Environmental drivers of spatial variation in myrtle rust development on a critically endangered tree species

The spread of invasive plant pathogens is on the rise globally, introducing additional threats to forest ecosystems and their constituent species above those already caused by climate change and other anthropogenic stressors. Austropuccinia psidii is a widespread invasive fungus that infects hundreds of species in the Myrtaceae plant family, causing myrtle rust. It was first detected in New Zealand in 2017, where the threatened endemic tree Lophomyrtus bullata is one of the worst affected hosts. Recent studies have described heavy disease severity on this species but environmental drivers of spatial variation in disease severity have not been explored, including the possible role of edge effects which are prevalent across much of this species' highly fragmented natural range. In this study, we tracked changes in disease incidence and severity across a forest edge gradient over three years. Individuals further from the forest edge, where forest canopies were more intact, forest structure more complex, and with greater understorey humidity, had greater disease severity in the first year of infection. In subsequent years, the influence of edge was no longer evident as myrtle rust severity became more even across the site, but disease was more prevalent in denser L. bullata populations. Only small individuals were asymptomatic across all three years. Since all available myrtle rust control techniques are currently feasible only at small scales, our research implies that their application needs to be as early as possible before infection spreads and across all suitable microclimates, and without bias toward easily accessible forest edges.