Pathogenic and genetic variation in Xanthomonas axonopodis pv. Phaseoli and its fuscans variant in Southern Africa

Abstract

Common bacterial blight (CBB) caused by Xanthomonas axonopodis pv. phaseoli and its fuscans variant, X. axonopodis pv. phaseoli var. fuscans is a widespread disease of dry beans in South Africa. Variation within pathogen populations has been reported and in order to breed for resistance it is important to investigate whether variation exists within the local pathogen population. One hundred and forty three common bacterial blight isolates from 44 localities in four countries, were inoculated onto eight Phaseolus acutifolius lines that differentiate between pathogenic races. This differential set was expanded to include resistant genotypes, XAN 159, GN #1 Nebr. sel 27, Wilk 2, Wilk 6, Vax 4, Vax 5 and Vax 6 and cv. Teebus as susceptible check. Genetic variation within nine selected Xap and Xapf isolates and a non-pathogenic Xanthomonas isolate, was studied using RAPD and AFLP analysis. Genotypes XAN 159, Wilk 2, Wilk 6, Vax 4, Vax 5 and Vax 6 were resistant to all isolates, while GN #1 Nebr. sel 27 and cv. Teebus were susceptible. Isolates varied in aggressiveness on cv. Teebus; however, the pathogenic reaction on the set of differentials indicated that all, but one isolate, grouped in what has been reported as race 2. Thus, results based on reaction of the majority isolates, suggest the absence of different races. However, the distinct differential reaction recorded for a single isolate, may prove to represent another, as yet unrecorded, race of this pathogen. Both RAPD and AFLP analyses revealed high frequency of DNA polymorphism among isolates and could distinguish between Xap, Xapf and a non-pathogenic isolate. Differences between Xap and Xapf isolates demonstrate that these are two distinct groups of bacteria.