280 m, partly on veins, ~3.IX.1989, B. Mauer (H.B. 3841, sq.). – Baden-Württemberg, 6 km NE of Tübingen, 0.5 km ENE of Pfrondorf, Tiefenbach, 400 m, partly on veins, 14.IX.1991, E. Weber & H.O. Baral (ø). – ibid., on primary veins of ‘?Acer’, 3.IX.1988, H.O. Baral (ø). – 12 km W of Stockach, N of Aach, 550 m, 20.VIII.1976, H.O. Baral (H.B. 1950, sq.). – ~4 km NNE of Emmendingen, Stilzerfritz, 350 m, 2.IX.1975, H.O. Baral (H.B. 410ø, #). – ~8 km N of Breisach, ~1 km NW of Burkheim, Rheinauen, 190 m, 6.IX.1975, H.O. Baral (H.B. 411ø, #). – Konstanz, 4 km NE of Radolfzell, 1 km SE of Möggingen, Mindelsee, 410 m, 30.VII.1975, H.O. Baral (STU, ex H.B. 376, H.H. 10409). – 1.6 km SE of Möggingen, Mindelsee, 410 m, 28.VIII.1976, H.O. Baral (ø, #). – 8 km NE of Radolfzell, 2.7 km SE of Bodman, Steckenloch, 440 m, 18.VIII.1976, H.O. Baral (ø, #). – 6.5 km NE of Radolfzell, 1.7 km SW of Bodman, Dettelbach, 500 m, 22.VIII.1976, H.O. Baral (ø, #). – ibid., 460 m, 17.VIII.1977, H.O. Baral (ø, #). – Bayern, 13 km ENE of Ulm, Unterfahlheim, Rühmerhalbinsel, 460 m, 15.VIII.1979, M. Enderle (ø, #). – Liechtenstein: 7 km NNW of Vaduz, 1.4 km SW of Bendern, Bannriet, 445 m, 5.VII.1997, E. Weber & J.P. Prongué (H.B. 5834, sq.). – Switzerland: Fribourg, 15.5 km NW of Bulle, 1 km SE of Villars-Bramard, Bois de Boulogne, 820 m, 14.VIII.2011, R. Dougoud (ZT, ex R.D. 32.28.094.11, n.v.). – 1.1 km E of Villars-Bramard, Bois de Boulogne, Le Mothey, 800 m, 14.VIII.2011, R. Dougoud (ZT, ex R.D. 32.29.094.11, d.v.). – Bern, 4.5 km WNW of Bern, 0.4 km ENE of Eymatt, Bremgartenwald, 508 m, 5.VIII.2009, B. Senn-Irlet (ZT 3300, sq.: GU586878). – 0.5 km E of Eymatt, 530 m, 5.VIII.2009, B. Senn-Irlet (ZT 3299, sq.: GU586877). – Luzern, 24 km NNW of Luzern, 1.2 km E of Aesch, Gitzitobel, 530 m, 4.IX.1978, F. Müller (ZT, H.B. 9454ø, ex NMLU 0409–78 FM 1, sq.: GU586910). – Zug, 3 km SSW of Unterägeri, 0.3 km E of Ochsenfeissi, Haslen, 830 m, 21.VII.2007, U. Graf (ZT 3891, ex NMLU 2107–07, sq.: GU586897). – Ticino, 0.8 km S of Quinto, 0.4 km WNW of Varenzo, 990 m, 26.VII.2009, V. Queloz (ZT 3296, sq.: GU586887, 586888, 586889). – 1.6 km N of Aquarossa, 0.7 km ENE of Castro, 597 m, 28.VII.2009, V. Queloz (ZT 3295, sq.: GU586882, 586883). – 0.3 km SSW of Lavorgo, 616 m, 27.VII.2009, V. Queloz (ZT 3294, sq.: GU586884, 586885, 586886). – France: Basse-Normandie, Calvados, (?) c. 4 km NE of Caen, (?) Bois de Lébisey, (?) 65 m, summer ca. 1850, M.R. Roberge (Desm., Pl. Crypt. N. France, Ed. 1, n° 2004, FH, lectotype; isolectotypes in GENT, K, PC, d.v., sq.: HM193466; Ed. 2, n° 1604, isolectotypes in GENT, FH, K, PC, d.v., sq.: HM193465). – Orne, 3 km NNW of Bellême, near La Herse, 190 m, 28.IX.1995, R. Dougoud (ZT 3293, ex R.D. 22.11.094.95, sq.: GU586876). – Poitous-Charentes, Deux Sèvres, 12.5 km WNW of Niort, 1 km ENE of Le Vanneau-Irleau, Marais Poitevin, 3 m, 24.VI.2007, M. Hairaud (ø, #, d.v.). – 10.5 km SSW of Niort, 1.8 km NW of Granzay-Gript, La Courance, 25 m, 16.VI.2007, M. Hairaud (M.H. 220607ø, #, d.v.). – ibid., 2.VII.2012, M. Hairaud (M.H. 10712, H.B. 9699). – ibid., 8.VII.2014 (ø, n.v.). – 2 km E of Chives, N of Le Vivier Jusseau, 85 m, 5.IX.2012, M. Hairaud (M.H. 050812, n.v.). – ibid., 10.VIII.2013 (ø, n.v.). – S of Bassac, 18 m, 5.VII.2012, M. Hairaud (ø, n.v.). ― Spain: Vasca, Gipuzkoa, 2.2 kmWNW of Aia, 2 km ESE of Urdaneta, Mindi erreka, 295 m, 25.VII.2014, J. Teres, vid. I. Olariaga (ARAN-Fungi 000082, d.v.). ― Atxondogoikoa (Zeanuri), 9 km WNW of Otxandio, 13 km SE of Orozko, 600 m, 18.VIII.2014, J.V. Fernandez (M.B. 10/2014).

Characterization

Morphologically, Hymenoscyphus albidus can be differentiated from Hy. fraxineus with certainty only by the absence of croziers at the ascus base. Further features include comparatively small apothecia (up to 4 mm diam.) growing on often rather small, insular pseudosclerotia, slightly smaller ascospores, and a rather late start of fructification, also the absence of an anamorph in pure culture.

Nomenclature

Korf (1982) and Hosoya et al. (1993) cited Desmazières’s exsiccatum ‘Plant. Crypt. N. France, #2004, 1850’ as basionym of Peziza albida Roberge in Desm., whereas other authors considered the first valid publication to be that of Desmazières (1851, p. 323). A description was only supplied in 1851, where Desmazières referred to the exsiccatum as ‘Rob. in herb.’. The label on the exsiccatum comprises notes on the substrate (on blackened areas of half-rotten rachises of F. excelsior) and phenology (summer), as well as a comparison with Peziza inflexa (=Cyathicula coronata), which is said to resemble in its pale colour and cyathiform shape. Since the Code does not require a minimum text for a diagnosis, Desmazières’ label from his widely distributed edition of exsiccata of Pl. Crypt. N. France meets the requirements for a valid publication of the new taxon. The date of 1850 is somewhat uncertain, however. According to Stafleu and Cowan (1976), the title pages of the fascicles of Pl. Crypt. N. France were preprinted in some cases before distribution and may give a too early date, thus the exact citation of Peziza albida remains unsettled.

Nevertheless, the name Peziza albida is illegitimate because it is a homonym of several older binomials. Therefore, the first legitimate name of the fungus at the species level is Phialea albida Roberge ex Gillet (1881) or Phialea albida Gillet, and this binomial is treated as a nomen novum attributed to Gillet. Likewise, He. albidum (Roberge ex Gillet) Pat. is illegitimate because of homonymy to older binomials. Dennis (1956) created the epithet robergei in order to avoid such homonymy within the genus Helotium Pers. The name He. robergei was in use until Helotium was abandoned by Dennis (1964) in favour of Hymenoscyphus, because of the older Helotium Tode (=Omphalina Quél.). Sharma (1976) and Svrček (1979) combined the epithet robergei in Hymenoscyphus, which is impermissible because here no older competing homonym with the epithet albidus exists.

Some workers have cited the authors of Hymenoscyphus albidus as ‘Roberge ex Desm.’, which would imply that Desmazières validated an invalid diagnosis of Roberge. On the label of Desmazières ‘Plantes Cryptogames Nord du France’ (1850) and in his ‘Notice sur les Plantes Cryptogames’ (1851), the authorship of the diagnosis and discussion to each species is actually difficult to know. We arrived at the conclusion that Desmazières considered himself as the author of a taxon only in those cases of his 1851 paper where ‘Desm.’ or ‘Nob.’ is mentioned after the fungal name. This is not the case in Peziza albida where he wrote ‘Rob. in herb.’. Also we believe that the author of the printed diagnoses of 1850 and 1851 is Desmazières, unless we would gain knowledge of a manuscript or correspondence proving different authorship. The remark ‘Desmaz.’ after the ecology in the 1851 paper is found in all those cases where the remark ‘Rob. in herb.’ or ‘Nob. in herb.’ appears after the fungal binomial, irrespective of whether or not an exsiccatum is cited. We interpret the remark ‘Desmaz.’ as an indication of the authorship of the diagnosis but not of the collection. The phrases ‘Rob.’ or ‘Rob. in herb.’ after the binomial on the printed label of 1850 and the article of 1851 indicate that Desmazières attributed the authorship of the taxon to Roberge, who was probably merely the collector, while Desmazières supplied the description, including the microscopical data.

Shortly after the demise of Roberge and Desmazières, authors were still quite consistent in their citation. Indeed, many of those in the nineteenth century followed Desmazières by citing the author of the taxon as ‘Rob.’, e.g., Roumeguère (1870, p. 130), Berkeley and Broome (1878, p. 29), Patouillard (1885, p. 173), Phillips (1887, p. 138), Rehm (1893, p. 797), and Massee and Crossland (1905, p. 285). Consistent with this citation is the variant ‘Rob. in Desm.’, which can earliest be found in Phillips (1887), and was adopted by Korf (1982) and Hosoya et al. (1993). In contrast, Roumeguère (1891, 1892) cited the authors as ‘R. et D.’ (Roberge & Desm.), hence he considered Desmazières as a coauthor. This version was followed by Boudier (1907, p. 111, 1908, p. 287), Schröter (1893, p. 74), Migula (1913, p. 1167), Dennis (1956), Arendholz (1979), and Carpenter (1981).

A very different view was adopted by Karsten (1871, p. 112), Cooke (1876, p. 132), and Gillet (1881, p. 105), who merely used the citation ‘Desm.’. The similar version ‘Rob. ex Desm.’, used by White (1944) and Cannon et al. (1985), considers Desmazières as a validating author.

We are inclined to adopt here the version ‘Roberge in Desm.’, because Desmazières attributed the authorship to Roberge. In any case, the original binomial was invalid due to homonymy; therefore, the correct author citation for combinations other than Peziza albida is Roberge ex Gillet or merely Gillet.

History of type studies

In or shortly before 1850 a rich collection was made, on which the description of Hy. albidus in Desmazières (1851, p. 323) is based. The sample was divided already in 1850 in a number of duplicates, and disposed as two exsiccata series in Pl. Crypt. N. France (Ed. 1, n° 2004; Ed. 2, n° 1604). For each of the two exsiccata, one duplicate was deposited in the herbarium of CN (Caen, Basse-Normandie), one came in Desmazières’ herbarium which is preserved at PC, and others are now to be found in herbaria such as FH, GENT, and K.

Desmazières (1851) mentioned the two exsiccata in his description, but without specifying which is the type. Apart from this rich collection, a herbarium specimen exists in K that bears Roberge’s handwriting and is apparently not part of the two distributed exsiccata series, but a different, later collection by Roberge, as it bears the note ‘Peziza albida Roberg. (Desm. 2004)’.

Nylander (1868, p. 40) examined two duplicates of Hy. albidus, possibly those from PC, since he was working at the Muséum national d’histoire naturelle, Paris in 1850–1858, and finally moved to Paris in 1863. He mentioned this only in a note under Peziza albidula (Hedw.) P. Karst. [as ‘(Hdw.) Whlnb.’, ≡ Phialea cyathoidea var. albidula (Hedw.) Rehm], in which he gave data on ascus and spore dimensions different from those of Desmazières. Karsten (1871, p. 112) made a description, apparently from type material (presumably in H), because he stated that the species was not yet collected in Finland.

Cooke (1876, p. 132, pl. LXV fig. 297) presented an uncommented microscopic drawing based on Desmazières n° 2004. Massee (1895, p. 260) supplied a description of an authentic specimen of Roberge, and also examined Desmazières n° 2004 which he considered conspecific (both in K). Like Nylander and Cooke, also Massee did not select a type. In the nineteenth century, it was not common usage to specify a single specimen as type when erecting a new taxon.

The American mycologist White (1944, p. 608, fig. 19–24) appears to be the first in the twentieth century who re-examined the type collection of Hy. albidus. He considered the two syntypes in FH as conspecific, and gave a thorough and precise redescription. His detailed illustration was based on n° 2004, and in the legend he wrote ‘all from type material, Desm. Pl. Crypt. Fr., Ed. I, 2004 (FH)’. We believe that this statement can be taken as a lectotypification (Art. 7.11, 9.21 ICBN).

Figure 19. — *Hymenoscyphus aesculi*: a–b. petiole in median section, showing black stroma and erumpent apothecial growth; pseudosclerotial plate occurring between cortical parenchyma and sclerenchyma but also between sclerenchyma and vascular bundle (phloem/xylem). – a–b. H.B. 9701 (DE-HS, Darmstadt).

Figure 24. — Known records of *Hymenoscyphus fraxineus* in Eastern Asia.

When Arendholz (1979, p. 79) studied three duplicates of the two exsiccata (n° 2004 in K and PC, n° 1604 in K), he stated that n° 2004 is the ‘type’, apparently following White’s lectotypification. Also Carpenter (1981, p. 187) examined the two exsiccata in PC (one erroneously citing as n° 2005). By overlooking White’s lectotypification and unaware of Arendholz’ study, he designated n° 1604 (PC) as lectotype.

When erecting the new species Hy. pseudoalbidus, Queloz et al. (2011, p. 141) examined those two duplicates of Hy. albidus deposited in GENT. Unaware of the previous designations, the authors designated n° 2004 in GENT as lectotype.

Of course, the two latter designations must be rejected. According to Laundon (1979), the personal herbarium of J.B.H.J. Desmazières (1786–1862) is deposited in PC. Although the Code recommends to select material as lectotype that was in the personal herbarium of the author (Recommendation 9A.4., ICBN), White’s selection of a duplicate in FH must be followed.

Desmazières’ (1850) two exsiccata appear to be part of a single collection. This can be concluded from the numbers in each of the two editions, which refer over a large range consecutively to the same taxa. Desmazières (1850, 1851) did not indicate date and location of the collection (except for ‘summer’). However, the handwritten label of Roberge’s specimen preserved at K bears the note ‘Caen’ [dépt. Calvados, France], the city where Roberge lived. Although no locality is mentioned for the type specimen, it seems probable that Roberge collected also this in the vicinity of Caen.

Husson et al. (2011) surprisingly reported more detailed specimen data: ‘collected by Roberge in 1850 from the Bois de Lébisey, close to Caen’. However, these data are actually an assumption which J.P. Rioult (director of the Départerment de Botanique, Mycologie et Biotechnologies, University of Caen) conveyed to the authors (C. Husson, personal communication) and also to us, reasoned from his personal experience about the most frequent origin of Roberge’s collections. Indeed, Roberge mostly collected in the parc de Lébisey, where he discovered his most important cryptogams (Morière 1866).

Misidentifications

Trustworthy records of Hy. albidus are actually not many. Quite a few were reported on leaves from hosts other than Fraxinus, such as Acer (Trail 1887, p. 173 [but inn Trail 1889, p. 63 changed to Fraxinus], FRDBI [Graddon, Spooner]), Aesculus (FRDBI [Crossland, Clark, Leedal]; Thind & Singh 1969; Thind et al. 1983; Sharma 1991), Betula (FRDBI [J.P. Blunt]), Corylus (FRDBI [Manvell, Shorten]), Eucalyptus (Beltrán Tejera et al. 2008), Filipendula (FRDBI [anon.]), Platanus (Feltgen 1903), Quercus (Massee & Crossland 1905, FRDBI [Hawley, Hunt]), Robinia (Roumeguère 1892), Rhododendron (FRDBI [Weir]), and Salix (Allescher 1898). These records probably belong to different species of Hymenoscyphus, or concern misidentified hosts.

Misidentified hosts reported for Hymenoscyphus Albidus

Declercq (personal communication) reported collections of Hy. albidus from Belgium on Juglans (unpreserved) and Pyrus (B.D. 94/108). However, re-examination of the latter collection revealed the host to be Fraxinus. Two unpreserved records included in our description of Hy. albidus were thought by the first author to be on Acer. Both showed asci without croziers and unquestionably concern Hy. albidus: in that from Luxembourg (24.IX.1990, ‘on blackened petiole of Acer pseudoplatanus’) druses of crystals were noted in the stipe, and in that from Tübingen (3.IX.1988, ‘on blackened primary veins of skeletonized leaf of ?Acer’) the spores contained large LBs. These notes on the presence of crystals or large LBs exclude Hy. vacini. Based on the general observation of a strict occurrence of Hy. albidus on Faxinus, we assume that the host was misidentified here.

Records on Fraxinus leaves misidentified as Hymenoscyphus albidus

Among the records on Fraxinus, possibly all those can be excluded in which the apothecia grew verifiably on unblackened parts of the rachises. Hy. albidus was actually confused in the years before ca. 1950 with Cyathicula fraxinophila (Svrček) Baral. According to Arendholz (1979, p. 80), almost all German records found in herbaria differ from British specimens in a textura oblita and represent at least partly a Cyathicula. Cy. fraxinophila resembles Hy. albidus in external view and is also confined to rachises (including petioles) of Fraxinus. Based on personal observations of the first author, this species deviates in a strongly gelatinized ectal excipulum (textura oblita) covered by rhomboid crystals, much narrower asci and spores, the former arising from croziers and with a more conical apex with an amyloid apical ring that resembles the Calycina-type, the latter homopolar and with a low lipid content that consists of minute LBs [spore size *(10–)14–18(–20) × (2.2–)2.5–2.8(–3) µm], also in multiguttulate paraphyses (VBs smaller and always globose), and particularly in the absence of a black stromatic tissue on the rachis and stipe base. The whitish apothecia also deviate in having a concave disc (0.6–1.2 mm diam.) with a finely crenulate margin.

Indeed, Rehm’s (1893, p. 797) description under the name He. albidum, with spores 15–18 × 3 µm with one oil drop at each end, undoubtedly refers to Cy. fraxinophila. Rehm mentioned the species from three localities: Bayern (Franken), Switzerland, and Südtirol; yet, he excluded the collection from Südtirol (leg. G. Bresadola) as deviating from the others, although he noticed here only the characteristic black stroma described by Desmazières. Rehm mentioned the Bresadola specimen also under Helotium virgultorum (p. 782), a lignicolous taxon which he described as growing on blackened wood surface, and he was sure that it belongs here. So only this specimen seems to represent true Hy. albidus.

Fautrey (in Roumeguère 1891, p. 124) described Phialea albida f. microspora Fautrey on ash rachises, with spores 10–12 × 2 µm, which again appears to concern Cy. fraxinophila. Also Schröter (1893, p. 74) reported under the name Hymenoscypha albida (Silesia, Oct.–Nov.) obviously Cy. fraxinophila (spores 13–18 × 2–3 µm, with homogeneous contents). Likewise Velenovský (1934, p. 205, pl. XX fig. 39) described and illustrated under the name He. albidum in fact Cy. fraxinophila (spores 12–18 × 2 µm, both ends obtuse and with granular content). Oddly enough, Velenovský confused Cy. fraxinophila and true Hy. albidus: only two of his four specimens preserved at PRM represent Cy. fraxinophila, according to their revision by Svrček (1985) and O. Koukol (personal communication). However, Svrček identified only one of these two (Mnichovice, XI.1933, PRM 147478) correctly (as Conchatium fraxinophilum), while he misidentified the other collection (Zvánovice, 24.X.1929, PRM 148804) as Hy. albidus, although also here the rachises were not blackened.

Without description, Le Gal (1938) reported He. albidum from Île-de-France (Villecresnes, Bois de la Grange et de l’Etoile) and stated the species to be very common in summer. It is possible that she was dealing in fact with Cy. fraxinophila. Grelet (1949, p. 35) mentioned this record, but his diagnosis is more or less a copy of Boudier’s, and it seems that Le Gal’s material has never been re-examined. Also Jaap’s (1910) report on Fraxinus rachises is without description; the stated association with Cy. coronata suggests that he was dealing with Cy. fraxinophila, a species which we found three times in association with Cy. coronata, whereas Hy. albidus grew always alone on the rachises. However, during survey in Czechia in 2013 Hy. fraxineus and Cy. fraxinophila were found sometimes on the same petiole (O. Koukol, personal communication). Also Gillet (1881) might have treated Cy. fraxinophila, because he did not mention a dark stroma.

This confusion includes also the leaf-inhabiting Hymenoscyphus caudatus (P. Karst.) Dennis. White (1944) stated that Hy. albidus is ‘known with certainty only from the material of Desmazières’, whereas he found all the many other European records under this name to be largely based upon specimens better referred to Hy. caudatus or He. scutula (Pers.) W. Phillips. This statement is in contradiction to Arendholz (1979), who mentioned only the genus Cyathicula as misidentification.

Records on leaves of Platanus and Aesculus misidentified as Hymenoscyphus Albidus

Feltgen (1903, p. 59) identified a collection from Luxembourg, on rotten leaves and petioles of Platanus orientalis, as Phialea albida. The small (0.2–0.5 mm diam.), yellowish, cyathiform apothecia and the narrow, obtusely fusoid spores (12–16 × 2–3.5 µm) might refer to a Cyathicula or Allophylaria. Apothecia could not be found by us in the preserved specimen (LUX 045915).Under the names He. robergei or Hymenoscyphus albidus, a taxon was reported on fallen petioles and leaf blades of Aesculus indica from the Northwestern Himalaya (India and Nepal), where it seems to be frequent (Thind & Singh 1969, p. 251, fig. 1; Thind et al. 1983, p. 278; Sharma 1976, p. 338, pl. 39 fig. 5–6, pl. 57 fig. 98, 1991, p. 123, pl. 4 fig. 5–6). Due to the similarity of Hy. albidus to both Hy. aesculi and Hy. honshuanus, which typically occur on Aesculus, this determination appears to be questionable and needs reinvestigation, particularly concerning the ascus base. The apothecia are described as cream to yellowish-brown when fresh, with a dark brown to black stipe base. Yet, the authors did not mention blackening of the substrate. Apart from the different substrate, Thind and Singh (1969, p. 253) noted that the Indian collections deviate from European Hy. albidus in narrower asci and ascospores, whereas Sharma (1976, p. 339) mentioned also ‘their more robust ascocarps’ and ‘longer ascospores’. However, the ascus and spore size given by Sharma [†(64–)75–95(–102) × 7–9.5(–11) µm, †15–22(–22.3) × 3–5 µm] fits all these species, and the fusoid-clavate spore shape would quite well match that of Hy. honshuanus. Those given by Thind and Singh are distinctly narrower (†85–100 × 7–7.5 µm, †16–20 × 3–3.5 µm) and the spores drawn rather cylindrical, which might better correspond to Hy. aesculi. However, Sharma’s remark that the apothecia become entirely dark brown to almost black on drying would fit the present observations on Hy. aesculi.

Figures 5–6. — *Hymenoscyphus fraxineus*. 5a. paraphysis containing VBs; 5b. freshly ejected ascospores containing LBs, 6c. do., from inside asci; 5d–e. ascus apices with euamyloid apical ring of the *Hymenoscyphus*-type; 5c, 6a–b. ascogenous hyphae with crozier formation (in 6a–b containing LBs). – Living state: 5a–c; dead state: 5d–e, 6a–c.

Hymenoscyphus fraxineus (T. Kowalski) Baral, Queloz & Hosoya, IMA Fungus 5(1): 80 (2014) – Figures 5–11

Figure 11. — *Hymenoscyphus fraxineus* (Japan). a–g, u. Apothecia emerging from black pseudosclerotia in rachises of *Fraxinus mandshurica* (u: fresh; a–g: rehydrated); i–k. pseudosclerotium in cross section (i–j: several stromata in one rachis, delimited by black demarcation lines that occur in the xylem along radial rays and through the mark parenchyma); l. median section of stipe base (with black basal surface and internal crystals); m. surface view on ectal excipulum near margin (with yellow exudate); n–t, v–w. ascogenous hyphae with croziers. – Dead state (h–k, m in H2O; l in KOH; n–t KOH+CR; v–w in phloxine B). – Nagano: a, e, k. TNS-F-40074 (Sugadaira); b, g. TNS-F-12761 (ibid.); f, h. TNS-F-17817 (ibid.); v. TNS-F-12503 (Yuzawa); Hokkaido: c, m, s–t. TNS-F-52060 (Tomakomai); d, l, n–r, u. TNS-F-40043 (ibid.); i, j, w. TNS-F-52061 (Sapporo). – Phot. u: YJ Zhao.

≡ Chalara fraxinea T. Kowalski, Forest Pathology 36, p. 265 (2006) [anamorph]
= Hymenoscyphus pseudoalbidus Queloz, Grünig, Berndt, T. Kowalski, T.N. Sieber & Holdenr., Forest Pathology 41, p. 140 (2010) [teleomorph]