The Effects of Ustilago sporoboli-indici on Giant Rat’s Tail Grass (Sporobolus natalensis) under Field Conditions

Abstract

Giant Rat’s Tail grass (GRT; Sporobolus natalensis) is a major invasive pasture species in subtropical Australia, with limited management options. The smut fungus Ustilago sporoboli-indici (USI) was previously considered for classical biocontrol but rejected due to low host specificity. Since its first detection in Australia in 2017, USI has expanded more than 1,500 km from New South Wales to northern Queensland, yet its impact on GRT and the factors influencing pathogen development and spread under field conditions remain unknown. Understanding these dynamics is essential to evaluate USI’s potential as a biocontrol agent. This study aimed to quantify the field impacts of USI on GRT growth and reproduction and examine how grazing influences infection dynamics. Two adjacent survey grids, each with twelve 2 square metre plots, were established at Conondale in October 2025, one fenced (ungrazed) and the other exposed to grazing. All GRT plants were mapped and assigned IDs for repeated measurements, with data collected on GRT’s growth, reproduction, and USI infection. Preliminary results (October 2025–January 2026) indicate that mean USI incidence was low in October (15.2% in grazed and 28.1% in ungrazed plots), while incidence increased significantly by January, rising to 74.7% and 49.5%, respectively, with infection intensity amplified by grazing. Stunted inflorescences decreased over time, occurring in a small proportion of tillers per plant by January, possibly because many infected tillers failed to reach the reproductive stage. The study will extend to a second site at Bundaberg, with monitoring to be undertaken over two years at three-month intervals. Findings will provide field-based evidence on the establishment, spread, and potential suppressive effects of USI on GRT.