Pathogenic Diversity in and Sources of Resistance to Uromyces appendiculatus in Southern Africa

Abstract

The reaction of the first (1983) common bean international differential set and other germplasm to 248 single pustule isolates of the rust fungus Uromyces appendiculatus, collected from various southern African countries, was evaluated. Eleven of the most important isolates were re-purified and re-inoculated, this time also on the second (2002) revised and smaller international differential set. The 248 isolates could be grouped into 44 race-groups. These were subjected to principal coordinates analysis (PCoA). A second PCoA was carried out using 25 of the most important of the 44, together with 34 African races reported by previous authors. Isolates were generally avirulent on accessions with the resistance genes Ur-3+, -5 or -11, as well as Compuesto Negro Chimaltenango (CNC) and A 286, all small seeded, and the most useful sources were accessions carrying both Ur-3 and Ur-11, for instance BelMiNeb-RMR-7, BelDakMi-RMR-14 and -18. Isolates were generally virulent on large seeded accessions (with, among others Ur-4, -6 or -9), reflecting the preference for large seeded beans in southern Africa and co-evolution of host and pathogen. No large seeded accessions showed broad resistance. The least susceptible was Plant Introduction 260418, which rated resistant to moderately susceptible to the 11 races. These observations were confirmed by field ratings on the same accessions over multiple seasons. According to the PCoA, which proved useful for the identification of differentiating accessions, southern African isolates fell into three main clusters, for which Redlands Pioneer and the South Africa cultivar Teebus were the most discriminating differentials. Other accessions that showed particularly useful differentiating ability were Olathe and 51051. Of these, only Redlands Pioneer has been included in the 2002 differential set. The PCoA grouping of the African races was similar to that of the southern African race-groups.