Cypress canker is killing exotic cypress trees in South Africa and poses a serious threat to native cedars, setting alarm bells ringing among members of the scientific community …
Welkom, the second-largest city in the Free State province of South Africa, is often referred to as a ‘city within a garden’ thanks to its numerous parks and trees. Unfortunately, many of these trees are dying from a disease known as ‘Cypress canker’.
Italian cypress (Cupressus sempervirens) is a popular ornamental tree that has been widely planted in Welkom, and is found on traffic circles, in parks, private gardens and along entire avenues in several neighbourhoods. Italian cypress is, however, also particularly susceptible to cypress canker.
The disease is caused by a number of fungal species in the Xylariales and specifically in the genus Seiridium. Symptoms include bleeding cankers that deform stems and cut off the water supply above the canker. As a result, diseased trees show typical “flagging” symptoms resulting in the death of affected branches.
Dr Janneke Aylward, a Research Fellow at the Forestry & Agricultural Biotechnology Institute (Fabi) based at the University of Pretoria, recently visited Welkom as part of a project to investigate the cause of cypress canker in South Africa. This project was prompted by the recent discovery of cypress canker on native Widdringtonia nodiflora cedar trees in South Africa, which poses a real threat to native cedar species.
During her visit Janneke, assisted by staff of the Matjhabeng Local Municipality’s Department of Parks, Sports and Recreation, took samples of diseased cypress trees at 11 sites across Welkom. She will isolate the canker fungus present in the diseased branches and investigate their identity and diversity.
Native cedars at risk
According to Janneke, the presence of cypress canker in one of our native cedars - the Mountain cedar Widdringtonia nodiflora – has set the alarm bells ringing in scientific circles. The disease was recently identified in Mountain cedar at one site in Franschoek. To date the disease has not been found on Widdringtonia wallichi (Clanwilliam cedar) in the Cederberg, and the Fabi team has not yet visited the Baviaanskloof in the Eastern Cape to look for the presence of cypress canker in Widdringtonia schwarzii (the Willowmore cedar).
The Clanwilliam cedar is critically endangered and the Willowmore cedar is near threatened, according to IUCN. Janneke believes that these two cedars are only found in a very specific range, so the chances are good that they have had little or no contact with other cedars that may carry the disease.
Janneke believes that the Mountain cedar will survive the canker because it's a species that re-sprouts after fire. When the fynbos vegetation burns, the pathogen should be destroyed and the infected trees will likely re-emerge. The other two native cedars, the Clanwilliam cedar and Willowmore cedar, do not re-sprout so cypress canker - combined with the fact that their populations are already in decline - could have devastating consequences.
According to a recent article in the scientific journal, Plant Pathology, in July 2022: ‘Cypress canker has been known in South Africa for many decades, where it causes a serious disease on non-native species of Cupressus, but it has never been found on native Cupressaceae. The newly discovered disease caused by a probable alien pathogen is of particular concern because only three species of Widdringtonia occur in South Africa and they are important components of the native flora.’ (https://doi.org/10.1111/ppa.13614)