New Legume Hosts of Phakopsora pachyrhizi Based on Greenhouse Evaluations

Phakopsora pachyrhizi, the causal organism of soybean rust, was first found in the continental United States in 2004 and has been found on soybean, kudzu, Florida beggarweed, and three Phaseolus species in the field. The pathogen has been reported to occur on more than 90 legume species worldwide and it is likely to infect native and introduced legume species in the United States. The objective of this study was to determine if 176 species representing 57 genera of legumes, the majority of which are either native or naturalized to soybean-growing areas of the United States, could be hosts of P. pachyrhizi. Between one and three accessions of each species, a total of 264 accessions, were inoculated with a mixture of four isolates of P. pachyrhizi. Severity and sporulation were rated on a 1-to-5 scale at 14 and 28 days after inoculation. P. pachyrhizi was confirmed by the presence of sporulating uredinia and/or immunological assay on 65 new species in 25 genera; 12 of these genera have not been reported previously as hosts. Many of the newly identified hosts grow in the southern United States, and like kudzu, could serve as overwintering hosts for P. pachyrhizi.