The compounding impacts of disease and weeds after the 2019–20 wildfires on Australian vascular plants and communities

Many Australian plant species and ecological communities are threatened by weeds and diseases. The diseases affecting the most species are Phytophthora dieback (mostly Phytophthora cinnamomi) and myrtle rust (Austropuccinia psidii). • For many plant species and ecological communities, weeds and disease compound the impacts of fire. Following the 2019–20 wildfires, 186 fire affected plant species were considered to be highly vulnerable to the compounding impacts of disease, and three species to the compounding impacts of weeds. • The interactions of fire, weeds and disease may be complex. The impacts of Phytophthora and myrtle rust can be magnified by fire, partly through greater vulnerability of regenerating plant tissues, and because post-fire environments provide conditions more conducive to Phytophthora and myrtle rust. Fires may mobilise the sporeload of myrtle rust, facilitating its broad scale dispersal. • At least transiently, recently burnt areas may provide suitable conditions favouring some weed species, through flushes of nutrients, reduced competition with native species and increased light, and spread of weeds may then suppress post-fire recovery of native plants. Weed impacts after fire may be especially pronounced at or near modified environments. In some cases, weeds may increase fire severity; invasive grasses may trigger changes in fire regimes and transform ecosystems. Post-fire germination of weeds may deplete their soil seed bank, providing a time-bound opportunity to then implement effective management. • There are risks of spreading or facilitating disease and weeds associated with fire control operations (e.g. movements of vehicles from infected to uninfected areas, and application of retardants), and such risks need to be managed through appropriate planning, operations and standards. • Management of disease and weeds has many challenges, but adequate resourcing of pre- and post-fire monitoring and management can improve conservation outcomes