Department of Plant Pathology, University of Nebraska, Lincoln, NE 68583-0722, USA; Novartis Seeds, 7240 Hoisclaw Road, Gilroy, CA 95020 USA

Background and objectives
Rustcaused by Uromyces appendiculatus (Pers.) Unger var. appendiculatus (Syn. U. phaseoli) is a major constraint to production of common bean (Phaseolus vulgaris L.). Although found in bean-producing areas worldwide, it is most severe in tropical and semi-tropical regions on an annual basis. Options for disease management include fungicide control, cultural practice modifications and host resistance. Host resistance offers the least costly but most effective strategy for farmers, particularly small landholders. High pathogenic variability of U. appendiculatus has been documented [1]; understanding this variability will be necessary for developing durable resistance. The objective of this study was to identify pathogenic variation in Africa and the Americas and relate this to use of resistance genes derived from a standard set of bean rust differential lines.

Materials and methods
Rust-infected bean leaves were collected from wild P. vulgaris and diverse bean landraces, breeding lines and cultivars. Countries in which collections were made were: Honduras, Colombia, Argentina, Jamaica, Dominican Republic, Morocco, Egypt, Malawi as well as Southern Africa and Puerto Rico of the USA. Single uredinium isolates were amplified on a susceptible host cultivar and inoculated onto the primary leaves of 19 standard bean rust differentials. Disease reaction was determined 14 days after inoculation by use of a 1-6 uniform bean-rust grading scale.

Results and discussion
Domesticated common bean occurs in two major gene pools, Andean and Middle American. The two forms were independently domesticated in pre-Columbian times from biologically and geographically distinct wild populations. The Andean form originated in the Andean region of South America, while the Middle American form was domesticated from wild populations in Mexico and Central America. We have found pathotypes of U. appendiculatus with specificity for Andean bean hosts [2].

Latin America: The most extensive rust collections were made in Honduras where wild beans (P. vulgaris aborigineus) and a common bean relative (P. coccineus) were sampled. The P. coccineus and wild bean rust collections ranged from broadly virulent to weakly virulent; similar to the diversity found in landrace and cultivar isolates from local landraces (Middle American) and snap beans (Andean). No single resistance genes in the differentials were effective against the wide diversity (>90) of pathotypes found. The isolates from wild P. vulgaris in Argentina were typed as Andean-specgic [2] yet the isolates from commercial cultivars were both Andean-specific and nonspecific. A combination of Middle American derived resistance genes such as Ur-3 and Ur-5 and Andean genes such as Ur-4 would be needed for broad resistance.

Africa: In Egypt and Morocco where beans are grown in limited acreages the rust pathotype diversity was narrow and both the Ur-3 and Ur-5 genes were effective. In southern Africa, predominantly Andean-specific pathotypes were found and so the Middle-American resistance genes would be effective.

The deployment of bean rust resistance genes is specific for each country or geographic area.

1. Mmbaga MT, Steadman JR, Eskridge KM, 1996. Journal of Phytopathology 144, 533-541.
2. Sandlin CM, Araya CM, Steadman JR, Coyne DP, 1996. Phytopathology 86, 1132.