Comparative susceptibilities of legume species to infection by Phakopsora pachyrhizi

Knowledge of the host range of Phakopsora pachyrhizi is important to agriculture in the United States because of the distinct possibility that economic losses could occur to crops other than soybean. Furthermore, it is possible that alternative hosts could provide a means of overwintering of the pathogen, providing inoculum to initiate epidemics in future years. To clarify the potential importance of soybean rust on nonsoybean legumes and their role in overwintering of the disease, multiple accessions of clover, cowpea, pea, kudzu, lima bean, snap bean, and single accessions of coffee senna, Florida beggarweed, hemp sesbania, hyacinth bean, partridge pea, and showy crotalaria were inoculated under greenhouse conditions with urediniospores of P. pachyrhizi; infected soybean plants served as a control. The four criteria used to assess susceptibility were lesion density, proportion of lesions with sporulating uredinia, average number of uredinia per lesion, and average uredinia diameter, each determined 2 weeks following inoculation. Based on lesion densities, percentage of lesions with sporulation, and average numbers of uredinia per lesion, soybean, kudzu, and pea were the most susceptible species, followed by snap bean. However, because infected pea plants defoliated rapidly, urediniospore production presumably was limited, lessening the potential for epidemics on pea. Cultivars of snap bean produced numerous brown to reddish-brown lesions, many of which sporulated, but numbers of uredinia per lesion were lower than on soybean, kudzu, or pea. The presence of both tan (susceptible) and reddish-brown (resistant) lesions on kudzu demonstrated physiological differentiation on that host. Some kudzu plants appeared to be potentially excellent hosts for overwintering of the disease. The average number of uredinia per lesion appeared to be a valid measurement with which to compare host susceptibilities, and may have epidemiological significance. High susceptibility of a host was characterized by numerous uredinia with a wide range of sizes within individual lesions. In contrast, low susceptibility to rust was characterized by no or a few small uredinia.