Seiridium insolitum sp. nov.: a new canker pathogen on Cupressus and Hesperocyparis in Southern Africa

Cupressaceae trees in many parts of the world are affected by cypress canker, a disease caused by fungi in the genus Seiridium . A recent survey of diseased Cupressaceae in South Africa reported five of the seven described Seiridium canker pathogens from the country. In addition, an unknown Seiridium species was found on visible cankers of Cupressus sempervirens and Hesperocyparis glabra trees at two locations on either side of the Lesotho-South Africa border. The aim of this study was to describe the fungus and consider its possible pathogenicity. The identity of the strains was investigated with maximum likelihood multi-gene phylogenetic analyses and by morphological comparisons with most closely related taxa. The genome of one strain was sequenced on the Illumina platform, and the pathogenicity of that strain tested in stem inoculations on Leyland cypress (× Hesperotropsis leylandii ). Phylogenetic analyses resolved a well-supported monophyletic lineage, with S. cupressi as its sister taxon. The fungus grew optimally at 25 °C and conidiomata bearing conidia larger than those of S. cupressi formed at 30 °C. The species, described as Seiridium insolitum sp. nov., produced significant necrotic lesions in inoculations and could be re-isolated from the diseased tissues. Seiridium insolitum is the sixth species known to cause cypress canker in South Africa and the only one known exclusively from the Southern Hemisphere. Its discovery highlights the extent to which the trade in exotic Cupressaceae , primarily for ornamental use, has introduced Seiridium species into new regions of the world.