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.

由柑橘黄单胞菌亚种引起的棉花细菌性枯萎病。锦葵(Xcm )是一种种子传播的疾病，对棉花种植构成全球威胁。在Xcm的几个种族中，race18是在印度遇到的最毒的一个。本研究侧重于两个方面：(i) 揭示 18 种族在印度的优势和流行程度；(ii) 破译 Xcm 18 种族内的遗传和致病变异。从不同地区共采集了56个样本，其中小种鉴定分析显示45株属于Xcm小种18菌株。此外，这些分离株使用 RAPD、IS1112 元素分析和 RFLP 技术进行指纹识别。这是第一份显示 IS1112 元素分析在研究 Xcm 18 种内遗传变异性的应用的报告。使用 NTSYS 软件通过生成树状图进行的高级分析将 Xcm 的 45 个种族 18 分离株聚集成 3 个主要簇。指定为 XcmR18_26 和 XcmR18_40 的两个分离株在遗传上更远 (43.6%)，并在 IS1112 和 RAPD 指纹分析中保持了各自的身份。使用 RFLP 分析进一步对每个集群的代表性成员进行基因分型。RFLP 标记可以验证使用 RAPD 和 IS112 元素分子标记形成的三个变异组。此外，测试了这三种变体，即 XcmR18_13、XcmR18_9 和 XcmR18_32 的利福平抗性代表成员的 使用 RFLP 分析进一步对每个集群的代表性成员进行基因分型。RFLP 标记可以验证使用 RAPD 和 IS112 元素分子标记形成的三个变异组。此外，测试了这三种变体，即 XcmR18_13、XcmR18_9 和 XcmR18_32 的利福平抗性代表成员的 使用 RFLP 分析进一步对每个集群的代表性成员进行基因分型。RFLP 标记可以验证使用 RAPD 和 IS112 元素分子标记形成的三个变异组。此外，测试了这三种变体，即 XcmR18_13、XcmR18_9 和 XcmR18_32 的利福平抗性代表成员的对易感棉花品种 Acala 44进行植物内生长测定。通过计算每 cm ²受感染叶组织的菌落数来测量这些代表性小种 18 分离株的侵袭性水平。在接种后第 7 天，最高、中等和最低侵袭性的变异记录为 3 × 10 ⁷、1.5 × 10 ⁷和 0.8 × 10 ⁷  CFU/cm ²。总而言之，本研究破译了 Xcm 18 种族内人口的致病和遗传多样性的详细说明，并证实在印度的 18 种族人群中至少进化出 3 种独特的生物型。此外，我们得出结论，Xcm 18 品系已成为印度棉花种植区的主要品种。