Fusarium kuroshium is the primary fungal symbiont of an ambrosia beetle, Euwallacea fornicatus, and can kill mango tree in Japan

Zi-Ru Jiang, Momo Tanoue, Hayato Masuya, Sarah M. Smith, Anthony I. Cognato, Norikazu Kameyama, Keiko Kuroda & Hisashi Kajimura
Scientific Reports volume 13, Article number: 21634 (2023)

Abstract
This study identifies fungi associated with Euwallacea fornicatus and determines whether these fungal species play the role of primary symbiont. E. fornicatus adults that emerged from the branches of infested trees in Okinawa main island, Japan, were collected and used to isolate fungi. Fusarium kuroshium and Penicillium citrinum were the most dominant fungal associates of females and males, respectively. F. kuroshium was much more frequently isolated from the head, including mycangia (fungus-carrying organs), of females than any other body parts. We inoculated healthy mango saplings with F. kuroshium or F. decemcellulare, both of which were symbionts of E. fornicatus females infesting mango trees. F. kuroshium decreased leaf stomatal conductance and rate of xylem sap-conduction area and increased length and area of xylem discoloration of the saplings, thereby weakening and killing some. These results suggest that F. kuroshium, a mycangial fungus of E. fornicatus, inhibits water flow in mango trees. This study is the first to report that F. kuroshium causes wilt disease in mango trees and that it is a primary fungal symbiont of E. fornicatus.

https://www.nature.com/articles/s41598-023-48809-8

Introduction
Polyphagous shot-hole borer, PSHB (Euwallacea fornicatus) is a tiny exotic wood boring beetle native to south-east Asia (Figure. 1). The beetle is highly invasive, attacking a wide range of both healthy and dead or dying trees. It burrows into tree trunks and branches where it lives in association with an ambrosia fungus that can cause infection of the plant tissues (Figure. 1). Eventually the beetles chew their way out of the wood to find a new host, producing shot holes in the tree.

In 2003, the beetle was detected in California and has since spread to Hawaii (2007), Israel (2009), South Africa (2017) and Palestine (2019). All incursions have proved challenging to manage. The impact has been predominantly in urban environments but there has been some cross-over into agriculture and natural ecosystems.

In 2021, PSHB was detected for the first time in Australia in a Box Elder Maple tree in East Fremantle, Western Australia and has since been found throughout the Perth metropolitan region. Western Australia is currently undertaking an extensive management and eradication program.

Polyphagous shot-hole borer is not present in Victoria
Close up of pest beetle sitting on fungus.Figure 1. Close up of PSHB sitting atop the ambrosia fungus.

PSHB life history
PSHB female beetles are slightly less than 2 mm long or about the size of a sesame seed (Figure. 2) Males are even smaller, around 1.6 mm. he egg, larvae, pupa and male adult spend their entire life within the tree. Only the females disperse to new hosts where they penetrate the wood to lay 20-50 eggs. Development time from egg to adult usually takes 30-40 days but can be shorter in hot weather (22 days at 24°C). A female is able to produce males from unfertilised eggs and is able to start a whole new colony on her own.

A small beetle next to 5 cent piece.Figure 2. PSHB next to five cent piece showing relative size.

Relationship with the ambrosia fungus
The beetle has a close, mutually beneficial relationship with the ambrosia fungus which it cultivates as a food source. Fusarium euwallaceae is the most commonly associated ambrosia fungal species discovered in association with PSHB. However, in Australia the beetle is associated with an unnamed species, Fusarium sp. [AF-18], which means any potential impacts may vary from overseas experience.

PSHB have small sacs in their mouth which store the spores of the fungus. Once the beetle has bored its way into a tree, it creates pathways (called ‘galleries’) within the branches and trunks of the tree (Figure. 3). The fungus is planted in these galleries and left to germinate and grow.

PSHB and the Fusarium fungus cannot survive without each other. Without the fungus, the beetle does not have a food source. Without the beetle, the fungus has no way of being transported to different places within the tree or new hosts. Combined, the two species cause branch dieback and tree death by disrupting the plants vascular system and flow of water and nutrients (Figure. 4).

Tree branch section showing beetle galleriesFigure 3. Galleries produced by the beetle burrowing into a tree branch.

Host trees
The PSHB has a host range of more than 600 tree species.

Trees in which the beetle is able to complete its life cycle (“reproductive hosts”) are preferred and are more likely to be heavily damaged through infestation. In California, South Africa and Israel the Box Elder Maple tree (Acer negundo) has been observed as the highest risk reproductive host.

Western Australia has identified the Box Elder Maple (Acer negundo) and the Black Locust (Robinia pseudoacacia) as its two highest risk host trees.

Other high risk reproductive hosts found in WA include:

Coral Tree (Erythrina x sykesii, Erythrina spp.)
Morton Bay Fig (Ficus macrophylla)
Port Jackson Fig (Ficus rubiginosa)
London Plane (Platanus x acerifolia)
White mulberry (Morus alba)
Black mulberry (Morus nigra)
Poinciana (Delonix regia)
Mirror bush (Coprosma repens)
Trees inhabited by the beetle but do not support beetle reproduction (“non-reproductive hosts”) can often tolerate PSHB damage. The fungus does not thrive or become established in these trees.

What to look for
Tree monitoring plays an important role in the early detection of PSHB which is vital in any eradication program. Unfortunately, the beetle’s size and habit of spending most of their life inside the tree makes them hard to spot. There are, however, a few symptoms to look for which indicate the beetle could be present.

With an established infestation you will see multiple entrance holes (shot holes) on the trunk or branches that are approximately the size of a ballpoint pen tip (0.85 mm in diameter) (Figures. 5 & 6).

The occurrence of frass (fine powdery refuse produced by the boring insects) on the trees indicates the presence of the insect activity (Figure. 7).

Noodles, which are thin tubules of compacted frass emerging from shot holes, are often a sign that the tree is heavily invested (Figure. 8).

Gumming is caused by excessive thick resin or sap being pushed out of the shot holes. In the process the sap can push the beetle out of the gallery aiding the monitoring process (Figure. 9)

Discoloration or staining of the outer bark around the shot holes is caused by resin (Figure. 10).

Sugar volcanoes are caused by the trees response to the disruption of the vascular system by the borer. A sugary substance is exuded from the entry and exit holes. As it dries it forms a white crystalline ring around the shot hole. (Figures. 11& 12). Sugar volcanoes are common on avocado trees overseas.

Tree with dying branches.Figure 4. Branch dieback is a common symptom of PSHB infestation

Tree trunk covered with small holes.Figure 5. Shot holes in a Box Elder Maple tree trunk

Man holding ruler near small hole in a tree trunk.Figure 6. Recently infected tree showing relative size of a shot hole.

Tree covered in powdery residue (frass) on trunk.Figure 7. Excessive frass on a Box Elder Maple tree trunk.

Tree trunk with thin tubules of frass (noodles) protruding from holes.Figure 8. “Noodles” protruding from PSHB shot holes.

Tree branch with gummy resin oozing from holes.Figure 9. Gumming refers to resin being expelled from the shot holes.

Tree with brown staining on trunk.Figure 10. Discolouration on the trunk of a Box Elder Maple tree.

Tree with white dried sugar on trunk.Figure 11. Sugar volcano on an avocado tree.

Tree with white sappy material on trunk.Figure 12. Sappy sugar volcano on a Morton Bay fig tree.

Impact of PSHB
Many recorded hosts are commonly planted as street, amenity and ornamental trees or are established, often iconic, trees gracing city parks and botanic gardens. Large scale removal of such trees is likely to be socially unacceptable as the trees provide significant advantages, particularly in an urban setting. Such benefits include aesthetics, noise and air pollution mitigation and maintenance of biodiversity. Removal of street trees would also negatively affect house prices and increase home heating and cooling costs. In California and South Africa serious damage to urban trees from PSHB infestation has raised concerns about public safety.

In a commercial setting, the beetle has emerged as a damaging pest of avocado in Israel. Predominantly non-reproductive hosts from the nursery, forestry and fruit and tree nut industries, can become heavily infested when the reproductive host population is saturated.

The presence of the beetle will also have an impact on internal markets affecting movements of wood and plants for planting. PSHB can be present in trunks and branches as small as 2.5 cm in diameter resulting in restrictions of the movement of woody plants in the nursery trade.

How does PSHB spread?
Long distance spread of PSHB is facilitated by movement of infested plants and plant products.

Any trees with branches greater than 2.5 cm in diameter could harbour the beetle and move it to a new location. PSHB has also been found in wood, wood products and untreated wood packaging materials (pallets, dunnage, crates) and can be spread via long haul transport.

The beetle has the ability to survive in felled wood for many months. Firewood, green waste and woodchips larger than 2.5 cm should not be moved around.

Female beetles can fly and average of approximately 40 m and will naturally disperse into the immediate environment when seeking new host trees.

Management of PSHB
There is no known effective chemical treatment for the extermination of PSHB. As the beetle spends almost its entire life hidden in galleries, insecticides have a limited efficacy. The biology of beetle also makes it a challenging candidate for biological control.

Controlling the spread of the beetle is best managed by removal of heavily infested reproductive host trees, pruning of infested branches and continued monitoring round high impact sites.

What should I do if I think I have found PSHB?
If you think that you have found PSHB and/or PSHB symptoms report them immediately to using the Agriculture Victoria public online reporting form or contact the Exotic Plant Pest Hotline 1800 084 881.

Take clear photographs of the tree symptoms and beetles, if you can find them, and upload them with your report. Good quality photos are important for rapid identification which is vital to any eradication program.

Reporting an unusual pest or disease of plants or honey bees
Report any unusual plant pest or disease immediately using our online reporting system. Early reporting increases the chance of effective control and eradication. Please take good quality photos of the pests or damage to include in your report where possible.

Alternatively, you can call the Exotic plant pest hotline on 1800 084 881.

Report online
Photo Credits
All photos courtesy of the Western Australian Department of Primary Industries and Regional Development.

https://agriculture.vic.gov.au/biosecurity/pest-insects-and-mites/priority-pest-insects-and-mites/polyphagous-shot-hole-borer#h2-8

The shot-hole borer is ravaging the city’s trees from within, while bark-strippers attack from without
BL PREMIUM - UNLOCKED BY RMB
27 APRIL 2023 - 05:00
By: ANNAMIA VAN DEN HEEVER

Cape Town’s leafy suburbs are in danger of losing up to 40% of their canopies to the same insect predator that has infected trees across the land, the tiny shot-hole borer beetle. There’s also a threat from people who strip the Cape Peninsula’s trees of their bark for medicinal purposes.

The beetle was first spotted in Newlands, Cape Town, in January and is spreading rapidly, often in firewood or pallets. “It could potentially be one of South Africa’s largest ecological tragedies,” according to TreeKeepers, an association dedicated to conserving Cape Town’s urban forest, which gives the 40% tree-loss estimate.

Though international and local researchers are working on solutions, there is no known way of eradicating the pests, other than felling susceptible reproductive host species and disposing of the wood responsibly.

The City of Cape Town has begun a social media campaign to inform residents. About 260 infested trees have been identified in the southern suburbs, according to mayoral committee member Patricia van der Ross. The number is increasing daily. Infected trees can contain more than 100,000 beetles.

The city has asked residents to report infestations to its invasive species unit, which provides assistance. Sightings are also encouraged on the social media platform iNaturalist.

“I don’t think that it’s any exaggeration to say that this is the most calamitous thing that has happened to the urban treescape in South Africa’s history. It’s like the rinderpest of trees,” arborist Francois Krige tells the FM.

“Civil society needs to get behind this. People often realise the severity of what they are facing when it’s too late.

“There is so much we can do right now: stopping the movement of firewood from region to region; not moving plant material in and out of zones of infestation; and supporting planting programmes, so that we will not be remembered as the generation that cut everything down.”

Krige and others suspect the beetle arrived in Newlands when an out-of-town rugby fan brought in wood for a braai near the sports stadium.

He believes the most important management tool available is the pre-emptive felling of box elder trees, the beetle’s preferred host. “The beetle jumps from one box elder to another,” he says. “So the initial stage of infestation is primarily a disease of box elders.” Other common hosts include oak, plane, poplar and coral trees. These are all suburban favourites.

Krige’s work includes tree management in Kirstenbosch Botanical Gardens and he is the chair of Friends of the Arderne Gardens. The Arderne Gardens in Claremont, which have one of the most diverse and valuable collections of exotic trees in Africa, are home to what Krige considers “the most iconic tree in Cape Town”, a towering Moreton Bay fig that has been a backdrop for wedding photographs for many decades.

This “wedding tree” may be at risk. The beetles have been found within 100m of the Arderne Gardens and Moreton Bay figs have been infested in Australia, where the tree comes from. An experimental protection programme, if fully implemented, could cost about R350,000 a year. “The responsibility weighs heavily on me,” says Krige.

The other calamity, illegal bark stripping, has caused serious damage in Newlands forest. Krige estimates it would take 100 years of zero tree felling, coupled with a well-funded alien clearing programme, to bring the forest to a safe and healthy state.

The Table Mountain National Park has an “integrated strategy” to combat the stripping of bark, which traditional healers use for a variety of ailments. The strategy includes intelligence gathering, law enforcement and “proactive solutions” such as painting mature and seed-producing trees with grey, water-based PVA paint, to make the bark less attractive for consumption.

“[The park] will [also] continuously implement law enforcement patrols, facilitate educational awareness campaigns through workshops with traditional healers, and set up a nursery to grow forest species for replanting trees,” according to park manager Megan Taplin.

Indigenous trees that are commonly stripped of their bark include the Cape beech, Cape chestnut, wild peach, assegai and stinkwood. “This has a huge impact on forest diversity as trees die a slow death due to the interruption of the nutritional transport system,” Taplin says.

SANParks says rangers arrested three bark strippers “in the act” last week soon after midnight in Newlands forest.

Bark- and root-harvesting have increased throughout the city’s southern and northern suburbs, CBD and conservation areas, says deputy mayor Eddie Andrews. The city has also resorted to painting trees.

He says bark stripping seems to increase before religious holiday periods such as Easter and Christmas.

The city is considering a new tree bylaw to address this issue more effectively. “As this is an extensive exercise, it may take some time to introduce,” according to Andrews.

https://www.businesslive.co.za/fm/fm-fox/2023-04-27-bored-to-death-in-cape-town/

Since it was discovered in KwaZulu-Natal in 2017, the Polyphagous Shot Hole Borer (PSHB) (Euwallacea fornicates) has spread rapidly into every province in South Africa except Limpopo, and has infested thousands of trees in urban areas, native forests and more recently in fruit crops.

This tiny invasive ambrosia beetle, native to Southeast Asia, has set alarm bells ringing as it is able to infest a wide variety of tree species – including seemingly strong, healthy trees - and is capable of spreading far and wide with the aid of human activity. It bores into the sapwood of trees and brings a damaging fungus, Fusarium euwallaceae, along with it.

The economic impact of PSHB in South Africa is still in its infancy, but is expected to run into the billions if allowed to continue unchecked. The environmental impacts are also potentially huge as a number of native trees are susceptible to infestation.

One of the management options being explored by scientists at the Forestry and Agricultural Biotechnology Institute (FABI) at the University of Pretoria, is to identify a natural enemy of PSHB in its native range and then introduce it into South Africa where it could play a role in controlling the PSHB population.

This led to a recent trip to northern Vietnam by Prof Brett Hurley and Dr Wilma Nel of FABI and PhD candidate Garyn Townsend to collect material from trees infested by PSHB with the aim of finding, identifying, rearing and testing natural enemies. The visit was highly successful and the team brought PSHB-infested Acacia back to the FABI quarantine facility, after obtaining the required permits from South Africa and Vietnam.

The plan is to monitor the material for the emergence of natural enemies, specifically parasitoids, which could be reared and deployed in South Africa to serve as an effective biocontrol agent for PSHB. This is a lengthy process that involves extensive testing to ensure that the introduced species is sustainable and does not cause any collateral damage to native species in South Africa.

The project has been made possible through the funding of the Department of Forestry, Fisheries & the Environment and through collaboration with the Vietnamese Academy of Forestry Sciences, specifically Prof Pham Quang Thu.

According to a recent article in the South African Journal of Science by a group of FABI scientists, the South African PSHB invasion represents the largest outbreak of this beetle in its invaded range anywhere in the world. It has infested 130 plant species in urban, agricultural, and native ecosystems in South Africa, including 44 previously unreported hosts.

In South Africa, PSHB is currently mostly confined to urban environments, but its presence has also been confirmed in the natural forests around Knysna and George where it is feared it could do extensive environmental damage.

Virgilia oroboides, a native tree species found in the southern Cape natural forests, is a reproductive host for PSHB.

“It is an important pioneer forest species in the southern Cape that protects forests from severe climatic fluctuations and fire and houses a large number of native organisms. Elimination of this single species could have irreversible consequences for native forest integrity,” the scientists write.

PSHB control measures using pesticide sprays have been effective in laboratory conditions, but are currently not considered practical in the wild. Deployment of a biocontrol agent looks to be the best management option at this stage, provided a suitable candidate can be found, and the necessary authorisation for its release in South Africa obtained.

The only agricultural crops that appear to be affected by PSHB so far are pecan and macadamia, but they are not reproductive hosts and so the impacts are expected to be limited. Other orchard crops that may be vulnerable to PSHB are cherry, apple, citrus, peach, guava, olive, grape and prune crops.

Initial surveys in invaded urban areas of Johannesburg, Knysna, George and Somerset West indicate that a high percentage of English oak, London plain trees, box elder and other maples will die when infested by PSHB. The cost of disposing of these urban trees and replacing them will be considerable.

Urban trees play a vital role in providing shade, moderating temperatures and creating an attractive environment that enhances the quality of life and boosts the hospitality and tourism sectors. They also provide refuge for numerous birds and insects.

How to detect PSHB infestation

The first signs of PSHB infestation are the presence of tiny holes penetrating the bark of trees and leading directly into the sapwood. These random holes, less than 1mm in diameter, look a bit like they could have been caused by shotgun pellets – hence the common name ‘shot hole borer’.

Lesions, wet patches and gum exudation may be visible on the outside of the tree, and a pinkish-brown stain caused by the associated fungusmay be seen spreading from the gallery into the vascular tissues below the bark. PSHB colonisation of reproductive hosts often results in wilting and death of the infested branch - or the entire tree.

The FABI team has had a lot of success with the introduction of biocontrol agents for a number of tree pests in South Africa in the past, and this experience will be invaluable in finding an effective biological enemy for PSHB. The stakes are high as this tiny hitchhiking beetle continues to spread across southern Africa.

Sources:

FABI News;

Article by W. Nel, B. Slippers, M. Wingfield, N. Yilmaz & B. Hurley in South African Journal of Science, April 2023

https://saforestryonline.co.za/articles/in-search-of-a-biocontrol-for-the-shot-hole-borer/?fbclid=IwAR01umpqyTlAI9AvsVQ8F8_8zw-HHBNjbxJlDqDeAR_72bZbRNiOmHoCmKg

In California, surveys in botanical gardens revealed that more than 200 tree species have been infested, damaged and often killed, by these organisms. An important distinction is being made between different types of infestations. Reproductive host trees are trees that the beetle infests and where it successfully establishes a breeding gallery in which the fungus grows, where eggs are laid, and larvae develop into mature adults, thus completing its life cycle. The majority of reproductive hosts eventually succumb to the disease symptoms caused by the fungus. Non-reproductive host trees are attacked by the beetle, but the beetles do not establish breeding galleries. The fungus may, or may not cause disease and these trees are generally not expected to die.

A problem when compiling host tree lists is that sometimes PSHB can infest a stressed tree (e.g. as result of drought, too much water, root damage, etc.). Such an individual, stressed tree might then become a reproductive host, whereas healthy growing individuals of the same species are barely affected. Thus, when trees are assessed for Fusarium Disease or whether it is a reproductive host, other stress factors on the tree should always be considered.

To date 161 tree species have been found infested in South Africa. These include 83 reproductive hosts and 78 non-reproductive hosts. The COMPLETE HOST TREE LIST (Last updated 2023-04-18 can be downloaded here: https://www.fabinet.up.ac.za/images/PSHB/PSHB_host_list_v6_20230417.pdf