Introduction

Species of Tubakia B. Sutton are common leaf endophytes and pathogens on Quercus (oak) and other Fagaceae (Harrington and McNew 2016; Harrington et al. 2012). The genus is characterized by unique pycnothyria, consisting of pigmented, radiating, setae-like cells (scutellum) on top of a columella, with small phialides on the underside of the scutellum producing ellipsoid, hyaline to brown conidia that are forced out from under the pycnothyrium for rain dispersal. The majority of the recognized species of Tubakia are found in eastern Asia (Kaneko and Kobayashi 1984; Kobayashi et al. 1979; Liu and Liu 2010; Yokoyama and Tubaki 1971; Yun and Rossman 2011). Traditionally, American species have been lumped into a single, broadly defined species, Tubakia dryina (Sacc.) Höhn. (Limber and Cash 1945), which has been reported most commonly from Europe (Boroń and Grad 2016; Harrington et al. 2012; Kowalski 2006). A comprehensive treatment of the genus is lacking, and the group is in need of phylogenetic analyses and taxonomic revision (Harrington and McNew 2016; Harrington et al. 2012).

Actinopelte was erected by Saccardo (1913) to accommodate fungi with fruiting bodies of radiating setae-like cells, though Saccardo (1913) mistook the large conidia of Actinopelte japonica Sacc. as asci. Höhnel (1925) transferred the European Leptothyrium dryinum Sacc. (Saccardo 1878) to Actinopelte and described Actinopelte americana Höhn. from material collected by Ellis and Everhart in New Jersey. The genus name Actinopelte was previously occupied by a lichen taxon, so Sutton (1973) erected Tubakia, using A. japonica as the type species. Besides Tubakia japonica and T. dryina, Yokohama and Tubaki (1971) reported three new species in Japan: Tubakia castanopsidis (Yokohama and Tubaki) B. Sutton, Tubakia rubra (Yokohama and Tubaki) B. Sutton and Tubakia subglobosa (Yokohama and Tubaki) B. Sutton. More recently, Tubakia seoraksanensis Yun and Rossman was described from oak leaves in Korea (Yun and Rossman 2011), Tubakia iowensis T.C. Harr. and McNew was described from oak leaves in USA (Harrington et al. 2012), and Tubakia chinensis U. Braun, S. Bien and Hantsch was described from Castanea leaves in China (Braun et al. 2014).

Numerous described species of leaf fungi from North America that could be placed in Tubakia, including A. americana (Höhnel 1925), have been considered synonyms of T. dryina (Gilman and Archer 1929; Glawe and Crane 1987; Limber and Cash 1945). Aside from host species in the Fagaceae, Tehon (1924) described eastern USA species with typical Tubakia-type pycnothyria as Actinothyrium gloeosporioides Tehon from leafspots on sassafras (Sassafras albidum) and Pirostoma nyssae Tehon from leafspots on black tupelo (Nyssa sylvatica). Tehon and Stout (1929) described Leptothyrella liquidambaris Tehon and Stout from leaf spots on sweetgum (Liquidambar styraciflua). These three species were considered probable synonyms of the oak leafspot pathogen A. dryina by Limber and Cash (1945), and Tehon (1948) later agreed. Studies of a new leaf blight disease on bur oak (Quercus macrocarpa) led to a new species, T. iowensis T.C. Harrin and McNew, which was distinguished from a more narrowly defined T. dryina by morphology and DNA sequence analyses (Harrington et al. 2012). The association of T. iowensis with a serious leaf disease on Q. macrocarpa and observations of other undescribed species causing leaf diseases of other Quercus spp. (Harrington and McNew 2016) prompted a more thorough evaluation of the genus Tubakia.

This study provides a re-evaluation of the genus Tubakia, especially of the species common to eastern USA. Cultures and specimens were studied morphologically and phylogenetically and compared to the other known species of Tubakia from Eurasia. Three new species are described from eastern USA.

Materials and methods

Specimens and isolates

Either green and non-symptomatic leaves or leaves with necrotic spots or veins were collected primarily in Iowa and surrounding states. Cooperators throughout the eastern USA and California provided other fresh samples for study. For isolations from green or necrotic plant tissues without conidiomata, excised pieces were surfaced-sterilized with undiluted household bleach (5.25% sodium hypochlorite)for 3 min with stirring, followed by 30 s in 95% ethanol, and rinsed in sterile water before plating on malt extract agar (MEA; 1.5% Difco malt extract and 2% agar). If present, conidiomata could be easily dislodged from spots on leaf blades or along necrotic veins by scraping with a dissecting needle, and isolations were made by placing several conidiomata with conidia in a drop of water and streaking the drop across the surface of streptomycin malt extract agar (SMA, MEA with 100 mg/L streptomycin sulphate added after autoclaving). Crustose pycnothyria that formed on leaves, petioles or twigs were moistened with sterile water until swollen, the crustose covering was cut open with a scalpel, and the conidial contents streaked on SMA. Subcultures of developing colonies were made after 1–3 days onto malt yeast extract agar (MYEA, 2.0% Difco malt extract, 0.2% Difco yeast extract and 2.0% agar).

Culture numbers beginning with “A” are those of the Iowa State University collection (maintained by the senior author), and representative cultures were deposited in the Centraalbureau voor Schimmelcultures (CBS, now Westerdijk Fungal Biodiversity Institute, Utrecht, The Netherlands). Aside from our collections, cultures were obtained from CBS, and these cultures were also assigned “A” numbers (Table 1).

Table 1 Collection numbers, substrate, location, and DNA sequences of representative cultures and specimens of Tubakia spp
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Specimen numbers beginning with RO and WO are those of the authors, and representative specimens were deposited in the Ada Hayden Herbarium (ISC) at Iowa State University. We examined herbarium specimens at ISC, the US National Fungus Collection (BPI), the Illinois Natural History Survey (ILLS), and the New York Botanical Garden (NY). Several of the exsiccati collections of Ellis and Everhart were examined, including a copy of Fungi Colombiani in the McClatchie Herbarium at NY.

Characterization of specimens and isolates

Isolates were grown on MYEA plates at room temperature (21 to 23 °C) or at various set temperatures (20, 25, 30, and 35 °C). For comparing growth rates, a plug of mycelium 4 mm in diam from the advancing margin of a 7-day-old colony was placed on a MYEA plate, and the radial growth rate (mm/d) was determined between 2 and 7 days. Three replicate plates were used for each isolate/temperature combination. For formal description, cultures were grown on MYEA at 25 °C in a dark incubator.

Material was mounted in 20% lactic acid or in lactophenol with cotton blue for microscopic examination. Digital photographs and measurements were taken on an Olympus compound microscope (Olympus BH-2, Nomarski DIC optics) fitted with a digital camera (DFC295) and software from Leica (Bannockburn, Illinois).

DNA sequencing

In most cases, DNA was extracted from specimens or cultures using PrepMan™ Ultra (Applied Biosystems, Foster City, California) following the manufacturer’s instructions. Isolates were grown on MYEA for 7–14 days at room temperature, and surface growth on the MYEA plates was used for DNA extraction. Attempts were also made to extract DNA from conidiomata on leaf surfaces of fresh field material or dried herbarium material.

The large subunit (LSU, 28S) rDNA region was amplified with primers LROR and LR5, and the PCR products were sequenced with primers LR0R and LR3 (published online: https://sites.duke.edu/vilgalyslab/rdna_primers_for_fungi/) as described in Harrington et al. (2010). Sequencing of portions of internal transcribed spacer region of rDNA (ITS) were attempted from DNA extracted from all cultures and from some conidiomata scraped from leaf surfaces. The primers ITS1-F and ITS4 (Gardes and Bruns 1993, White et al. 1990) and cycling conditions were as described earlier (Harrington et al. 2014). For conidiomata scraped from herbarium specimens, DNA was similarly extracted, but PCR and DNA sequencing utilized the fungal specific primer ITS1-F with the Tubakia-specific reverse primer TubITS4Rb (5′-ATC CGA GGT CAA CCA GTA AA). A portion of the translocation elongation factor 1-alpha (TEF-1α) was amplified with primers that we developed for Ceratocystis fimbriata, namely, EFCF1 (5′-AGT GCG GTG GTA TCG ACA AG) and EFCF6 (5′-CAT GTC ACG GAC GGC GAA AC) and sequenced with the same primers and EFCF2 (5′-TGC TCA CGG GTC TGG CCA T) and EFCF3 (5′-ATG GCC AGA CCC GTG AGC A), using the cycling conditions and recipe of Oliveira et al. (2015).

All sequencing was conducted at the Iowa State University DNA Sequencing and Synthesis Facility. Complementary sequences were compared with Sequence Navigator v 1.0.1 or Auto Assembler v 1.3.0 (Perkin Elmer Applied Biosystems, Foster City, California). Sequences of LSU-rDNA were generated for 33 Tubakia isolates, sequences of ITS-rDNA for nearly 500 isolates and specimens, and sequences of TEF-1α sequences for 56 Tubakia isolates. Sequences of isolates representing putative taxonomic units were used in phylogenetic analyses, and a select number of representative sequences were deposited in GenBank (Table 1). Generated sequences were compared to those of other fungi using BLAST searches (v. 2.2.24, National Centre for Biotechnology Information, National Institute of Health, Bethesda, MD), and the closest matches were downloaded and included in the datasets for phylogenetic analyses.

Phylogenetic analyses

The sequences in each of the three datasets were manually aligned, with little ambiguity among the ingroup taxa (Tubakia spp.), except that there was some ambiguity in the alignment of the ITS sequences due to short indels in ITS1. The LSU dataset had 41 sequences, including those of Tubakia spp., representative Diaporthales that had sequences most similar to those of the Tubakia, and Chaetomium brasiliense (Sordariales). Of the 527 aligned LSU characters, 424 were constant and 52 were parsimony informative. The ITS dataset had 59 sequences representing the diversity of ITS sequences of Tubakia spp., along with the sequences of Gnomonia setacea and Greeneria uvicola, whose sequences were most easily aligned with those of Tubakia. The ITS alignment had 581 characters, including the 5.8 S gene, with 346 characters constant and 128 characters that were parsimony informative. The TEF-1α dataset had 53 sequences that we generated, as well as sequences of C. globosum and Metarhizium guizhouense (Hypocreales) as outgroup taxa, which were selected because their sequences showed the greatest similarity to those of Tubakia in BLAST searches. The 1261 aligned TEF-1α characters included the entire second exon and most of the third exon, as well as the intervening intron of about 170 aligned bp, which was unambiguously aligned for the Tubakia sequences. Of the 1261 characters, 957 were constant and 133 were parsimony informative.

The aligned sequences were analysed for maximum parsimony (MP) using PAUP v.4.0b10 (Swofford 2002). Gaps were treated as a fifth base, all characters had equal weight, and the heuristic searches utilized simple stepwise addition and tree-bisection-reconnection. Bootstrap analyses also were conducted in PAUP with 1000 replications, but the maximum number of trees in each bootstrap replication in the ITS analysis was set to 100 per run. Posterior probability (Bayesian) estimates for branches were determined with Mr. Bayes 3.2.1 (Ronquist and Huelsenbeck 2003), in which gaps are treated as missing data. The settings included a general time-reversible (GTR) model and gamma distribution. There were 1,000,000 (ITS) or 2,000,000 (LSU and TEF-1α) generations, a diagnosis frequency of 5000, and a sample frequency of 1000. The first 25% of samples were discarded (burninfrac = 0.25), and Bayesian posterior probability estimates were calculated by majority rule consensus of the trees after burn-in.

Results

Morphological comparisons

The characteristic scutella of radiating, setae-like hyphae were found on symptomatic leaves of Quercus spp., Castanea dentata (chestnut), Lithocarpus densiflorus (tanoak), and Liquidambar styraciflua in eastern USA and California (Fig. 1). A range of leaf symptoms was seen (Fig. 1a, d, h), but there was only limited variation in the diameter of scutella and the size of conidia, which ranged from hyaline to brown, often with a finely punctate, thick outer wall (Fig. 1j). Naked sporodochia were also seen in some samples, especially on necrotic tissue along the midveins on the underside of Quercus leaves, and the conidia from these sporodochia were similar in size and coloration to those formed under the radiate scutella. A second type of pycnothyria with crustose coverings (Harrington et al. 2012) was occasionally found on living and dead leaves, petioles, twigs, or acorns of Quercus spp. (Fig. 1l). Conidia from crustose pycnothyria (Fig. 1g, m) were similar in appearance to those under radiate scutella, though the sizes of conidia from the two types of pycnothyria typically differed. Herbarium samples from Illinois (ILLS), Iowa State University (ISC) and the New York Botanical Garden (NY) also showed Tubakia spp. on leaves of oaks and numerous other hosts that were similar to those on fresh leaves. Some herbarium samples showed fruiting structures of two Tubakia species that differed by the size and shape of their conidia on different leaf spots of the same leaf (Fig. 2).

Fig. 1
figure 1

Leaf symptoms, conidiomata and spores of three new Tubakia species. a–c T. hallii. a Irregular leaf spots on Quercus stellata. b Pycnothyrium with conidia on leaf of Q. stellata, from the holotype. c Microconidia from a culture (CBS 129015 = A762) on media with sterile leaf pieces. d–g T. macnabbii. d Veinal necrosis (arrows) on leaf of Q. rubra. e, f Poorly developed pycnothyrium with conidia and microconidia and mature pycnothyrium with conidia, from the holotype. g Conidia from a squashed crustose-type pycnothyrium (from ISC 453295, strain A1258). h–m T. tiffanyae. h Necrotic flecks, small circular leafspots, and larger irregular leaf spots on a leaf of Q. rubra. i–k Pycnothyrium, conidia, and poorly developed pycnothyrium with conidia and microconidia, from the holotype. l, m Crustose-type pycnothyrium on leaf vein of Q. rubra and conidia from a crushed pycnothyrium (ISC 453298). Bars in all photos = 20 µm, except l, where the bar = 1 mm

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Fig. 2
figure 2

Tubakia americana and a second Tubakia sp. on lectotype material. a, b Pycnothyria and conidia of T. americana from a leafspot on the lectotype. c Pycnothyrium and conidia of a second Tubakia sp. on a separate leafspot on the lectotype material. d–f T. americana on Quercus macrocarpa leaves and twigs in Iowa. d, e Underdeveloped pycnothyrium with microconidia and fully-developed pycnothyrium with conidia from a leaf of Q. macrocarpa (ISC 453301, strain A1158). f Conidiophores and conidia from a squashed, crustose pycnothyrium taken from a twig of Q. rubra (ISC 453304, strain A1007). Bars in all photos = 20 µm

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Pure cultures were obtained from over 500 collections of Tubakia spp., mostly by isolating from surfaced sterilized leaves or twigs of Q. macrocarpa in Iowa. In contrast to pycnothyria and conidia on leaves collected in the eastern USA, which showed limited variation, cultures of Tubakia spp. showed a wide range in growth rate and pigmentation of mycelium on MYEA. Most cultures were fast growing and had scalloped margins, initially white and later darkening grey to black (Harrington et al. 2012). Sporodochia with conidia rarely or commonly formed on MYEA, depending on the putative species, and aerial mycelium often obscured the sporodochia. The CBS isolates of Tubakia spp. from Japan showed a wide range in pigmentation and growth rate, but isolates from other countries appeared similar to the USA isolates.

Phylogenetic analyses

The LSU-rDNA sequences showed limited variation among the Tubakia collections from the USA, although the Japanese sequences were more variable (Fig. 3). There were 18 most parsimonious trees of 157 steps, with the homoplasy index (HI) = 0.2229 (= 0.3398 excluding uninformative characters), retention index (RI) = 0.7697, and rescaled consistency index (RC) = 0.5981. In the Bayesian analysis, the level of convergence from two parallel runs after 2,000,000 generations had a mean standard deviation of split frequencies of 0.005440. The MP trees (Fig. 3) grouped the putative Tubakia spp. in a single clade with moderate support (88/.99 bootstrap/posterior probability support). There were only two LSU sequences among the Tubakia spp. found in the eastern USA, though a GenBank sequence (HM122939) of an unidentified isolate recovered from a dead oak leaf in Arizona differed by 1 or 2 bp, respectively, from the LSU sequences of the other American species. The base of the Tubakia clade comprised four Japanese species and the deposited sequence (AF277127) of a sexual species, originally described as Apiognomonia supraseptata S. Kaneko and Ts. Kobay. With the tree rooted to C. brasiliense at an internal node with basal polytomy, the LSU sequences of the Tubakia clade grouped most strongly with Diaporthales with 2-celled ascospores, including representatives of Apioplagiostoma, Cryphonectria, and Gnomonia, as well as the asexual G. uvicola (Fig. 3). The LSU sequence of an Italian isolate (A683 = CBS 639.93) deposited as Diacarpella dryina Belisario and Barr was placed among the USA isolates of Tubakia spp., but Diacarpella has single-celled ascospores, unlike A. supraseptata and the sexual relatives of Tubakia spp.

Fig. 3
figure 3

One of 18 most parsimonious trees of Tubakia spp. and representative Diaporthales using partial sequences of the large subunit (28S) rDNA gene. The tree is rooted to Chaetomium brasiliense. Bootstrap values (greater than 50%, 1000 replications)/posterior probability estimates are given above the branches

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Analysis of the ITS rDNA sequences gave 49 MP trees of 441 steps, HI = 0.2404 (= 0.3365 excluding uninformative characters), RI = 0.9011, and RC = 0.6845. In the Bayesian analysis, the level of convergence from two parallel runs after 1,000,000 generations had a mean standard deviation of split frequencies of 0.008518. Strong support (99/.97) for the branch comprising 13 Tubakia spp. was seen (Fig. 4), with three Japanese species (T. rubra, T. subglobosa, and T. castanopsidis) in a group sister to the other Tubakia spp. Aside from these three Asian species, the Tubakia sequences formed two sister clades. One of these clades comprised primarily collections from the USA and Europe, mostly from Quercus sect. Quercus, including collections considered to be T. dryina sensu stricto (Harrington et al. 2012). This clade was further separated into the T. dryina group (T. dryina sensu stricto along with an isolate from an oak tree in Iowa and two sequences deposited in GenBank from Q. rubra in New York, designated as Tubakia sp. E) and a second group that included the lectotype (FC 286) of A. americana from Q. coccinea in New Jersey, USA. The latter sequence differed by only one or 2 bp from those of isolates from sect. Quercus in and around Iowa, and it was also similar to sequences of some Eurasian collections (Fig. 4).

Fig. 4
figure 4

One of 49 most parsimonious trees of Tubakia spp. and two representative Diaporthales (Gnomonia setacea and Greeneria uvicola) using partial sequences of the internal transcribed spacer regions (ITS) of rDNA. The tree is unrooted. Bootstrap values (greater than 50%, 1000 replications)/posterior probability estimates are given above the branches

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The other major clade of Tubakia spp. included two subclades (Fig. 4). One of these clades was primarily on the white oak group (sect. Quercus) and included the recently described T. iowensis (Harrington et al. 2012). The second subclade included most of the USA collections, which primarily were isolated from members of the red oak group (Quercus sect. Lobate). Two isolates from sweetgum (considered to represent L. liquidambaris Tehon and Stout), a USA isolate from chestnut, and the Asian species T. japonica (the type species for the genus) and T. seoraksanensis were within the USA red oak group (Fig. 4).

The TEF-1α analyses resulted in trees with topologies similar to those of the ITS trees. There were 23 MP trees of 488 steps in the TEF-1α analysis, with HI = 0.2193 (= 0.3262 excluding uninformative characters), RI = 0.8856, and RC = 0.6914. In the Bayesian analysis, the level of convergence from two parallel runs after 2,000,000 generations had a mean standard deviation of split frequencies of 0.009960. The sequence of the Japanese species T. castanopsidis was outside of the other Tubakia spp. in the tree (Fig. 5). The sequences of the other collections grouped similarly to the groupings found with the ITS rDNA sequences. Three broad groups were recognized among the American isolates. One included T. dryina, a second included T. iowensis, and a third comprised most of the collections from Quercus sect. Lobatae, a collection from chestnut, T. japonica, and the Korean species T. seoraksanensis. Two isolates from sweetgum grouped outside of the red oak group in the TEF-1α analysis (Fig. 5), but they were within the red oak group in the ITS analysis (Fig. 4).

Fig. 5
figure 5

One of 23 most parsimonious trees of Tubakia spp. using partial sequences of the translocation elongation factor 1α (TEF-1α). The tree is unrooted, but Chaetomium globosum and Metarhizium guizhouense are not in the Diaporthales. Bootstrap values (greater than 50%, 1000 replications)/posterior probability estimates are given above the branches

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Adding the respective ITS sequences to the TEF-1α dataset yielded maximum parsimony trees that were nearly identical to the TEF-1α tree, including placement of the sweetgum isolates outside the American red oak group. The Asian species T. seoraksanensis and T. japonica were still placed inside the American red oak group in the combined trees, but T. iowensis and its most closely related species were more clearly separated as sister groups (not shown).

Each group of collections with similar ITS and TEF-1α sequences were morphologically examined as potential species. Those USA groups with unique phenotypes (morphology, host range or symptom type) were considered to be species, a number of which could be accommodated within previously described species. Three new species are described, and six other species are transferred to Tubakia.

Taxonomy

Three new American species of Tubakia

Tubakia hallii T.C. Harr. and McNew, sp. nov. (Fig. 1a–c), MycoBank No. MB 822391


Etymology: named for Richard B. Hall, deceased forest geneticist at Iowa State University.


Conidiomata (pycnothyria) superficial, hypophyllous or epiphyllous on necrotic interveinal spots and along necrotic mid and lateral leaf veins, radiate scutella 45–155 µm diam, composed of a series of dark-brown, thick-walled cells originating from a central cell, ending in a blunt to acute point. Sporodochia superficial with only a few radiating dark hyphae, mainly hypophyllous, on necrotic interveinal tissues and along leaf veins. Conidiophores on underside of scutella or on top of sporodochia. Conidia hyaline, turning light brown with age, smooth to slightly varicose, aseptate, obovoid to ovoid, 9.5–14.5 (16) × 7.5–10 (11) µm (mean 12.4 × 8.7 µm). Microconidia not seen on leaves.

Colonies on MYEA with optimal growth between 25–30 °C, diam 50–70 mm after 7 days, initially white with dense aerial mycelium, smooth to scalloped at edge, turning cream to light grey at 10 days, developing concentric rings of dense mycelium, underside yellow, becoming golden yellow to brown at 10 days, sometimes with dark cell masses on surface or subsurface. Conidiophores rare to abundant, short, hyaline, aggregated (sporodochia) on agar surface, producing conidia in dark brown to black, wet masses. Conidia hyaline, becoming light brown, thick-walled, aseptate, ellipsoidal to obovate, 12.5–16.5 (17) × 5.3–7.5 µm (mean 14.5 × 6.2 µm). Microconidia not seen on agar media but produced from scutella developing on autoclaved pieces of leaves of Q. macrocarpa placed on MEA, hyaline, aseptate, fusiform, 3.5–7.5 × 1.0–2.5 µm.


Holotype: USA, Missouri, Kirbyville, on leaf of Quercus stellata, 2 Sep 2008, D. Brandt (Holotype, ISC 453286; ex-type strain, CBS 129013 = A666).


Other material examined: USA, Arkansas, Bella Vista, leaf of Q. stellata, 27 Aug 2009, D. McNew, N36°46′43″W94°29′39″, (ISC 453288; strain CBS 129015 = A762); Iowa, Marion, on leaf of Q. macrocarpa, 25 Aug 2010, D. McNew (ISC 453287; strain CBS 137348 = A949); Wisconsin, Lafayette County, leaves of Q. macrocarpa, 23 Sep 2011, M. Guthmiller, N42°78′06″W90°00′01″, (ISC 453289; strain A1102).


Comments: This species was called Tubakia sp. B in earlier publications (Harrington and McNew 2016; Harrington et al. 2012). It is similar in DNA sequences and morphology to T. iowensis. However, the two species differ in both ITS and TEF-1α sequences; in analysis of the combined dataset, the two species form monophyletic sister groups and appear to have recently diverged. In contrast to T. iowensis, T. hallii has not been associated with bur oak blight (Harrington et al. 2012) and appears to have broader host and geographic ranges. In contrast to T. iowensis, no crustose pycnothyria of T. hallii have been seen on the petioles of overwintering leaves. Radiate scutella tend to be larger in T. hallii (up to 155 µm) than in T. iowensis (up to 110 µm). In culture on MYEA T. hallii tends to produce conidia, but T. iowensis does so rarely. Whereas T. iowensis is mostly restricted to Q. macrocarpa and causes necrosis of leaf veins, T. hallii has been associated with both leaf spots and necrotic leaf veins of numerous members of Quercus sect. Quercus: Q. alba, Q. bicolor, Q. macrocarpa, Q. muehlenbergii, and Q. stellata in Wisconsin, Illinois, Iowa, Missouri, Kansas and Arkansas.


Tubakia macnabbii T.C. Harr. and McNew, sp. nov., Fig. 1d–g, MycoBank No. MB 822392


Etymology: named for H. Sande McNabb, deceased forest pathologist at Iowa State University.


Conidiomata (pycnothyria) superficial, hypophyllous or epiphyllous, on necrotic spots and along mid and lateral leaf veins, radiate scutella (45) 65–135 (178) µm diam, composed of a series of dark brown, thick-walled cells radiating from a central cell, ending in a blunt to acute point. Sporodochia with no scutella or poorly developed scutella, epiphyllous or hypophyllous on necrotic veins and tissue, light to dark brown due to mass of conidia. Conidiophores on underside of scutella or on sporodochia. Conidia from radiate pycnothyria and sporodochia hyaline, turning light brown with age, smooth to slightly varicose, aseptate, obovoid to ellipsoidal, 9.5–14.5 × (6) 7–9 (10) µm (mean 11.8 × 8.2 µm). Microconidia sterile, hyaline, aseptate, fusiform, 3.5–9 (11.5) × 1–3 µm, from small, radiate pycnothyria, alone or along with macroconidia. Crustose conidiomata (pycnothyria) hypophyllous or epiphyllous on necrotic mid and lateral veins of late–season, current years leaves and overwintering leaves still hanging from tree, black, pulvinate, irregularly shaped, 0.7–1.5 mm diam, single or grouped, covered with thick-walled cells, breaking open with swelling when wet. Conidia hyaline to light brown, aseptate, ellipsoidal to obovate to irregular in shape, (8) 10–15 (19) × (6) 6.5–8 (9) µm (mean 13 × 7.1).

Colonies with optimal growth on MYEA at 25 °C, diameter 50–65 mm after 7 days, creamy-white aerial mycelium, smooth to scalloped at edge, may develop concentric rings of dense mycelium, underside golden yellow to slightly darker with age, sometimes with dark cell masses on surface or subsurface. Conidiophores rare to abundant, short, hyaline, sometimes aggregated (sporodochia) on agar surface. Conidia hyaline to dark brown, thick-walled, smooth to slightly varicose, aseptate, obovoid to ellipsoidal, 8.5–15 × 5.5–10 µm.


Holotype: USA, Missouri, Jackson County, leaves of Quercus palustris, Sep 2010, D. Brandt, (ISC 453290; ex-type strain CBS 137349 = A989).


Other material examined: USA, Arkansas, Bella Vista, leaf of Q. muehlenbergii, 27 Aug 2009, D. McNew (ISC 453291, strain CBS 137346 = A810); California, Los Osos, twig of Q. agrifolia, 27 April 2012, S. Latham, (strain A1177); Florida, Alachua County, leaf of Castanea ‘Dunstan,’ Nov 2009, T. Schubert (ISC 453292, strain CBS 137347 = A852); Illinois, Nauvoo, leaf of Q. rubra, 28 Jul 2010, D. McNew (ISC 453293, strain A929); Iowa, Montgomery County, leaf of Q. macrocarpa, 27 Aug 2010, T. Harrington (ISC 453294 strain A954); Ohio, Dublin, leaf of Q. rubra, Mar 2014, T. Hollowell (ISC 453295, strain A1258); ITALY, near Rome, isolated from overwintered leaf of Q. rubra, Dec 1993, A. Belisario (strain CBS 639.93 = A683); ITALY, Unknown, dead leaf, Jul 1989, H. A. van der Aa (strain CBS 387.89 = A868).


Comments: This was the most commonly encountered Tubakia species in the eastern USA, where it appears to be indigenous and widespread. It was reported as Tubakia sp. D in earlier work (Harrington and McNew 2016) and has been found primarily on Quercus sect. Lobatae. We have examined material or isolates from Quercus in Kansas, Missouri, Oklahoma, Arkansas, Louisiana, Florida, Maryland, New Hampshire, Ohio, Illinois, Wisconsin, Minnesota, Iowa and California. Oak hosts in eastern USA include Q. alba, Q. hemisphaerica, Q. imbricaria, Q kelloggii, Q. laurifolia, Q. macrocarpa, Q. marilandica, Q. muehlenbergii, Q. nigra, Q. palustris, Q. rubra, Q. stellata, Q. velutina, and Q virginiana. We have also examined material from Castanea spp. in Florida and Iowa. Although no species of Tubakia had been identified west of the Great Plains, California cultures and specimens of T. macnabbii from leaves and twigs of Q. agrifolia (San Luis Obispo County), Q. wislizenii and Q. kelloggii (Richland and Shasta Counties), and Lithocarpus densiflorus (tanoak) from Santa Clara County were sent to us by S. Latham.

Two CBS cultures from Q. rubra (sect. Lobatae) leaves collected in a nursery near Rome, Italy were also identified as T. macnabbii, but it is unlikely that the fungus is native to Europe because members of sect. Lobatae are not native there, and this is the only report of a Tubakia sp. on sect. Lobatae outside of North America (Harrington et al. 2012). The fungus may have been transported to the nursery on acorns collected from eastern USA. A sexual state, Dicarpella dryina, was reported in a leaf of Q. rubra collected from the same nursey (Belisario 1991), but as discussed later, it is not thought that the perithecium was actually that of a Tubakia sp.

Tubakia macnabbii is distinguished from T. dryina by the production of crustose pycnothyria on vein and leaf tissue, rather than twigs (Harrington et al. 2012). The conidia from crustose pycnothyria of T. macnabbii are larger, and cultures lack the concentric rings typical of T. dryina. The conidia of T. macnabbii from radiate scutella are slightly smaller than those of T. hallii and T. iowensis, which are found on species in Quercus sect. Quercus. Unlike T. iowensis, T. macnabbii readily produces conidia in culture.

Sequence analyses suggest that there is substantial genetic variation in T. macnabbii, and there may be other cryptic species in the group. Surprisingly, two Asian species, T. japonica and T. seoraksanensis, form a monophyletic group within T. macnabbii according to ITS and TEF-1α analyses (Figs. 4, 5), but the Asian species are readily distinguished from the American T. macnabbii by their larger conidia. Further phylogenetic analyses are needed to resolve this anomaly. With the Asian species excluded and the ITS and TEF-1α sequences combined, isolates from the red oak group form two sister clades: T. macnabbii and the following new species. Two isolates of Tubakia from sweetgum in Arkansas, which appear to represent Leptothyrella liquidambaris (Tehon and Stout 1929), are sister to the red oak species (Fig. 5), and the sweet gum isolates differ from those of T. macnabbii in culture morphology. Two other species with similar pycnothyria and conidia were described on other host families, but no cultures or DNA were available, and these will be presented as new combinations in Tubakia. A second species on red oaks has slightly larger conidia and a more limited geographic range, and it is described below.


Tubakia tiffanyae T.C. Harr. and McNew, sp. nov., Fig. 1h–m, MycoBank No. MB 822393


Etymology: named for Lois H. Tiffany, deceased mycologist at Iowa State University.


Conidiomata (pycnothyria) superficial, hypophyllous or epiphyllous, on circular leaf spots or along necrotic leaf veins, radiate scutella (40) 60–150 µm diam, composed of a series of dark brown, thick-walled cells originating from a central cell, ending in a blunt to acute point. Sporodochia with no scutella or poorly developed scutella epiphyllous or hypophyllous on necrotic vein tissue, light to dark brown due to masses of conidia. Conidiophores on underside of scutella or on sporodochia. Conidia hyaline, turning light brown with age, smooth to slightly varicose, aseptate, obovoid to ovoid (9) 10–15.5 × (8) 8.5–11.5 (12) µm (mean 12.9 × 9.6 µm). Microconidia sterile, hyaline, aseptate, fusiform 3–8 × 1–2.5 µm may develop from small, pycnothyria, alone or along with macroconidia.

Crustose conidiomata (pycnothyria) developing in late summer to fall on underside of necrotic veins and found on overwintering leaves still hanging from twigs, erumpent, brown to black, irregularly shaped, 80–580 µm diam, single to grouped, covered with dark, thick-walled cells that break open in fissures due to swelling when wet. Conidia hyaline to brown, ellipsoidal to obovate to irregularly shaped, aseptate, (11) 13–18 (20) × (6) 7–9.5 µm (mean 15.2 × 7.9 µm).

Colonies with optimal growth on MYEA at 25 °C, 40–58 mm diam after 7 days, creamy white aerial mycelium, smooth to scalloped at edge, developing concentric rings of dense mycelium, underside golden yellow to slightly darker with age, sometimes with dark cell masses on surface or subsurface. Conidiophores rare to abundant, short, hyaline, sometimes aggregated (sporodochia) on agar surface. Conidia hyaline to dark brown, thick-walled, smooth to slightly varicose, aseptate, obovoid to ellipsoidal, 10–16 × 7–11 µm.


Holotype: USA, Iowa, Ames, leaf of Quercus rubra, 5 Sep 2009, T. Harrington, N42°04′47″W93°65′22″, (ISC 453296, ex-holotype strain CBS 137345 = A803).


Other material examined: USA, Iowa, Ames, leaf of Q. imbricaria, 27 Jul 2011, D. McNew, N42°29′99″W93°64′45″, (ISC 453297, strain CBS 137352 = A1052); leaf of Q. rubra, Mar 2014, D. McNew, N42°02′87″W93°63′21″, (ISC 453298); Iowa County, leaf of Q. rubra, 29 Jul 2011 (ISC 453299, strain CBS 137351 = A1042); Hardin County, Steamboat Rock, leaf of Q. ellipsoidalis, 9 Sep 1913, J. P. Anderson (ISC 319897); Minnesota, Washington County, leaf of Q. rubra, Sep 2013, J. Pokorny (ISC 453300, strain A1252).


Comments: This species is closely related to T. macnabbii but has wider conidia than any of the other American Tubakia spp. It was referred to as Tubakia sp. C by Harrington and McNew (2016), where it was noted that affected leaves usually show characteristic circular spots with light-coloured centres (Fig. 1h) in addition to the veinal necrosis more typical of symptoms caused by the other Tubakia spp. It has been identified in Iowa and Minnesota on leaves of Q. ellipsoidalis, Q. imbricaria, and Q. rubra.

New Combinations in Tubakia

Tubakia americana (Höhn.) T.C. Harr. and McNew, comb. nov., Fig. 2, MycoBank No. MB 822395

≡ Actinopelte americana Höhn., Mitt. Bot. Inst. Tech. Hoechst. Wein 2:68. 1925, MB 256634


Conidiomata (pycnothyria) with radiate scutella epiphyllous or hypophyllous on leaf spots or along necrotic leaf veins of Quercus. Scutella convex, 40–110 μm diam, of brown, thick-walled cells radiating from a central point, ending in a 2–3 μm blunt tip. Sporodochia hypophyllous under leaf spots or along necrotic veins, irregular in shape, sometimes with small, poorly developed scutella. Conidiophores short, forming under developing scutellum or on sporodochia. Conidia of pycnothyria and sporodochia thick-walled, finely varicose to smooth, initially hyaline, later light brown, obovoid to ellipsoidal, aseptate, 9–13 (14) × 6–8 (8.5) μm (mean 11.1 × 6.9 μm). Microconidia sometimes produced from same pycnothyrium as macroconidia or from smaller pycnothyria, hyaline, fusiform, curved, aseptate, (3.5) 4–7 × 2–3 μm. Crustose conidiomata (pycnothyria) forming in winter or spring, erumpent on twigs or inside of acorn caps, black, irregular in shape, 150–500 μm diam. Scutella of thick-walled cells, textura angularis, dehiscing marginally or by irregular fissures. Conidiophores lining inside of crustose pycnothyrium. Conidia thick walled, hyaline to light brown, obovoid to ellipsoidal, aseptate, 9.5–14 × 5.5–7.5 μm.

Colonies with optimal growth at 25 °C on MYEA, attaining a diameter of 56–64 mm after 7 days, initially with a distinct ring of dense aerial mycelium, later developing concentric rings of fluffy white to grey aerial mycelium with wet conidial masses that are initially hyaline, becoming olive green then black, coalescing into large areas. Conidia in culture abundant, thick walled, smooth to finely varicose, hyaline to dark brown, obovoid to ellipsoidal, aseptate, 9–15 (16.5) × (4) 4.5–7.5 (8) μm.


Lectotype: USA, New Jersey, Newfield, leaf of Quercus coccinea, Aug 1883, Ellis and Everhart (designated here as FC 286 NY ex McClatchie Herbarium, MBT 379548; Isolectotypes: Ellis and Everhart FC 286 = ISC 453285; Ellis and Everhart FC 286, NY ex Mr. F. S. Earle Herbarium; Ellis and Everhart FC 286, NY Ellis collection).


Other material examined: USA, Iowa, Ames, Brookside Park, leaves of Q. macrocarpa, 12 Oct 2011, D. McNew N42°03′10″W93°63′24″, (ISC 453301, strain CBS 137353 = A1158); from leaf spot on Q. macrocarpa, Jun 2009, D. McNew (strain CBS 129014 = A749); acorn cup of Q. macrocarpa, 19 May 2009, D. McNew, N42°02′98″W93°63′21″, (ISC 453302, strain A693); twig of Q. rubra, 6 Apr 2011, D. McNew, N42°02′95″W93°63′22″, (ISC 453304, strain CBS 137350 = A1007); Missouri, Lewis County, from leaf of Q. bicolor, 5 Aug 2012, D. McNew (ISC 453305, strain A1201); Wisconsin, Green County, from leaf of Q. macrocarpa, 23 Sep 2011, M. Guthmiller, N42°78′69″W89°63′32″, (ISC453306, strain A1105); Racine, from leaf of Q. rubra, 14 Jul 1887, J. J. Davis (Ellis collection NY ex Herb. J. J. Davis). FRANCE, Seignosse le Penon, from Cynips kollari gall on Q. pedunculata (= Q. robur) leaf, H.A. van der Aa (strain CBS 335.86 = A682).


Comments: Höhnel (1925) described Actinopelte americana from specimen FC 286 in Ellis and Everhart’s Fungi Columbiani, distinguishing this species from the European A. dryina Sacc. by its smaller conidia. The particular FC 286 specimen that Höhnel examined appears to be lost (personal communication, Farlow Herbarium). A copy of Fungi Columbiani FC 286 from the McClatchie Herbarium (NY), which we designate as the lectotype, shows pycnothyria of two Tubakia spp. associated with different necrotic spots. One of these Tubakia species has relatively small conidia, as described by Höhnel (1925) for A. americana, and the tips of the setae of the scutella are blunt (Fig. 2a, b). The second type of pycnothyria on the lectotype material has scutella with acute tips and larger conidia (Fig. 2c), matching our concept of T. macnabbii. The FC 286 material from four other sets of Fungi Columbiani (in NY and ISC) each contained leaves with shields of the two Tubakia species.

Pycnothyria of the small-spored species were scraped from a leaf spot of the lectotype and used for DNA extraction and PCR using a ITS rDNA primer pair specific to Tubakia spp. The PCR product was directly sequenced with the same primers, and the sequence matched that of Tubakia sp. A (Harrington and McNew 2016; Harrington et al. 2012), which has been commonly isolated as a twig and leaf endophyte in Q. macrocarpa and other Quercus spp. in Iowa (Fig. 4). Tubakia americana also is associated with leaf spots and necrotic veins on Q. macrocarpa in Iowa.

The T. americana conidia from pycnothyria with radiate scutella are the narrowest of all the species of Tubakia in the USA. Phylogenetic analyses suggest that T. americana is closely related to T. dryina (Figs. 3, 4, 5), and both species produce crustose pycnothyria on twigs (Harrington et al. 2012). In culture, the mycelium of T. americana is much lighter in colour than the dark mycelium of T. dryina (Harrington et al. 2012). Species of Quercus sect. Quercus (Q. bicolor, Q. macrocarpa and Q. robur) are the primary hosts of T. americana in Iowa. However, the fungus also has been found on Q. rubra, and the lectotype material was from Q. coccinea, also a member of Quercus sect. Lobatae. Specimens and cultures have been examined from Wisconsin, Illinois, Iowa, Missouri, and New Jersey. Phylogenetic analyses suggest that the Eurasian population of T. americana is distinct (Figs. 4, 5), but strain CBS 335.86 from France appears similar to the USA strains of T. americana. Boroń and Grad (2016) reported two distinct ITS rDNA sequences from T. dryina collections in Poland, and their Hp2 sequence appears to be that of the European T. americana.


Tubakia gloeosporioides (Tehon) T.C. Harr. and McNew, comb. nov., MycoBank No. MB 822396

≡ Actinothyrium gloeosporioides Tehon, Mycologia 16:136. 1924, MB 221305


Specimens on Sassafras albidum from Illinois (holotype ILLS 3311, as well as ILLS 2972, ILLS 3671, ILLS 29748, and ILLS 17547) and New Jersey (as Leptothyrium dryinum f. sassafras, NY E&E Fungi of New Jersey) have pycnothyria 45–132 µm diam (reported as 50–95 µm diam by Tehon 1924) and conidia 9.5–12.5 × 7–8.5 µm (reported as 11–12 × 6–7.5 µm by Tehon 1924), which are similar to pycnothyria and conidia of T. macnabbii. Without cultures or DNA, it is not possible to distinguish T. gloeosporioides from T. macnabbii, but the former is probably distinct based on its unique host, which is in the family Lauraceae. Glawe and Crane (1987) reported the holotype ILLS 3311 as being lost and unnecessarily designated a lectotype, which is no longer valid.


Tubakia liquidambaris (Tehon and Stout) T.C. Harr. and McNew, comb. nov., MycoBank No. MB 822396

≡ Leptothyrella liquidambaris Tehon and Stout, Mycologia 21:192. 1929, MB 188944


Herbarium specimens on Liquidambar styraciflua from Arkansas (ISC 453303), Illinois (holotype ILLS 1445 and ILLS 29743), Maryland (BPI 391888), and Mississippi (BPI 840865) have pycnothyria 60–138 µm diam (91–112 µm diam by Tehon and Stout 1929) and conidia 9–14 × 6.5–9 µm (8.4–10.2 × 6–6.8 µm by Tehon and Stout 1929), similar to those of T. macnabbii. Cultures from necrotic spots on leaves of L. stryaciflua in Bella Vista, Arkansas (CBS 139744 = A771 and CBS 139745 = A830, both from ISC 453303) are distinct from cultures of T. macnabbii in having slower growth, a flat surface, and abundant production of conidia formed from sporodochia in concentric rings. In addition to the different host, differences in DNA sequences (Figs. 4, 5) of the two Arkansas strains also support recognition of this species as distinct from T. macnabbii.


Tubakia nyssae (Tehon) T.C. Harr. and McNew, comb. nov., MycoBank No. MB 822398

≡ Pirostoma nyssae Tehon, Mycologia 16:137. 1924, MB 196764


Specimens from Illinois (holotype ILLS 2940 and ILLS 8871) and Virginia (BPI 391890 = ILLS 41874 and BPI 391891) are from Nyssa sylvatica, which is outside of the Fagaceae, suggesting that this is a distinct species of Tubakia. Morphologically T. nyssae is similar to T. macnabbii, with pycnothyria in discreet leaf spots, the scutella 70–116 µm diam (60–95 µm diam according to Tehon 1924) with acute tips. Conidia are hyaline to light brown, ellipsoid to subglobose, 10–14 × 7–9 µm (12 × 7 µm according to Tehon 1924). The designation of a lectotype by Glawe and Crane (1987) was unnecessary as the holotype (ILLS 2940) is still available.


Tubakia stellata (Farr) T.C. Harr. and McNew, comb. nov., MycoBank No. MB 822399

≡ Actinopelte stellata Farr, Mycopathologia 31:63. 1967, MB 325826


Known only from the holotype (BPI 391941) on leaves of Byrsonima coriacea (= B. spicata) in Pará, Brazil (Farr 1967), T. stellata is the only species of Tubakia, other than T. dryina, from the Southern Hemisphere. This is also the only record of a Tubakia from the tropics and the tropical host family (Malpighiaceae). The scutella of the holotype are made up of unbranching cells that expand from the centre and then abruptly narrow to an acute tip. The scutella are reddish brown, 47–70 µm diam, with conidia formed underneath, hyaline, aseptate, globose to irregularly globose, 6.5–8 µm diam. No microconidia were seen. The shape of the scutella and conidial development sufficiently resemble other Tubakia species to warrant the new combination. No cultures or DNA sequences are available.


Tubakia supraseptata T.C. Harr. and McNew, comb nov., MycoBank No. MB 822400

≡ Apiognomonia supraseptata Kaneko and Kobayashi, Trans. Mycol. Soc. Japan 25:11. 1984, MB 105925


Kaneko and Kobayashi (1984) isolated an endophyte from Quercus that formed perithecia but no conidia in culture. Aside from the production of perithecia, the illustrated culture had a mycelial morphology typical of Tubakia. The deposited LSU rDNA sequence from a culture taken from the holotype (strain CBS 632.92) clearly places this fungus within the genus Tubakia as defined herein (Fig. 3). As pointed out by Kaneko and Kobayashi (1984), Apiognomonia Höhn. has two celled ascospores in which the top cell is larger than the bottom cell, but in A. supraseptata the top cell is smaller. Also, Apiognomonia spp. have Discula-like asexual states (Sogonov et al. 2007).

Based on Kaneko and Kobayashi’s (1984) LSU sequence obtained from the ex-holotype strain, T. supraseptata is the only Tubakia sp. with a clearly demonstrated sexual state. Although many species of Tubakia produce small conidia that do not germinate and appear to be spermatia, no clear connection between a Tubakia asexual state and a perithecial state had been made. As discussed below under doubtful and excluded species, the connection of Dicarpella dryina (with one-celled ascospores) to Tubakia dryina (Belisario 1991) is questioned.

Previously recognized species of Tubakia

Tubakia castinopsidis (Yokoyama and Tubaki) B. Sutton, Trans Brit. Mycol. Soc. 60:165. 1973, MB 530653

≡ Actinopelte castanopsidis Yokoyama andTubaki, IFO Res. Comm. 5:50. 1971, MB 308270


Yokoyama and Tubaki (1971) described T. castinopsidis from leaves of Castanopsis cuspidata in Japan, scutellum 100–150 µm diam, conidia hyaline, oblong to cylindrical, 12–13 × 7–8 µm. The culture from the holotype (CBS 189.71 = IFO-9263) does not produce conidia or pycnothyria, but DNA sequences generated from this culture are clearly placed in Tubakia (Figs. 3, 4, 5).


Tubakia chinensis U. Braun, S. Bien and Hantsch, Schlechtendalia 28:23. 2014, MB 809784


This species was described from leafspots on Castanea henryi in China as having scutella up to 200 µm diam and conidia 25–40 × 20–30 µm (Braun et al. 2014). The measurements are similar to those of T. japonica, which is found in Japan on Castanea. No culture or DNA sequence was available at the time of this study.


Tubakia dryina (Sacc.) B. Sutton, Trans Brit. Mycol. Soc. 60:165. 1973, MB 325072

≡ Leptothyrium dryinum Sacc., Michelia 1:201. 1878, MB 24303

≡ Actinopelte dryina (Sacc.) Höhn., Mit. Bot. Inst. Tech. Hochsch. Wein. 2:69. 1925, MB 263534


As redefined by Harrington et al. (2012), T. dryina has radiate scutella, 60–125 µm diam, producing hyaline to light-brown, thick-walled conidia, 10–15 × 7–10.5 µm. It can apparently kill small twigs of Quercus spp., on which it produces crustose pycnothyria (Harrington et al. 2012; Holdenreider and Kowalski 1989). Conidia from the crustose pycnothyria are 10–16 × 6–7.5 µm, which are narrower than those from radiate pycnothyria. The DNA sequences of T. dryina are unique but similar to those of T. americana and an unidentified species (Tubakia sp. E) from eastern USA (Fig. 4). Tubakia dryina appears to be primarily a European species and found east to Iran (Zahedi et al. 2011), and it commonly causes a leaf spot on Q. alba in the eastern USA (Harrington and McNew 2016; Harrington et al. 2012). It has been found on the European species Q. robur in various locations around the world, including New Zealand, which is the only report of Tubakia in the Southern hemisphere besides Tubakia stellata in Brazil.

Belisario (1991) reported perithecia of a new species, Dicarpella dryina, as the sexual stage of T. dryina. The perithecia were in leaves of Q. rubra collected in a nursery near Rome, Italy affected by an introduced strain of T. macnabbii. However, the connection of the Dicarpella to either T. dryina or T. macnabbii is questioned.


Tubakia iowensis T.C. Harr. and McNew, Mycologia 104:86. 2012, MB 561043


Compared to T. hallii, T. iowensis has a narrower geographic range and a much narrower host range (Harrington and McNew 2016; Harrington et al. 2012), but the two species are difficult to distinguish morphologically and phylogenetically (Figs. 3, 4, 5). Pycnothyria of T. iowensis with radiate scutella are somewhat smaller (40–110 µm diam) than those of T. hallii (45–155 µm diam). The crustose pycnothyria of T. iowensis on the petioles of leaves overwintering on twigs of bur oak trees is considered the most critical feature of bur oak blight, which occurs only on Q. macrocarpa var. oliviformis. Veinal necrosis caused by T. iowensis on Q. bicolor leaves in proximity of bur oak trees with bur oak blight has been seen, but it is not common. T. iowensis has been identified in Iowa and each of the states surrounding Iowa, and in northeastern Kansas (Harrington and McNew 2016).


Tubakia japonica (Sacc) B. Sutton, Trans Brit. Mycol. Soc. 60:165. 1973, MB 325073

≡ Actinopelte japonica Sacc., Annales Mycologici 11:315. 1913, MB 247526


This is the type species for the genus and has exceptionally large pycnothyria and conidia. Only T. chinensis is reported to have such large conidia (Braun et al. 2014). Specimens of T. japonica (on leaf spots on Castanea spp. in Japan, holotype: BPI 391948 = Sydow’s Fungi Exotica Exsiccate no. 526; and BPI 391940) have large scutella (75–210 µm diam) and very large conidia (30–48 × 28–38 µm). These measurements fall within Saccardo’s (1913) original description of A. japonica as having large asci (conidia), 35–42 × 28–33 µm and superficial perithecia (pycnothyria) 220–240 µm diam. Yokoyama and Tubaki (1971) reported microconidia 5–10 × 1–2 µm. In 2014, Schwarze deposited specimens of A. japonica on Quercus spp. from New Jersey, USA, but his collections (BPI 391916, BPI 391923 and BPI 391899) have conidia 10–15 × 5–7.5 µm, similar to those of T. americana. Another collection (BPI 391900) of A. japonica from West Virginia, USA has conidia 10–14 × 7.5–10 µm, similar to those of T. macnabbii.

The DNA sequences of a culture of T. japonica from Japan (CBS 191.71 = A872) are similar to those of T. seoraksanensis from Korea and T. macnabbii of the USA (Figs. 4, 5), but these species are morphologically distinct.


Tubakia rubra (Yokoyama and Tubaki) B. Sutton, Trans Brit. Mycol. Soc. 60:165. 1973, MB 325074

≡ Actinopelte rubra Yokoyama and Tubaki, IFO Res. Comm. 5:47. 1971, MB 308271


The ex-holotype culture (CBS 192.71 = IFO-9271) from Q. phillyraeoides in Japan has a distinctive red pigmentation, as described by Yokoyama and Tubaki (1971), but the culture no longer sporulates. Yokoyama and Tubaki (1971) reported that the fungus does not produce symptoms on leaves, but when green or senescent leaves are incubated in a moist chamber they readily produce red pycnothyria, 60–110 µm diam, with conidia hyaline to yellow orange, ellipsoidal to ovate, 12–15 × 10–13 µm. Microconidia are hyaline 8–10 × 1 µm. Tubakia rubra, T. subglobosa and T. castanopsidis have ITS rDNA sequences that differ significantly from the other Tubakia spp. (Figs. 3, 4).


Tubakia seoraksanensis H.Y. Yun, Mycotaxon 115:371. 2011, MB 519212


This species causes leaf spots and veinal necrosis on leaves of Q. mongolica in Korea (Yun and Rossman 2011). Specimens examined include the holotype (BPI 880799, ex holotype strain CBS 127490 = A785) and the isotype (BPI 880798, ex isotype strain CBS 127492 = A787). The scutella are 90–128 µm diam with hyaline to light brown, with thick-walled conidia 12–19 × 9–14 µm (scutella 90–160 µm diam and conidia 13–25 × 10–15 µm according to Yun and Rossman, 2011). The conidia are larger than all other described Tubakia spp. except T. japonica and T. chinensis, and the DNA sequences of T. seoraksanensis and T. japonica are similar (Figs. 4, 5).


Tubakia subglobosa (Yokoyama and Tubaki) B. Sutton, Trans Brit. Mycol. Soc. 60:165. 1973, MB 325075

≡ Actinopelte subglobosa Yokoyama and Tubaki, IFO Res. Comm. 5:49. 1971, MB 308272


This species was originally described as having subglobose to globose conidia 10–13 × 9–11 µm and cultures with a distinctive cinnamon-red colour (Yokoyama and Tubaki 1971). A culture from the holotype (CBS 193.71 = IFO-8931 = A685) from leaves of Q. glauca in Japan no longer produces conidia or the distinctive colour, but its ITS rDNA sequence is unique and groups T. subglobosa with two other Japanese species, T. castanopsidis and T. rubra (Figs. 3, 4). Tubakia subglobosa was recently reported from Korea (Yun and Kim 2017).

Doubtful and excluded species

Leptothyrium castaneicola Ellis and Ev., Jour. Myc. 4:103. 1888. nomen dubium, MB 187659

This species was described from leaves of Castanea as having scutella-like perithecia 115–150 µm diam on the green parts of the leaf (Ellis and Everhart 1888). No spore measurements were given. Glawe and Crane (1987) found no fruiting structures in the probable holotype of L. castaneicola (NY specimen 009278112 from the Ellis collection), and we also could find no Tubakia-like fruiting structures in that specimen.


Dicarpella dryina Belisario and Barr, Mycotaxon 41:154. 1991, MB 128110

Belisario (1991) found perithecia and ascospores of a fungus on overwintering leaves of Quercus rubra in a nursery in central Italy, and a single-ascospore culture from one of the leaves was thought to be conspecific with T. dryina. However, the Tubakia associated with leaf spots on Q. rubra in the same nursery (CBS 387.89) and Belisario’s deposit (CBS 639.93) are T. macnabbii, not T. dryina (Figs. 3, 4, 5). T. macnabbii will produce conidia in abundance on overwintered leaves of Q. rubra, so it is possible that conidia of a Tubakia contaminated the putative single-ascospore isolation deposited as D. dryina.

Dicarpella Syd. and P. Syd. has one-celled ascospores (Reid and Dowsett 1990) instead of the two-celled ascospores found in T. supraseptata, Apiognomonia (Sogonov et al. 2007), and other sexual relatives of Tubakia spp. based on LSU sequences (Fig. 3). If the perithecia described by Belisario (1991) were those of a Tubakia sp., Dicarpella Syd. and P. Syd. (1921) is an illegitimate homonym of Dicarpella Bory (1824), and thus, D. dryina is an illegitimate combination. Also, the basionym of D. dryina could not be transferred to Tubakia as that combination is occupied by Tubakia dryina (Sacc.) B. Sutton.

Discussion

The genus Tubakia is well defined by phylogenetic analyses and its unique pycnothyria with radiate scutella. The studied species in the USA also form a second type of pycnothyrium, with a crustose covering rather than a scutellum of radiating setae. The two fruiting structures appear to be evolving in parallel, as the conidia size and shape differ slightly in each of the studied species. Microconidia that do not germinate also may be produced from under the radiate scutella, and these spermatia-like spores suggest that most Tubakia species form ascomata. However, only T. supreseptata is known to produce ascomata, and the cultures of this species that produced the ascomata did not produce pycnothyria or another conidial state (Kaneko and Kobayashi 1984). The ascospores of T. supreseptata are two-celled, as are ascospores of the relatives of Tubakia based on LSU rDNA sequence analysis. If D. dryina Belisario and Barr is the sexual state of a Tubakia, as suggested (Belisario 1991), it would be the sexual state of T. macnabbii, not T. dryina.

The centre of genetic diversity of Tubakia appears to be in East Asia, where Quercus and other genera of Fagaceae are the most common hosts. Many Tubakia spp. are readily isolated from green leaves of Fagaceae, and all of the species may have an endophytic phase of growth. Pycnothyria may appear on leaf or twig tissue killed by other agents, but most of the species appear capable of causing necrotic leaf spots or necrosis along leaf veins (Harrington and McNew 2016). At least one species, T. iowensis, is capable of causing a serious leaf disease, bur oak blight (Harrington et al. 2012). T. macnabbii and T. hallii also have been associated with significant defoliation of Quercus spp. (Harrington and McNew 2016), and T. dryina may cause twig death of Quercus in Europe (Holdenreider and Kowalski 1989). As endophytes, it appears that some of the species can have broad host ranges.

Most of the recognized species of Tubakia in the USA differ only slightly from other species in sizes of conidia and pycnothyria. However, differences in leaf symptoms and relatively narrow host ranges distinguish some species. Three of the new combinations that we made were based on species described from hosts outside the Fagaceae, but the pycnothyria and conidia on leaves fit our concept of T. macnabbii. Cultures of T. liquidambaris were clearly distinct from those of T. macnabbii, but T. gloeosporioides and T. nyssae warrant further study. On the other hand, it appears likely that there are other cryptic species of Tubakia awaiting description.