International Year of Plant Health Part 1: The secret behind SA’s disappearing urban forests

20 July 2020

Dr Trudy Paap is a participant in the International Plant Sentinel Network who was tasked to do routine surveys for tree pests and diseases in the National Botanical Gardens of South Africa, a project funded by the South African National Biodiversity Institute. It was during one of these surveys that she noticed small lesions resembling shotgun marks on the stems and branches of mature London plane (Platanus x acerifolia) in the historical avenue of the KwaZulu-Natal National Botanical Garden in Pietermaritzburg.

The lane of London plane trees where the PSHB was first identified in Pietermaritzburg

"Upon closer inspection, I found that the lesions developed around entrance holes of small beetles. When I removed the bark, the sapwood was discoloured by a fungus. I brought samples back to UP’s Forestry and Agricultural Biotechnology Institute (FABI), and with the assistance of FABI team members, the beetle and fungus were identified based on DNA sequences as Euwallacea fornicatus, or polyphagous shot hole borer (PSHB), and Fusarium euwallaceae, respectively," said Dr Paap.

Polyphagous shothole borer

The PSHB, which is responsible for the destruction

Polyphagous refers to the ability of the PSHB to infest many different tree species. The results were published in the journal, Australasian Plant Pathology (Paap et al 2018).

Since its discovery in KwaZulu-Natal in 2017, the FABI team has confirmed the presence of the PSHB in eight of the nine provinces of South Africa.

The discovery of this beetle and fungus in South Africa is of major concern to farmers, foresters, landscapers, homeowners and ecologists, as together, these organisms can be aggressive tree killers.

"The most visible impact of the PSHB invasion in South Africa is in urban forests on street, park and garden trees, and this became the focus of many articles in the media. Many trees have been killed by the PSHB in Sandton, George, and Knysna, while reports from Sedgefield, Bloemfontein, Ekurhuleni, Jan Kempdorp, Hartbeesfontein, Pietermaritzburg and Durban suggest that the impact is becoming worse in those areas. The most common trees to be killed are English oak, Chinese maple, Japanese maple, boxelder, and coral trees," said Dr Paap.

Of great concern is the recent discovery of the PSHB on London plane and sweetgum trees in Somerset West in the Cape Peninsula. Oak trees appear to be a favoured reproductive host, and have been severely impacted by this invasive pest; this has been the case especially in the George and Knysna areas, where many large, old specimens have been killed in a relatively short period of time. As such, it seems inevitable that the famous oaks of Stellenbosch and the surrounding wine farms will be dramatically impacted. Other commercial crops like avocados and pecan nuts are also at risk

It is now known that the beetle is spreading from urban areas into native forests close to the towns of George, Knysna, and Durban. However, which species will be affected and to what extent is unpredictable.

   Outeniqua yellowwoods affected by PSHB

An Outeniqua yellowwood infested with a PSHB. Notice how the boring appears on the surface of the bark and how it extends deeper into the core.

Several native tree species were found to be infested in the gardens of Sandton, George, and Knysna, with species like the coral tree, keurboom and Cape willow being particularly vulnerable and often killed.

FABI researchers are working on several projects to collaborate and try to manage the invasion and to explore ways of possibly containing the spread from a multitude of angles with key role players from representing policy, industry and research. 

The lane of London plane trees where the PSHB was first identified in Pietermaritzburg