17 Sep Phaeoacremonium

Phaeoacremonium W. Gams, Crous & M.J. Wingf., Mycologia 88(5):789(1996)

For synonyms see Index Fungorum (2018)

Background

The hyphomycetous genus Phaeoacremonium was established by Crous et al. (1996) to accommodate six species with P. parasiticum (Ajello, Georg & C.J.K. Wang) W. Gams, Crous & M.J. Wingf. as the type, which was transferred from the genus Phialophora Medlar. It is morphologically similar to Acremonium Link and Phialophora Medlar, but can be distinguished from them by its aculeate phialides and inconspicuous, non-flaring collarettes and pigmented vegetative hyphae (Crous et al. 1996). The genus Phaeoacremonium together with Togninia Berl. were accommodated in the family Togniniaceae Réblová, L. Mostert, W. Gams & Crous and in the order Togniniales Senan., Maharachch. & K.D. Hyde (Maharachchikumbura et al. 2015). Gramaje et al. (2015) reduced Togninia to synonymy with Phaeoacremonium as 13 of 26 epithets are insufficiently known and some already have names in Phaeoacremonium. Currently, only Phaeoacremonium is retained in Togniniaceae (Wijayawardene et al. 2018).

Classification – Sordariomycetes, Diaporthomycetidae, Togniniales, Togninicaceae

Type species – Phaeoacremonium parasiticum (Ajello, Georg & C.J.K. Wang) W. Gams, Crous & M.J. Wingf., Mycologia 88(5):789(1996)

Distribution – Worldwide

Disease Symptoms – Brown wood streaking/Esca

Phaeoacremonium species are known as vascular plant pathogens causing wilting and dieback of several woody plants, e.g. P. fuscum L. Mostert, Damm & Crous, P. pallidum Damm, L. Mostert & Crous and P. prunicola L. Mostert, Damm & Crous which were isolated from necrotic woody tissue (Damm et al. 2008). Yellowing, wilting, dieback, canker and internal node discolouration can be observed from the trees that are affected by the species of this genus (Cloete et al. 2011; Mohommadi et al. 2013; Úrbez-Torres et al. 2014). In a cross-section of affected wood, wedge-shaped and circular wood necrosis can be observed (Sami et al. 2014).

Some species also cause human diseases, e.g. P. parasitica Ajello, Georg & C.J.K. Wang was described from a subcutaneous infection of a human patient (Ajello et al. 1974; Baddley et al. 2006). Considering its association with human infections and disease symptoms of several woody hosts, it is represented as an ecologically important group of fungi (Crous et al. 1996).

 Hosts – Woody plants with brown wood streaking, humans with phaeophyphomycotic infections, larvae of bark beetle, arthropods and soil. Species of Phaeoacremonium are associated with more than 50 plant genera.

Morphological based identification and diversity

To date, there are 65 epithets recorded in Index Fungorum (2018). Six species of Phaeoacremonium, i.e. P. aleophilum, P. angustius, P. chlamydosporum, P. inflatipes, P.  parasiticum and P. rubrigenum, were originally identified based on morphological features (Crous et al. 1996) and a key based on morphological and cultural characters was also provided, but some species were reported to have been misidentified. For instance, Phaeoacremonium chlamydosporum W. Gams, Crous, M.J. Wingf. & Mugnai was referred to a new genus, Phaeomoniella Crous & W. Gams based on its straight, pigmented conidia, dark green-brown conidiophores with light green to hyaline conidiogenous cells, a yeast-like growth in young colonies, a Phoma-like synanamorph, and producing chlamydospore-like structures in culture (Gams and Crous 2000). Subsequently, Mostert et al. (2005) re-examined all isolates of P. inflatipes and revised their taxonomy based on morphology and sequence data. Because of numerous incorrect identifications that have been made since 1996 (Crous et al. 1996; Gams and Crous 2000), it is difficult to use the key provided by Crous et al. (1996) for identification (Mostert et al. 2005). An updated multiple-entry electronic key was developed by Mostert et al. (2005). During 2006–2018, about 36 new species were described, most of which were identified based on DNA sequence data (Gramaje et al. 2009; Gramaje et al. 2014; Ariyawansa et al. 2015b; Gramaje et al. 2015; Crous et al. 2016).

Mostert et al. (2005) suggested that a combination of macromorphological characters (including colonial colour, growth rate, maximum growth temperature and sometimes the size and extent of mycelial warts can be distinguishing features in several species as well) and micromorphological characters (including conidiophores, phialides type, to a less extent the shape of conidia) proved useful in identification. The representative features are warty mycelium, pigmented conidiophores with phialidic conidiogenous cells and hyaline, aseptate conidia which vary from oblong-ellipsoidal to allantoid in shape. Normally the conidia gather in slimy heads at phialide apices (Gramaje et al. 2015). However, minor differences in cultural and microscopic features also cause misidentification for several species (Mostert et al. 2005). Therefore, molecular data is necessary to deeply understand these species.

Molecular based identification and diversity

Presently, Phaeoacremonium has been reported to represent a monophyletic group of taxa (Gramaje et al. 2015). There have been studies done to investigate phylogenetic relationships among a large number of species. Mostert et al. (2006) provided a rapid identification method for 22 species of Phaeoacremonium with 23 species-specific primers. It facilitates the understanding of indiscernible species in a plant as well as in human disease, however, is the key still needs to be validated. Phylogenetic analysis based on individual LSU and SSU sequence data have good performance in the study of generic placement. Analyses showed that Phaeoacremonium species form a distinct clade within Sordariomycetes and have a close affinity with Diaporthales and Calosphaeriales species (Mostert et al. 2003; Damm et al. 2008; Gramaje et al. 2015; Crous et al. 2016). Herewith, we update the phylogenetic relationship of Phaeoacremonium species by analysing the concatenated alignment of TUB2 and ACT sequence data. Molecular data of three species are not included in the phylogenetic analysis; for P. aquaticum and P. leptorrhynchum only ITS is available, for P. inconspicuum no ex-type culture or DNA sequence data exist (Gramaje et al. 2015). In the phylogenetic tree, three distinct clades were observed, and the topological structure is in accordance with Silva et al (2017).

Recommended genetic markers (genus level) – SSU, LSU

Recommended genetic markers (species level) – ACT, TUB2

Multigene phylogeny gives a deeper understanding of the phylogenetic relationships of Phaeoacremonium species. For example, combined ITS- TEF1-α – regions (Mostert et al. 2003), combined ITS-TUB2-ACT-TEF1-α dataset (Úrbez-Torres et al. 2014) and combined ACT-TUB2 regions can resolve intraspecific identification; of which ACT-TUB2 sequence data analysis was frequently used for the investigation of taxonomy and diversity among Phaeoacremonium as it provides topologies with greater resolution and well supported (Damm et al. 2008, Essakhi et al. 2008, Gramaje et al. 2015, Silva et al. 2017, Spies et al. 2018).

Accepted number of species: There are 65 species epithets in Index Fungorum (2018) under this genus. However, only 62 are accepted. This is because P.aleophilum and P. mortoniae were treated as basionym of P. minimum and P. fraxinopennsylvanicum, respectively (Gramaje et al. 2015). Phaeoacremonium chlamydosporum was transferred to a new genus, Phaeomoniella (Gams and Crous 2000).

References: Crous et al. 1996 (morphology and a key for Phaeoacremonium species), Mostert et al. 2005 (morphology, phylogeny and a key for Phaeoacremonium species), Úrbez-Torres et al. 2014 (detection, morphology, phylogeny and pathogenicity), Gramaje et al. 2015, Maharachchikumbura et al. 2016 (morphology and phylogeny).

Table  Phaeoacremonium Details of the isolates used in the phylogenetic analyses. Ex-type (ex-epitype) strains are in bold and marked with an * and voucher strains are in bold.

Fig.  Phylogenetic tree generated by maximum likelihood analysis of combined TUB2 and ACT sequence data of Phaeoacremonium species. Sequences were obtained from GenBank. Ninety-seven strains are included in the analyses, which comprise 815 characters including gaps. Single gene analyses were carried out to compare the topology of the tree and clade stability. The tree was rooted with Wuestneia molokaiensis (CBS 114877) and Pleurostomophora richardsiae (CBS 270.33). Tree topology of the Bayesian analysis was similar to the RAxML. The best scoring RAxML tree with a final likelihood value of -14544.681166 is presented. The matrix had 591 distinct alignment patterns, with 4.52% of undetermined characters or gaps. Estimated base frequencies were as follows; A = 0.225480, C = 0.307476, G = 0.225692, T = 0.241352; substitution rates AC = 1.148614, AG = 4.470582, AT = 1.079312, CG = 0.984112, CT = 4.144633, GT = 1.000000; gamma distribution shape parameter α = 1.818569. RAxML support values greater than 50% (left), Bayesian posterior probabilities greater than 0.90 (middle) and MP bootstrap value higher than 50% (right) are indicated near the nodes.. The scale bar indicates 0.08 changes. The ex-type strains are in bold.