Evaluating plant-pathogen interactions in classical biological control of weeds: Kordyana brasiliensis released against target weed Tradescantia fluminensis in Australia

Abstract

Tradescantia fluminensis is a significant environmental weed in Australia invading deeply shaded forests and riparian ecosystems reducing native vegetation species richness and abundance. Biotrophic leaf pathogen, Kordyana brasiliensis has been introduced into Australia and New Zealand as a biocontrol agent for T. fluminensis. Post-release surveys from initial releases have reported successful establishment and variable impacts on T. fluminensis populations, however, impacts on the weed's populations and invaded vegetation communities across broader contexts and temporal gradients remains to be evaluated. This thesis sought to explore aspects of the ecology of K. brasiliensis and T. fluminensis and evaluate short-term outcomes of the biocontrol program in eastern Australia. Chapter 1 examines how different local habitat and regional contexts influence the abundance-impact associations of T. fluminensis in native forests across eastern Australia. We modelled the association of T. fluminensis invasion with native species foliage cover and richness across two different habitat types (remnant vs replanted forests), two vegetation community types (wet sclerophyll vs river oak forests), and two regions (northern vs southern New South Wales). The variation in native species responses amongst different functional growth forms was also modelled. Impacts of T. fluminensis invasion on native species communities was most strongly explained by local site variables. This study highlights the need for site-level weed management plans to consider local community and habitat contexts and functional representation of resident species. Chapter 2 details a series of experiments with K. brasiliensis in controlled environments to investigate whether conidia can cause leaf lesions on T. fluminensis and the effects of temperature and wetness period on basidiospore germination processes (germ-tube formation and elongation, formation of conidia). Inoculations with conidia demonstrated that conidia can cause leaf lesions on T. fluminensis. Basidiospore germination and germ-tube elongation were significantly influenced by the interaction of both temperature and wetness period duration whereas conidia formation was strongly influenced by both factors separately. Chapter 3 details a short-term field experiment where K. brasiliensis was released at sites invaded by T. fluminensis in eastern Australia. Sites were monitored over 30-months to evaluate agent population dynamics, impacts of the agent on T. fluminensis populations, and the subsequent response of invaded vegetation communities. Kordyana brasiliensis rapidly established at all release sites and was strongly influenced by macroclimate variables. On average, T. fluminensis abundance declined significantly after 18-months and was strongly associated with increasing K. brasiliensis disease severity. Declines in T. fluminensis abundance was associated with increases in native species richness. This study demonstrates that K. brasiliensis is a highly damaging agent, however, further long-term monitoring is required to evaluate whether additional management and restoration activities are required. Chapter 4 examines the identity of white leaf lesions collected on two native Australian Commelinaceae hosts during pre- and post-release surveys of K. brasiliensis. The specimens identities were studied using multigene phylogenetic analyses to determine whether K. brasiliensis had extended its known host range, or whether they represented other Kordyana species. Three new species of Kordyana were described based on the phylogenetic analysis, morphology, and host species. Show more