Identity and pathogenicity of Seiridium species causing cypress canker in South Africa.

Atkins, S.*1, Roets, F.1, Wingfield, B. D.2, Wingfield, M. J.2, Aylward, J.1,2

1 Department of Conservation Ecology and Entomology, Stellenbosch University, Private Bag X1 Matieland, 7602, South Africa
2 Department of Biochemistry, Genetics and Microbiology, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Private Bag X20 Hatfield, 0028, South Africa

Cypress canker is an important fungal disease primarily affecting trees in the Cupressaceae. A recent taxonomic revision has shown that the disease can be caused by at least seven closely related Seiridium species. Cypress canker on species of Cupressus and Hesperocyparis has been known in South Africa since the 1980's. Its relevance was accentuated in 2021 when it was discovered on native Widdringtonia nodiflora trees in Franschhoek, Western Cape Province. The causal agent was identified as S. neocupressi, a previously undocumented species in South Africa. This highlighted a lack of information regarding cypress canker in South Africa and prompted the present study to document the occurrence of the disease throughout the country. Phylogenetic analyses of isolates from cypress canker symptoms revealed the existence of six Seiridium species, including an undescribed species. Seiridium cardinale was most abundant in the Western Cape, with S. neocupressi found sporadically. Seiridium unicorne predominated in the Gauteng and Free State provinces. The remaining species, S. cancrinum, S. kenyanum and the undescribed species, were each recovered only in localised areas. Stem inoculations using 11 isolates, representing four locally occurring species, on the susceptible host, x Hesperocyparis leylandii, showed that S. cardinale, S. neocupressi, and the undescribed species are highly pathogenic. In contrast, S. unicorne exhibited variable levels of pathogenicity and was less aggressive than the other species tested. This research represents the first extensive consideration of cypress canker in South Africa and one of few studies on this important disease in the Southern Hemisphere.

Keywords: fungi, pathogen, Cupressaceae, phylogenetics, inoculation, survey