16 Oct Sphaeropsis
Sphaeropsis Sacc., Michelia 2: 105. 1880.
The genus Sphaeropsis was introduced by Saccardo (1880) (for species of Diplodia with brown, aseptate conidia), with S. visci as the type species. Sphaeropsis is the asexual morph of Phaeobotryosphaeria (Phillips et al. 2008, 2013; Wijayawardene et al. 2017). Species in Sphaeropsis seem to be cosmopolitan in distribution since they have been recorded from both temperate and tropical countries (i.e. Germany, Namibia, New Zealand, South Africa, Thailand (Phillips et al. 2013; Slippers et al. 2014; Farr and Rossman 2019). The host specificity of Sphaeropsis has not yet been clarified and species have been recorded from various plant families.
Classification – Dothideomycetes, incertae sedis, Botryosphaeriales, Botryosphaeriaceae
Type species – Sphaeropsis visci (Alb. & Schwein.) Sacc.
Distribution – Worldwide
Disease symptoms – Blight, calyx-end rot, stem end rot
The decayed tissues in rot diseases are firm or spongy and brown in color. The skin of decayed areas generally remains brown or dark brown but may appear dark in aged areas (Kim et al. 2005). In blight disease browning, stunting and twisting of the new shoots and needle growth of pines can be observed. The lower branches are the first and most seriously affected. Infected twigs commonly exude resin resulting in the stunted, dead, brown, needles sticking to the twig. Female cones may be killed before full development becoming dark, shrunken and deformed (Rees and Webber 1988; Georgiev and Hlebarska 2017; Filiz et al. 2018; Hu et al. 2018).
Hosts – Broad range of hosts, including Myrtaceae, Pinaceae, Rutaceae, Santalaceae, and Vitaceae.
Morphological based identification and diversity
Over 600 species names are listed in Index Fungorum (2019), but few of them are currently in use and for most species cultures are not available except for S. citrigena (A.J.L. Phillips et al.) A.J.L. Phillips & A. Alves, S. eucalypti Berk. & Broome, S. porosa (Van Niekerk & Crous) A.J.L. Phillips & A. Alves, S. variabilis F.J.J. Van der Walt, Slippers & G.J. Marais and S. visci (Alb. & Schwein.) Sacc. Pycnidial paraphyses in Sphaeropsis species distinguish this genus from Diplodia species, which do not have paraphyses. The aseptate, smooth-walled conidia of Sphaeropsis species differentiate them from Lasiodiplodia species, which have 1-septate, striate conidia. Recently, S. variabilis was transferred to a separate genus, Oblongocollomyces due to distinct morphological differences (Yang et al. 2017).
Colony and conidial morphology are the primary characters to identify species within this genus (Ellis 1971, 1976; Simmons 1992). The sexual and asexual morphs connectivity of Sphaeropsis was established by Phillips et al. (2008) who obtained coelomycetes with large, brown, aseptate conidia typical of Sphaeropsis from a Phaeobotryosphaeria culture. Sphaeropsis species can be distinguished from Diplodia species in having pycnidial paraphyses. Lasiodiplodia differs from Sphaeropsis in having striate conidia, whereas Sphaeropsis has smooth conidia. Four Sphaeropsis species have been identified from culture. Sphaeropsis porosa differs from other species in having distinct pitted conidial walls. Sphaeropsis visci and S. citrigena can be distinguished from each other with their conidial pigmentation and swollen paraphyses tips (Phillips et al. 2013).
Molecular based identification and diversity
Phillips et al. (2013) suggested that phylogenetic analysis of combined SSU, LSU, ITS, tef1 and TUB2 genes provide better resolution compared to ITS alone. This study provides the phylogenetic analyses of combined ITS, LSU, SSU, tef1 and TUB2 sequence data (Table 10, Fig. 16). The topology of the Sphaeropsis species tree is identical to the phylogeny tree of Phillips et al. (2013).
Recommended genetic markers (genus level) – LSU and SSU
Recommended genetic markers (species level) – ITS, tef1 and TUB2
The accepted number of species: There are 624 species epithets in Index Fungorum (2019) under this genus. However, only four species have sequence data.
References: Phillips et al. 2013; Yang et al. 2017 (morphology, phylogeny).