As participant in the International Plant Sentinel Network (IPSN), Dr Paap is tasked to do routine surveys for tree pests and diseases in and around the National Botanical Gardens of South Africa in collaboration with SANBI. It was during such a survey that she noticed small lesions resembling shotgun marks on the stems and branches of mature London plane (Platanus x acerifolia) trees planted along the street outside the KZN Botanical Gardens. Upon closer inspection, she found that the lesions developed around entrance holes of small beetles. When she removed the bark, the sapwood was discoloured by a fungus. She brought samples back to FABI, and with the assistance of FABI team members, the beetle and fungus were identified based on DNA sequences as Euwallacea fornicatus (polyphagous shothole borer or PSHB) and Fusarium euwallaceae, respectively. The results were published in the Australasian Journal of Plant Pathology (Paap et al. 2018).
The discovery of this beetle and fungus in South Africa is of major concern to foresters, farmers and landscapers, as these organisms are known as aggressive tree killers. The PSHB is a 2 mm long ambrosia beetle that is native to Southeast Asia. The beetle carries several fungal species, one of which is Fusarium euwallaceae, with it when it infests new trees. It bores through the bark into the sapwood of the trees and inoculate the fungus into living wood. The fungus grows in the galleries (tunnels) of the beetle and serve as 'vegetable garden' for the beetle larvae, but in susceptible trees the fungus can spread through the sapwood causing disease or even death of the tree. In its native environment in Southeast Asia, it seems as if the beetle and fungus do not cause serious damage because tree species have evolved with the beetle-fungus complex and have resistance towards them. However, the beetle and fungus were somehow introduced into Israel and California during the past 15 years where they cause serious damage on especially Avocado trees.
In California, surveys in botanical gardens revealed that more than 270 tree species can be infested, damaged and often killed, by these organisms (Eskalen et al. 2013). The list of susceptible trees in California included several species native to southern Africa, including Cussonia spicata (cabbage tree), Calpurnia aurea (common calpurnia), Diospyros lycioides (monkey plum), Erythrina humeana (dwarf coral tree), Erythrina lysistemon (common coral tree), Schotia brachypetala (huilboerboon), Melianthus major (honey flower, kruidjie-roer-my-nie), Cunonia capensis (red alder), Nuxia floribunda (forest elder) and Bauhinia galpinii (red orchid bush). Most of these species showed some level of susceptibility to Fusarium dieback, except the last three species that were infested by the beetle but did not develop disease. According to Eskalen et al. (2013), several commercial crop trees that are also planted in South Africa, such as Persea americana (avocado), Macadamia integrifolia (macadamia nut), Carya illinoinensis (pecan), Prunus persica (peach), Citrus sinensis (orange) and Vitis vinifera (grapevine), are susceptible to PSHB infestation and Fusarium dieback. In addition, many exotic tree species planted as ornamentals in South Africa, are also susceptible. These include species of maple, holly, wisteria, oak and Camellia.
Dr Paap, together with a team of FABIans under the supervision of Profs Wilhelm de Beer and Noelani van den Berg, will continue to monitor the spread and host range of the beetle in South Africa. They will also assess the risks posed to crop trees (e.g. avocado), commercial forests (e.g. eucalypts, wattle) and ornamental trees (plane, maple, etc.) in South Africa and investigate possible control measures.