Pathogenicity variation of some Thai isolates of Xanthomonas campestris pv. glycines

Pathogenic variation among 100 isolates of Xanthomonas campestris pv. glycines, the causal agent of bacterial pustule of soybean (Glycine max), collected from various growing regions throughout Thailand was examined with three differential soybean cultivars. Based on the reactions of the differentials, ten pathotype strain (PS) designated were PS1 to PS10. These pathotype strains were almost established on SJ4 and SK1 cultivars of soybean (90 percent). PS8 to PS10 were widely distributed through the production area, while PS3 and PS6 were restricted to one or two locations. Three populations from north, northeast, and central regions consisted of 58, 26 and 16 isolates, respectively, were distinctly, different from each other. Pathogen populations in the north were dissimilar in pathogenicity to those of the central than to those of the northeast. The most complex pathogenicity were present at a higher frequency in the northeast than in the north and the central. Isolates with low virulence were widespread over the region, whereas isolates with high virulence were detected at a lesser frequency. When bacterial suspension of each pathotype strain was sprayed onto foliage of greenhouse-grown soybean plants, variability in leaf lesion size and colony forming unit (CFU) of the pathogen per lesion were observed. The most aggressive isolate produced the largest leaf lesion size of 0.18*0.4 square cm and the highest of 8.7*10*[9) CFU/lesion, where the least aggressive isolate produced the smallest leaf lesion size of 0.11*0.27 square cm and the lowest of 3.3*10*[9) CFU/lesion. The result also revealed that the number of leaf lesion or infected leaf area was the most convenience to be evaluated for severity of soybean bacterial pustule regardless size and CFU of lesion. The set of differential soybeans and the data of a high variability level for pathogenicity were effective for typing the virulence phenotypes of a large collection of X. campestris pv. glycines isolates which should be useful to other workers investigating this host/parasite system.