The relationship between trichothecene production and pathogenicity was investigated for 29 isolates of Fusarium tumidum, a potential bioherbicide for gorse (Ulex europaeus) and broom (Cytisus scoparius) in New Zealand. All isolates originally derived from broom produced high levels of T-2 tetraol derivatives when grown on ground maize kernels and pearl barley grains, compared with isolates from gorse. Low amounts of scirpentriol derivatives were also produced by both groups of isolates. No nivalenol and deoxynivalenol derivatives were detected in any of the culture extracts. A subset of isolates cultured on gorse and broom tissue produced only small amounts of T-2 tetraol derivatives relative to the amounts produced in grain cultures. Overall, isolates from broom were more aggressive towards both hosts than isolates from gorse, but the pathogenicity of isolates was not correlated with their capacity to produce large amounts of T-2 tetraol derivatives in culture. Two isolates from gorse were highly aggressive towards both weeds. These isolates offer prospects for the development of a safe bioherbicide that could target two major weeds in New Zealand, as trichothecenes were not detected from them at the higher concentrations.