Canada thistle decline, a biocontrol opportunity?

Abstract

Canada thistle (Cirsium arvense) poses a significant challenge in organic production systems, often forming expansive colonies that reduce crop yield and prevent effective harvest. To control this pest, our program seeks to weaponize thistle pathogens associated with colony collapse for use as bio-herbicides. Pathogens associated with colony collapse were Alternaria spp, Fusarium oxysporum, and Fusarium redolens. Controlled experiments, both in greenhouse and field conditions, were conducted to assess the pathogenicity and efficacy of the Fusaria isolates, while ensuring crop safety for Montana's predominant crops. Both F. oxysporum and F. redolens exhibited strong virulence, reducing thistle vigor by over 70% compared to untreated controls. Koch's postulates were fulfilled, confirming their pathogenicity. For the efficacy of the isolates greenhouse trials demonstrated a significant reduction in thistle health following inoculation with Fusarium species (p < 0.001). While Fusarium oxysporum isolates had no adverse effects on Montana's crops, Fusarium redolens isolate significantly reduced the emergence, vigor and dry weight of wheat, lentil, and chickpea in crop safety trials. Field trials showed no crop damage from the inoculation of these Fusarium species. We tested the field efficacy of three F. oxysporum isolates Bio-02, Bio-05, Bio-15, applied singly and in two-way combinations, at two locations: Highwood and Big Timber, Montana in the year 2023 and an additional Bio-42 isolate in the year 2024 in Highwood and Great Falls. In the year 2023, at Highwood, isolates Bio-02 and Bio-05 reduced thistle stands by 79.2% and 61.2%, respectively (p = 0.023). A negative correlation were observed between the reemergence of thistle in Highwood and necrosis in Big Timber (R 2 = 0.18, p = 0.01) in the year 2023. This research highlights the potential of Fusarium species to provide organic farmers with an effective, sustainable alternative to synthetic herbicides for controlling Canada thistle, advancing the goal of environmentally responsible agriculture in Montana.