Genomics and resistance assays inform the management of two tree species being devastated by the invasive myrtle rust pathogen

Myrtle rust is a plant disease caused by the invasive fungal pathogen Austropuccinia psidii (G. Winter) Beenken, which has a global host list of 480 species. It was detected in Australia in 2010 and has caused the rapid decline of native Myrtaceae species, including rainforest trees Rhodamnia rubescens (Benth.) Miq. (scrub turpentine) and Rhodomyrtus psidioides (G.Don) Benth. (native guava). Ex situ collections of these species have been established, with the goal of preserving remaining genetic variation. Analysis of reduced representation sequencing (DArTseq; n = 444 for R . rubescens and n = 301 for R . psidioides ) showed genetic diversity is distributed along a latitudinal gradient across the range of each species. A panel of samples of each species ( n = 27 for R . rubescens and n = 37 for R . psidioides ) were resequenced at genome scale, revealing large historical e]ective population sizes, and little variation among individuals in inferred levels of deleterious load. In Rhodamnia rubescens , experimental assays ( n = 297) identified individuals that are putatively resistant to myrtle rust. This highlights two important points: there are tangible pathways to recovery for species that are highly susceptible to rust via a genetically informed breeding program, and there is a critical need to act quickly before more standing diversity is lost.