The Etiology Of Common Bacterial Blight Study And Identification Of Germplasm Resistance On Common Bean (Phaseolus Vulgaris L.)

Common Bacterial Blight, caused by Xanthomonas axonopodis pv. phaseoli and Xanthomonasfuscans subsp. fuscans, is one of the most destructive diseases on common bean (Phaseolus vulgaris L.)worldwide. This disease could occur at any stage of plant development and cause yield losses of20-60%, and losses even reach80%on susceptible cultivars, especially under favorable environmentconditions. The results of common bean disease surveys from2009to2011indicated that the diseaseoccurred in all common bean production regions, and specially caused serious yield losses in somemajor production regions, where the disease incidence of some fields were100%. For investigating thespecies, distribution and population diversity of pathogen of CBB, this study was focused on pathogensidentification, disease epidemic, genetic diversity, pathogenicity diversity, resistant germplasmresources screening. The results were as follows:1. Xanthomonas spp. was isolated from the diseased samples of common bacterial blight collectedfrom9provinces in China during2009to2011. Based on morphology, pathogenicity assay, whole-cellfatty acid identification, physiological and biochemical tests, Xanthomonas campestris pv. phaseoliexpress diagnostic kit,16S rDNA and16S~23S rDNA ITS sequence analysis and species-specific PCR,the pathogenic bacterium were identified as X. axonopodis pv. phaseoli and X. fuscans subsp. fuscans.The separation frequency of Xap was56.1%, while Xff was43.9%in all isolates.This is the first reportof CBB caused by X. fuscans subsp. fuscans in China.2. In the seed detection,36out of60samples from Heilongjiang, Shanxi and Hebei provinces,respectively, were contaminated by X. axonopodis pv.phaseoli and X. fuscans subsp. fuscans withbacteria population sizes from2.49×102to5.20×107cfu per seed. The results of seed detection showedthat the commercial and research common bean seeds had been contaminated with pathogens ofcommon bacterial blight. In soil and plant debris test,25out of34samples from Heilongjiang, Shanxi,Hebei and Neimenggu provinces, respectively, were contaminated by X. axonopodis pv.phaseoli and X.fuscans subsp. fuscans.The study showed the pathogens of common bacterial blight can overwinter insoil and crops residues in the region from38.7°to45.8°north latitude and from111.57°to126.53°eastlongitude.The results above indicated that the pathogen could survive in seed, soil,and plant debris, andbecome the primary source of infection.3. The rep-PCR method and cluster analysis were used to study the genetic diversity of the142Xanthomonas isolates. ERIC-PCR primer amplified33polymorphism fragments. By UPGMA clusteranalysis,142isolates could be divided into four groups or thirteen groups, when divided by geneticsimilarity coefficient at0.33or0.5, respectively. REP-PCR primer amplified36polymorphismfragments. All isolates could be divided into six groups when divided by genetic similarity coefficient at0.33. The Ⅱ group could be divided into six groups at coefficient of0.56and Ⅲ group could be dividedinto six groups at coefficient of0.58. Through the different classification, Xap and Xff could bedistincted. Each clusterd group included isolates from different provinces and isolates from the same province were distributed into different groups. The results showed that the isolates from different areasof China appear high genetic diversity and there is no relationship between the variation andgeographical origins.4. The pathogenicity tests of62strains of Xanthomonas isolated from8different provinces wereconfirmed by inoculating four bean cultivars of different resistant levels in greenhouse. The resultsshowed that there was obvious difference in pathogenicity among those isolates tested. The averagepathogenicity of isolates from the same region on different cultivars was different and is not relevant.There was no direct correlation between the differentiation and geographical origins. No obviousdifference was observed among Xap and Xff.5. By inoculating Xap strain XS2,602accessions of common bean germplasm resources werescreened with multiple needle method. The result showed that10bean cultivars occupied the scales of1.1~3.0, which occupied2%of all. Cultivars with disease scales of3.1~5.0were192, which accountingfor32%. Disease degree of306resources that occupied51%ranges from5.1to7.0and the others were94, which occupied15%. No immune and high resistant cultivars were obtained.