Drivers of Barrier Permeability and Their Role in Determining Phosphite Treatment Efficacy for Kauri Dieback: An Epidemiological Analysis

ABSTRACT

Emergent forest Phytophthora pathogens are routinely managed in a context of uncertainty where basic epidemiological understanding must be developed alongside disease management experiments. Phytophthora agathidicida causes the emergent disease kauri dieback, which threatens kauri ( Agathis australis ), a southern‐hemisphere conifer comprising some of the world's largest trees. Disease management often includes phosphite injections to reduce kauri susceptibility and slow or prevent spread between invaded and uninvaded stands. We adapt an existing spatially‐explicit and multi‐host forest epidemiological model (SODDr) to understand where incompletely understood treatment and epidemiological mechanisms may result in disease emergence across a treated host barrier. We parameterize our model with published data to evaluate three key areas of uncertainty: (1) the impact of transmission from secondary hosts, (2) the epidemiological impact of phosphite treatments at the host level, and (3) the width (treatment area) of phosphite chemical barriers. Our model experiments routinely demonstrated the potential for unknown secondary hosts to overwhelm phosphite treatment benefits, including any secondary hosts that support relatively little sporulation. Also, it is unclear if phosphite treatments cure trees of infection and reduce transmission in real systems however, in the model when treated trees remain infected the barrier is rapidly overcome. Lastly, increasing the width of the disease treatment area resulted in consistent slowing of disease spread in the model landscape, regardless of the other factors. These results were consistent in a model landscape with uniform host densities and in a follow‐up simulation of 2250 unique host landscapes with random host distribution. Our analysis suggests understanding the outcome of chemical treatment at the individual tree level along with a robust understanding of secondary host competency will be critical to effective kauri dieback management.