Bean rust is a devastating disease of snap beans in Sri Lanka. Our study endeavored to develop snap beans with resistance to Uromyces appendiculatus, the bean rust pathogen reported to have numerous virulent races. Accordingly, we needed a detailed understanding of the virulence of U. appendiculatus and to identify genes conferring effective rust resistance. To investigate the virulence, we collected leaves of snap bean with rust symptoms from four snap bean producing districts of Sri Lanka. From these samples we established 57 single pustule isolates. Each isolate was inoculated on a set of 12 differential cultivars, six from the Andean and six from the Middle American gene pools of common bean. The virulence of the isolates segregated into four distinct groups that comprised the first four reported races of U. appendiculatus in Sri Lanka, 23-5, 31-1, 31-11, and 63-21. All races were significantly more virulent on the Andean than on the Middle American differential cultivars. PI 1819966, a Middle American differential cultivar containing the Ur-11 gene, conferred resistance to all four races. Three other Middle American cultivars were resistant to three races. Conversely, five Andean differential cultivars were susceptible to all four races. PI 160418 was the only Andean differential cultivar with resistance to three races. The results suggested that rust resistance genes from the Middle American differential cultivars could be combined in different ways with the resistance of Andean PI 260418 to develop snap bean varieties with broad resistance to all races of U. appendiculatus in Sri Lanka.
Keywords: Sri Lanka; Ur genes; Uromyces appendiculatus; bean rust; races; snap beans; virulence diversity.