Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2007
Publication Date: 1/1/2008
Citation: Bonde, M.R., Nester, S.E., Berner, D.K., Frederick, R.D., Moore, W., Little, S. 2008. Quantitative Susceptibility of Legume Species to Infection by Phakopsora pachyrhizi. Plant Disease. 92:30-36.
Interpretive Summary: In 2004, the dangerous plant disease soybean rust, known for many years in parts of Asia and Australia, spread to the United States for the first time. Many questions immediately followed, such as whether or not the pathogen could survive the cold winters. It had been believed by disease experts that soybean rust, in order to survive winters, must survive in infected leaves of legume hosts where infected leaves do not freeze. In our study, we inoculated various legume weeds common to southeastern U.S., in addition to important cultivated legume crops other than soybean. We then counted numbers of lesions that developed on infected leaves, and numbers and sizes of spore producing fructifications within lesions. Based on these criteria, we determined that soybean and kudzu were the most susceptible species, followed by snap bean and green pea. However, the rapid defoliation of infected pea suggested that pea might be of limited importance. Some snap bean cultivars produced many lesions that sporulated and therefore this crop should be monitored in the future.
Technical Abstract: Knowledge of the host range of P. pachyrhizi is important to U.S. soybean production because legume species other than soybean potentially could allow the pathogen to survive winters in southeast United States. Although host range studies have been conducted, results often are difficult to access because of a lack of a description of the methodology used, and whether or not sporulation was observed. To clarify the potential importance of non-soybean legumes, including susceptibility of non-soybean legume crops, multiple accessions of clovers, cowpea, green pea, kudzu, lima bean, snap bean, and single accessions of coffee sienna, hemp sesbania, hyacinth bean, partridge pea, and showy crotalaria were inoculated under greenhouse conditions with urediniospores of P. pachyrhizi. The four criteria used to access susceptibility were density of lesions, proportion of lesions producing urediniospores, average number of uredinia per lesion, and average diameter of uredinia. Based on numbers of lesions and percentage of lesions sporulating, soybean and kudzu were the most susceptible plant species, followed by snap bean and green pea. Some snap bean cultivars produced numerous lesions, many of which sporulated. Green pea produced approximately the same number of uredinia per lesion as soybean, but the rapid fall of infected leaves from plants suggested plants would not be important producers of inoculum. The presence of both Tan (susceptible) and RB (resistant) lesions on kudzu demonstrated race differentiation. The average numbers of uredinia per lesion appeared to be a valid measurement with which to compare host susceptibility and potential variation among accessions within a legume species, and had epidemiological significance. A highly susceptible reaction was characterized by a large range in uredinia sizes within lesions, whereas a resistant reaction was typified by smaller uredinia with a smaller range in uredinia sizes.