Identification and Genetic Characterization of Ralstonia solanacearum Species Complex Isolates from Cucurbita maxima in China

Abstract

<i>Ralstonia solanacearum</i> species complex is a devastating phytopathogen with an unusually wide host range, and new host plants are continuously being discovered. In June 2016, a new bacterial wilt on <i>Cucurbita maxima</i> was observed in Guangdong province, China. Initially, in the adult plant stage, several leaves of each plant withered suddenly and drooped; the plant then wilted completely, and the color of their vasculature changed to dark brown, ultimately causing the entire plant to die. Creamy-whitish bacterial masses were observed to ooze from crosscut stems of these diseased plants. To develop control strategies for <i>C. maxima</i> bacterial wilt, the causative pathogenic isolates were identified and characterized. Twenty-four bacterial isolates were obtained from diseased <i>C. maxima</i> plants, and 16S rRNA gene sequencing and pathogenicity analysis results indicated that the pathogen of <i>C. maxima</i> bacterial wilt was <i>Ralstonia solanacearum</i>. The results from DNA-based analysis, host range determination and bacteriological identification confirmed that the 24 isolates belonged to <i>R. solanacearum</i> phylotype I, race 1, and eight of these isolates belonged to biovar 3, while 16 belonged to biovar 4. Based on the results of partial <i>egl</i> gene sequence analysis, the 24 isolates clustered into three <i>egl-</i> sequence type groups, sequevars 17, 45, and 56. Sequevar 56 is a new sequevar which is described for the first time in this paper. An assessment of the resistance of 21 pumpkin cultivars revealed that <i>C. moschata</i> cv. Xiangyu1 is resistant to strain RS378, <i>C. moschata</i> cv. Xiangmi is moderately resistant to strain RS378, and 19 other pumpkin cultivars, including four <i>C. maxima</i> cultivars and 15 <i>C. moschata</i> cultivars, are susceptible to strain RS378. To the best of our knowledge, this is the first report of <i>C. maxima</i> bacterial wilt caused by <i>R. solanacearum</i> race 1 in the world. Our results provide valuable information for the further development of control strategies for <i>C. maxima</i> wilt disease.