Yu, Shao-Shuai

Areca catechu palm is an important cash plant in Hainan Island of China and even tropical regions worldwide. Areca catechu palm yellow leaf (AcYL) disease caused by the phytoplasmas is a devastating disease for the plant production. In the study, the phytoplasmas associated with the AcYL diseases were identified and characterized based on the conserved genes of the phytoplasmas, and genetic variation and phylogenetic relationship of the phytoplasma strains in the 16SrXXXII group was demonstrated. The results indicated that Areca catechu palm showing yellow leaf symptoms were single infected by ‘Candidatus Phytoplasma malaysianum’-related strains belonging to 16SrXXXII-D subgroup. BLAST and multiple sequence alignment analysis based on 16S rRNA and secA genes showed that the AcYL phytoplasmas shared 100% sequence identity and 100% homology with the ‘Ca. Phytoplasma malaysianum’-related strains. Phylogenetic analysis indicated that the AcYL phytoplasmas and ‘Ca. Phytoplasma malaysianum’-related strains belonging to 16SrXXXII group were clustered into one clade with a 100% bootstrap value. Based on computer-simulated digestions, 6 kinds of RFLP patterns within 16SrXXXII group were obtained and a novel subgroup in the 16Sr group was recommended to propose to describe the relevant strains in this 16Sr subgroup. To our knowledge, this is the first report that Areca catechu palm showing yellow leaf symptoms infected by ‘Ca. Phytoplasma malaysianum’-related strains belonging to 16SrXXXII group. And a novel 16Sr subgroup 16SrXXXII-F was proposed based on the systematical analysis of genetic variation of all the phytoplasmas within 16SrXXXII group. The findings of this study would support references for monitoring the epidemiology and developing effective prevention strategies of the AcYL diseases.

The pathogens associated with citrus Huanglongbing symptoms, including yellowing and mottled leaves in Citrus maxima, an important economic crop on Hainan Island of China, were identified and characterized. In the study, detection, genetic variation and phylogenetic relationship analysis of the pathogens were performed based on 16S rRNA and β-operon gene fragments specific to phytoplasma and Candidatus Liberibacter asiaticus. The results indicated that the pathogens—such as phytoplasma strains of CmPII-hn belonging to the 16SrII-V subgroup and CmPXXXII-hn belonging to the 16SrXXXII-D subgroup, as well as Candidatus Liberibacter asiaticus strains CmLas-hn—were identified in the diseased plant samples, with numbers of 12, 2 and 6 out of 54, respectively. Among them, mixed infection with the 16SrII-V subgroup phytoplasma and Candidatus Liberibacter asiaticus was found in the study, accounting for 7.4% (four samples). The phytoplasma strains of CmPII-hn—Tephrosia purpurea witches’ broom, Melochia corchorifolia witches’ broom and Emilia sonchifolia witches’ broom—were clustered into one clade belonging to the 16SrII-V subgroup, with a 99% bootstrap value. The phytoplasma strains of CmPXXXII-hn and Trema tomentosa witches’ broom belonging to 16SrXXXII-D, and the other 16SrXXXII subgroup strains were clustered into one clade belonging to the 16SrXXXII group with a 99% bootstrap value. There were 16 variable loci in the 16S rRNA gene sequences of the tested 16SrXXXII group phytoplasma strains, of which two bases had an insertion/deletion. The strains of Candidatus Liberibacter asiaticus, identified in the study and the strains that had been deposited in GenBank, were in one independent cluster with a 99% bootstrap value. To our knowledge, this is the first report showing that Citrus maxima can be infected by 16SrII-V and16SrXXXII-D subgroup phytoplasmas in China. Moreover, this is also the first report in which the plants are co-infected by 16SrII-V subgroup phytoplasmas and Candidatus Liberibacter asiaticus. More comprehensive and detailed identification and characterization of the pathogens associated with the diseased symptoms in Citrus maxima on the island in China would be beneficial for epidemic monitoring and for the effective prevention and control of related plant diseases.