New Zealand has experienced numerous introductions of exotic plant species since human arrival. Exotic plant invasions can alter native ecosystems by introducing pathogens that facilitate spillover to native species. This thesis investigates factors that could impact the likelihood of pathogen spillover from exotic plant species onto native species. The research comprises two components: an observational field study and a controlled experimental analysis.

The field study focused on the abundance of foliar fungal hyphae in five native plant species within areas dominated by exotic plants. Using a new, cost-effective method for fungal quantification, it was found that species effects had a stronger influence on fungal abundance than exotic plant proximity. Endophyte diversity and generalist fungal species occurrence also showed no significant correlation with exotic plant abundance. The results highlight host-specific traits as important factors in endophyte dynamics.

The experimental component examined the effect of exotic soil sources on the growth and disease prevalence of three native plant species. Findings suggested weak phylogenetic signals in pathogen dynamics, with low disease prevalence and mixed outcomes for biomass accumulation. This research demonstrates that while exotic plants contribute to environmental complexity, their role in driving fungal spillover to native species is nuanced and dependent on host traits and ecological context.

This work advances methodologies for fungal measurement and contributes to understanding pathogen-host interactions, informing conservation strategies for New Zealand’s ecosystems.