Field evaluation of Phomopsis amaranthicola, a biological control agent ofAmaranthus spp.

There are approximately 60 species in the genus Amaranthus, of which seven are used as grains, leafy vegetables, or ornamentals. The majority of the r emaining species are considered important weeds. A new fungal species, Phom opsis amaranthicola, isolated from stem and leaf lesions on an Amaranthus s p. plant, was found to be pathogenic to 22 species of Amaranthus tested. Th e efficacy of this fungus was tested in field trials using one or two poste mergent applications of the fungus consisting of two concentrations of coni dia or mycelial suspensions. Species tested for susceptibility in the field included Amaranthus hybridus, A. lividus, A. viridus, A. spinosus, and a t riazine-resistant A. hybridus. The cumulative disease incidence data for ea ch treatment within each species were plotted versus time using regression for lifetime data. Plant mortality was recorded 2, 4, and 6 weeks after ino culation. There were significant differences between the treatment effects in the control plots versus the plots treated with P. amaranthicola. The hi ghest level of control was obtained in the first trial when the fungus was applied at 6 x 10(7) conidia per mi. Final mortality of all species, except A. hybridus, reached 100% in inoculated plots 25 days earlier than in noni noculated control plots. Conidial suspensions were more effective in contro lling the species in the second trial than were mycelial suspensions. Sprea d of the pathogen to noninoculated control plots was faster in the second y ear than in other years. High levels of plant mortality were achieved in pl ots of A. spinosus, A. lividus, and A. viridis. A. hybridus and the triazin e-resistant A. hybridus were not effectively controlled in the second year due to the advanced stage of plant growth (8 to 10 true leaves) at the time of pathogen application. Results confirmed that P. amaranthicola is an eff ective biocontrol agent of some of the Amaranthus spp. tested.