Bio-herbicidal activity of fungal metabolites against broadleaf weeds in rice paddies

Abstract:

A single Alternaria cassiae leaf extract wiped out 94% of sicklepod seedlings in a Florida screenhouse trial back in 1981 and that finding sparked three decades of bio-herbicide research that has yet to deliver a dependable paddy-field product. This research screened culture filtrates from twelve soil-borne fungi isolated from Australian rice paddies for herbicidal activity against five broadleaf weed species commonly found in irrigated rice. Laboratory bioassays and pot experiments were conducted at the Southern Tablelands University of Agriculture, Casnberra, between March 2022 and November 2023. Crude ethyl acetate extracts of each fungal isolate were tested at 1, 5 and 10 mg mL⁻¹ on detached leaf discs and whole seedlings. Three isolates Fusarium equiseti (Cep-03), Phoma herbarum (Cep-07) and Curvularia lunata (Cep-11) showed broad-spectrum activity, inhibiting shoot growth by 62.7–89.4% across the five weed targets. The purified ethyl acetate fraction of Phoma herbarum Cep-07 was the most potent, reducing Sphenoclea zeylanica biomass by 89.4% and Eclipta prostrata by 78.3% at 10 mg mL⁻¹. Importantly, rice seedlings treated at the same concentration showed less than 8% growth reduction, indicating adequate crop safety. Radar-chart analysis of six efficacy parameters germination inhibition, shoot suppression, root suppression, chlorophyll reduction, biomass reduction and soil persistence placed the purified Phoma fraction above all other candidates. Phytotoxic metabolites tentatively identified by LC-MS included alternariol, tenuazonic acid and curvularin. These results suggest that Phoma herbarum Cep-07 filtrate merits advanced formulation development as a paddy-compatible bio-herbicide.