Biosecurity capacity building for the Australian avocado industry: Laurel Wilt

Abstract

Ambrosia beetles share one special feature with humans, the ability to farm. These weevil-sized beetles colonize wood but rather than consuming this rather nutrient-poor material, they inoculate the tree with a particular type of fungus, which then becomes the food for the beetle larvae. Many tree species have evolved specialized chemical defences against this type of insect infestation and normally the fungal symbiont is only weakly pathogenic. However, there are some notable exceptions to this rule where perfectly healthy trees have succumbed to ambrosia
beetle infestation and their fungal symbionts have acted as aggressive pathogens. One classic example occurs in the south-eastern states of the USA, where cultivated avocado and its wild relatives are afflicted by a lethal disease called laurel wilt. The culprit is the beetle Xyleborus glabratus, which transmits the fungus Raffaelea lauricola. X. glabratus is native to Asia and entered the USA as a stowaway in wooden crates. X. glabratus is considered one of the most serious biosecurity threats to the Australian avocado industry.
In a pre-emptive move to prevent X. glabratus entering Australia, scientists from Brisbane have visited their counterparts in the USA to learn about laurel wilt disease. As a result of this collaboration, a diagnostic manual for laurel wilt has been produced, which will become the national standard for Australia. During preparation of this manual, new molecular diagnostic assays for both X. glabratus and R. lauricola were developed. Armed with these new assays, ambrosia beetle surveys were done in subtropical and tropical avocado production areas of the east coast of Australia and thankfully no X. glabratus found, confirming Australia’s pest-free status. However, there is no room for complacency, as two more ambrosia beetle species were discovered (Euwallacea sp. aff. fornicatus and Microperus sp.) were found on the Sunshine Coast and Atherton Tablelands in Queensland. Infestations of
these beetles were associated with avocado canopy thinning and most likely yield loss. The fungal symbionts of these ambrosia beetles were isolated and shown to be new species in the genera Fusarium and Bionectria. When pure cultures of these fungi were injected into the stems of young avocado plants, large brown lesions were produced in the sapwood, confirming that these fungi were causing disease.