Genus-specific host range of Phomopsis amaranthicola (Sphaeropsidales), a bioherbicide agent for Amaranthus spp.

Phomopsis amaranthicola, a newly described species discovered in Florida, is under development as a biological control agent for weeds belonging to the genus Amaranthus. Host-range testing was performed on the fungus, using the centrifugal phylogenetic scheme, with Amaranthus hybridus as the focal plant. Forty-five accessions of 21 species in the genus Amaranthus were tested for susceptibility to infection by this fungus. Fifty-six plant species outside the genus Amaranthus were also tested. The latter included crops in which the fungus might be used as a bioherbicide, members of genera that are closely related to Amaranthus, and selections of species that have been reported as hosts of other species of the genera Phomopsis or Diaporthe. Phomopsis amaranthicola did not infect any of the plants from outside the genus Amaranthus but was highly pathogenic to several species in the genus Amaranthus. All Amaranthus species tested were infected to varying degrees by P. amaranthicola; the resulting mortality levels were also variable. Thus, P. amaranthicola appears to be host-specific to the members of the genus Amaranthus. The generic level of specificity of this pathogen renders it not only a safe bioherbicide agent but also one that could be used against several weedy Amaranthus species.