Research Features

The Polyphagous Shothole Borer (PSHB) and its fungus in South Africa

FABI information pamphlets on the PSHB

PSHB - Information sheet     PSHB - Life stages of the beetle     PSHB - How to sample

PSHB - Symptoms     PSHB - External symptom types     PSHB - How to photograph for diagnosis

Californian information pamphlets

Background

As participant in the International Plant Sentinel Network (IPSN), Dr Trudy 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 200 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, Brett Hurley, 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. A current list of hosts identified from these surveys are given below.


# The taxonomy of the Euwallacea fornicatus species complex was recently revised (Gomez et al. 2018). The correct species names that should thus be used are:

1. Euwallacea fornicatus (Eichhoff 1868) = TSHB-a ‘Tea Shot Hole Borer Clade a’

2. Euwallacea fornicatior (Eggers 1923) = TSHB-b 'Tea Shot Hole Borer Clade b’

3. Euwallacea whitfordiodendrus (Schedl 1942) = PSHB ‘Polyphagous Shot Hole Borer’ [the species in South Africa]

4. Euwallacea kuroshio Gomez and Hulcr = KSHB ‘Kuroshio Shot Hole Borer’


Current host trees on which infestations in South Africa have been confirmed

REPRODUCTIVE HOST TREES1

Exotic species

 

 

Native SA species

Latin name

Common name

 

 

Latin name

Common name

Acacia melanoxylon

Blackwood

 

 

Combretum krausii

Forest bushwillow

Acacia mearnsii

Black wattle

 

 

Erythrina caffra

Coast coral tree

Acer buergerianum

Trident (Chinese) maple

 

 

Podalyria calyptrata

Water blossom pea

Acer negundo

Boxelder

 

 

Psoralea pinata

Fountain bush

Acer palmatum

Japanese maple

 

 

Salix mucronata

Cape willow

Brachychiton discolor

Pink flame tree

 

 

 

 

Gleditsia triacanthos

Honey locust

 

 

 

 

Liquidambar styraciflua

American sweetgum

 

 

 

 

Magnolia grandiflora

Southern magnolia

 

 

 

 

Pearsea americana

Avocado

 

 

 

 

Platanus x acerifolia

London Plane

 

 

 

 

Quercus palustris

Pin oak

 

 

 

 

Quercus robur

English Oak

 

 

 

 

Ricinus communis

Castor bean

 

 

 

 

Salix alba

White willow

 

 

 

 

NON-REPRODUCTIVE HOST TREES2

Exotic species

 

 

Native SA species

Latin name

Common name

 

 

Latin name

Common name

Bauhinia purpurea

Butterfly orchid tree

 

 

Bauhinia galpinii*

Pride of De Kaap

Betula pendula

Silver birch

 

 

Buddleja saligna*

False olive

Camellia japonica

Common camellia

 

 

Calodendrum capense*

Cape chestnut

Carya illinoinensis

Pecan nut

 

 

Calpurnia aurea*

Geelkeurboom

Ceiba pentandra

Kapok

 

 

Combretum erythrophyllum*

River bushwillow

Cinnamomum camphora

Camphor

 

 

Cordia caffra*

Septee tree

Citrus limon

Lemon

 

 

Cussonia spicata*

Cabbage tree/ Kiepersol

Citrus sinensis

Orange

 

 

Diospyros dichrophylla*

Star apple

Eriobotrya japonicum

Loquat

 

 

Diospyros lycidioides*

Monkey plum

Erythrina livingstoniana

Aloe coral tree

 

 

Ekebergia capensis*

Cape ash

Eucalyptus camaldulensis

River red gum

 

 

Erythrina lysistemon*

Common coral tree

Ficus carica

Common fig

 

 

Ficus natalensis*

Natal fig

Fraxinus excelsior

European ash

 

 

Grewia occidentalis*

Cross berry

Jacaranda mimosifolia

Jacaranda

 

 

Gymnosporia buxifolia*

Spike thorn

Melia azedarach

Syringa

 

 

Halleria lucida*

Tree fuschia

Morus sp.

Mulberry

 

 

Harpephyllum caffrum*

Wild plum

Platanus occidentalis

American plane

 

 

Melianthus major*

Honey flower/ Kruidjie-roer-my-nie

Platanus racemosa

Californian plane

 

 

Nuxia floribunda*

Forest elder

Plumeria rubra

Frangipani

 

 

Olea europea subsp. africana*

Wild olive

Populus nigra

Lombardy poplar

 

 

Podocarpus falcatus*

Outeniqua yellowwood

Prunus nigra

Black plum

 

 

Podocarpus henkelii*

Henkel’s yellowwood

Prunus persica

Peach

 

 

Protea mundii*

Forest sugar bush

Psidium guajava

Guava

 

 

Rapanea melanophloeos*

Cape beech

Schinus molle

Pepper tree

 

 

Schotia brachypetala*

Weeping boerbean/ Huilboerboon

Taxodium distichum

Swamp cypress

 

 

Senegalia galpinii*

Monkey-thorn

Ulmus minor = procera

English elm

 

 

Vachellia karoo*

Sweet thorn

Ulmus parvifolia

Chinese elm

 

 

Vachellia sieberiana var. woodii*

Paper bark thorn

Viburnum sinensis

Viburnum

 

 

Virgilia divaricata*

Keurboom

Vitis vinifera

Grapevine

 

 

 

 

 

 

 

 

 

 

1 Host trees in which both the beetles and the fungus establish, and where the beetle successfully reproduce. In most cases the reproductive hosts will eventually be killed by the fungus.

2 Host trees that are attacked by the beetle and where the fungus establishes, but where the beetle does not successfully breed. The fungus might, or might not cause disease and kill the trees.


Paap T, de Beer ZW, Migliorini D, Nel W, Wingfield MJ. (2018) The polyphagous shot hole borer (PSHB) and its fungal symbiont Fusarium euwallaceae: a new invasion in South Africa. Australasian Plant Pathology 10.1007/s13313-018-0545-0 PDF

Gomez DF, Skelton J, Steininger MS, Stouthamer R, Rugman-Jones P, Sittichaya W, Rabaglia RJ, Hulcr J. (2018) Species delineation within the Euwallacea fornicatus (Coleoptera: Curculionidae) complex revealed by morphometric and phylogenetic analyses. Insect Systematics and Diversity 2(6):1-11. 10.1093/isd/ixy018 PDF

De Beer ZW. (2018) A tiny beetle and its deadly fungus is threatening South Africa’s trees. The Conversation (27 February) http://bit.ly/2F0J2Ln PDF