Evolution of biotypes within race 18 population of Xanthomonas citri subsp. malvacearum and their predominance in Indian cotton belts

Abstract

Bacterial blight of cotton, caused by Xanthomonas citri subsp. malvacearum (Xcm) is a seed borne disease that constitutes a global threat to cotton cultivation. Among the several races of Xcm, race18 is the most virulent one encountered in India. This study is focused on two aspects: (i) to reveal the predominance and prevalence of race 18 in India and (ii) to decipher genetic and pathogenic variability within race 18 of Xcm. A total of 56 samples were collected from various regions, among these the race identification analysis revealed 45 isolates belonged to the race 18 strain of Xcm. Further, these isolates were fingerprinted using the RAPD, IS1112 element analysis and RFLP technology. This is first report to show application of IS1112 element analysis for studying genetic variability within race 18 of Xcm. Advanced analysis by generation of dendrogram using NTSYS software clumped forty five race 18 isolates of Xcm into 3 major clusters. Two isolates, designated as XcmR18_26 and XcmR18_40 were genetically more distant (43.6%) and maintained their separate identity in IS1112 and RAPD fingerprint analysis. A representative member of each cluster was further genotyped using RFLP analysis. The RFLP marker could validate three variant groups formed using RAPD and IS112 element molecular markers. Additionally, the rifampicin resistant representative members of such three variants i.e., XcmR18_13, XcmR18_9, and XcmR18_32 were tested for in-planta growth assay on susceptible cotton cultivar Acala 44. The level of aggressiveness of these representative race 18 isolates was measured by counting colonies per cm2 of infected leaf tissue. The most, moderate and least aggressive variants recorded 3 × 107, 1.5 × 107, and 0.8 × 107 CFU/cm2 at 7 days after inoculation. Altogether, the present study deciphered a detailed account on pathogenic and genetic diversity of population within race 18 of Xcm and confirmed that at least 3 unique biotypes have evolved within the race 18 populations in India. Furthermore, we conclude that the race 18 strain of Xcm has emerged as a predominant race in Indian cultivated cotton regions.