The secret behind South Africa's disappearing urban forests

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 polyphagous shothole borer was first identified in Pietermaritzburg: The lane of London plane trees where the polyphagous shothole borer was first identified in Pietermaritzburg (FABI)

The PSHB: The polyphagous shothole borer which is responsible for the destruction (S Bush/FABI)

Infecting indigenous and non-indigenous trees: 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. (FABI)

The PSHB and how it affects trees by boring galleries through them and spreading a fungus it carries to feed its larvae.: The PSHB and how it affects trees by boring galleries through them and spreading a fungus it carries to feed its larvae. (FABI)

Avocado trees are among the most susceptible agricultural crop trees to PHSB infestation and Fusarium disease. : An affected avocado tree (FABI)

The polyphagous shot hole borer is nuts about pecan trees.: An affected pecan nut tree (FABI)

A healthy tree is lush and green: The work that researchers at FABI do is to ensure that we have healthy trees for a healthy environment, ecosystem, and food supply. (FABI)

The polyphagous shot hole borer (PSHB) manifests differently on species of trees: It is generally marked by the iconic holes it leaves in branches which look like the tree has been shot at with a gun. As the PSHB bores through the tree to create its galleries/tunnels where it lays its larvae to feed off the fungus it carries, the trees start to ooze sap as they slowly die. (FABI)

An example of a South African reproductive host tree: A series of images showing how a keurboom is affected by the polyphagous shot hole borer, right down to its core. (FABI)

An example of a South African non-reproductive host tree: A monkey plum tree affected by the PSHB (FABI)

The lane of London plane trees where the polyphagous shothole borer was first identified in Pietermaritzburg
The polyphagous shothole borer which is responsible for the destruction
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.
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.
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.
The PSHB and how it affects trees by boring galleries through them and spreading a fungus it carries to feed its larvae.
An affected avocado tree
An affected pecan nut tree
The work that researchers at FABI do is to ensure that we have healthy trees for a healthy environment, ecosystem, and food supply.
It is generally marked by the iconic holes it leaves in branches which look like the tree has been shot at with a gun. As the PSHB bores through the tree to create its galleries/tunnels where it lays its larvae to feed off the fungus it carries, the trees start to ooze sap as they slowly die.
A series of images showing how a keurboom is affected by the polyphagous shot hole borer, right down to its core.
A monkey plum tree affected by the PSHB

Researchers

Dr Trudy Paap

Dr Trudy Paap is a research fellow at the University of Pretoria's Forestry Agriculture and Biodiversity Institute. Worldwide forests are increasingly under threat from invasive pests and pathogens. While impacts of tree health disasters are experienced locally, the drivers of their emergence are global; therefore, a strategy for dealing with pests and pathogens is urgently required at a global level. The International Plant Sentinel Network (IPSN) has been established with the objective of functioning as an early warning system to identify new and emerging pest and pathogen risks, and her current research will use the IPSN as background to improve surveillance and identification of new and emerging pest and pathogen risks by using botanic gardens and arboreta as sentinel sites for tree health monitoring. The focus area of detection is central to the project, but in addition, using sentinel plants within botanic gardens also provides relevant information on a particular pest, including potential host range, which can then be used to determine the risk a pest poses and potentially aid in the development of appropriate eradication and/or containment programmes.

Her previous research included the detection and management of Phytophthora cinnamomi in natural ecosystems of south west Western Australia, and understanding the underlying causes of and determining practical management solutions to canker disease of Corymbia calophylla (marri), a keystone tree species of SWWA, caused by the endemic fungal pathogen Quambalaria coyrecup.

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