Temperature, humidity, weed species and age, and inducible responses in the host are factors that could limit the efficacy of fungal bioherbicides. The influences of these factors on the efficacy of the fungal bioherbicides Phomopsis amaranthicola and Microsphaeropsis amaranthi against Palmer amaranth (Amaranthus palmeri), smooth pigweed (Amaranthus hybridus), and redroot pigweed (Amaranthus retroflexus) were investigated in greenhouse and field studies under south Texas conditions. Despite plants being given an initial dew period, the bioherbicides, applied individually or in combination, did not cause mortality on any pigweed species in greenhouse or field environments. In greenhouse experiments, fewer than 5% of the leaves in six- to eight-leaf A. palmeri plants developed necrotic lesions within 2 weeks after bioherbicide treatment and only 8% or fewer of the plants developed stem lesions. Disease incidence was significantly higher in A. hybridus and A. retroflexus, with as much as 94% of leaves developing necrosis and 95% of the plants having stem lesions. New leaf production was reduced by biobherbicide treatment in A. hybridus. Combined-pathogen inoculation caused leaf and stem lesions on mature (13 to 36 leaves per plant) A. hybridus and A. retroflexus. Summer and fall field inoculations with M. amaranthi on A. hybridus and A. palmeri produced disease incidence levels similar to or higher than those in greenhouse tests. Infection of A. palmeri by P. amaranthicola increased the peroxidase activity level nearly twofold compared with the controls. Neither pathogen influenced leaf free amino acid content. The high temperatures and low humidity of south Texas and interspecific variation in resistance, possibly linked to peroxidase induction, limited the efficacy of these bioherbicides.