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First Report of Witches'-Broom Disease in a Cannabis spp. in China and Its Association with a Phytoplasma of Elm Yellows Group (16SrV)

Hemp fiber plants (Cannabis spp.) spread naturally in almost every climate zone in China and have a long history of cultivation in the country (1). While hemp stalks provide high-quality fibers for making ropes, clothes, and paper products, hemp seeds are a rich source of edible oil. During the summer of 2004, a disease characterized by witches'-broom symptoms was observed in wild hemp fiber plants growing in suburban Taian, Shandong, China. The diseased plants developed clusters of highly proliferating branches with much shortened internodes and leaves on the affected branches were significantly reduced in size. Phytoplasma infection was suspected in this hemp fiber witches'-broom (HFWB) disease because of the typical symptoms and because of its geographic location where other phytoplasmal diseases such as jujube witches'-broom (JWB), paulownia witches'-broom (PaWB), paper mulberry witches'-broom (PMWB), and Chinese wingnut witches'-broom (CWWB) diseases were previously reported (3,4). Total DNA was extracted from leaves of four diseased and four nearby healthy looking hemp fiber plants. Nested PCR were carried out on the DNA samples using phytoplasma universal 16S rDNA primers (P1A/16S-SR and R16F2n/R16R2) (2). Results revealed that all examined diseased plants were infected by phytoplasma, whereas nearby healthy looking plants were phytoplasma free. Subsequent restriction fragment length polymorphism (RFLP) analysis of the PCR-amplified 1.25-kb 16S rDNA R16F2n/R16R2 fragment indicated that the phytoplasma associated with HFWB disease belongs to subgroup 16SrV-B of the elm yellows (EY) phytoplasma group. Nucleotide sequence analysis of the cloned HFWB phytoplasma partial rRNA operon (GenBank Accession No. EF029092), spanning a near full-length 16S rRNA gene and a partial 16S-23S rRNA intergenic spacer, suggested that HFWB phytoplasma is most closely related to JWB and PMWB phytoplasmas, both members of subgroup16SrV-B. To further characterize the HFWB phytoplasma, a genomic segment covering full-length ribosomal protein genes rplV and rpsC was PCR-amplified using primer pair rp(V)F1A/rp(V)R1A (2), cloned, and sequenced (GenBank Accession No. EF029093). The nucleotide sequence of the HFWB phytoplasma rplV and rpsC locus is nearly identical (99.9%) to that of JWB phytoplasma. To our knowledge, this is the first report of a phytoplasmal disease in Cannabis spp. Since HFWB and JWB phytoplasmas share extremely high sequence identity and share the same eco-geographic location, further investigation is warranted to determine whether these two phytoplasmas are actually one species that can infect both plants, an issue having important implications in managing both diseases.

References: (1) S. Hong and R. C. Clarke. J. Int. Hemp Assoc. 3:55, 1996. (2) I. M. Lee et al. Int. J. Syst. Evol. Microbiol. 54:337, 2004. (3) Q. Liu et al. Plant Dis. 88:770, 2004. (4) Q. Liu et al. Plant Dis. 89:529, 2005.