|Lee, Ing Ming|
Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2006
Publication Date: 2/1/2007
Citation: Zhao, Y., Sun, Q., Davis, R.E., Lee, I., Liu, Q. 2007. First report of witches’-broom disease in a Cannabis species and its association with a phytoplasma of elm yellows group (16SrV). Plant Disease. 91:227.
Interpretive Summary: Phytoplasmas are small bacteria without a cell wall. These bacteria can cause serious diseases in vegetables, fruit trees and other agriculturally important plants. Hemp fiber plants grow naturally in almost every climate zone in China and have a long history of cultivation in the country for making ropes, clothes, and paper products. In summer 2004, a disease characterized by uncontrolled branching (witches'-broom disease) was observed in hemp fiber plants growing in suburban Taian, Shandong, China. DNA finger-printing analysis revealed that this disease is associated with infection by a phytoplasma that is closely related to the one causing a severe disease in jujube (jujube witches'-broom disease). This is the first report of a phytoplasmal disease in hemp fiber plants. Since this new disease occurred in the same geographic region where jujube witches'-broom disease is common, and since the DNA fingerprints of the phytoplasmas associated with the two diseases are nearly identical, determining whether these two phytoplasmas can cross infect jujube and hemp fiber plants will have significant implications in managing both diseases. This information will be of interest to scientists and extension personnel who are concerned with phytoplasma diseases and to regulatory agencies for implementation of new quarantine regulations.
Technical Abstract: Hemp fiber plants (Cannabis spp.) spread naturally in almost every climate zone in China and have a long history of cultivation in the country. While hemp stalks provide high quality fibers for making ropes, clothes, and paper products, hemp seeds are a rich source of edible oil. In summer 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; 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) and paper mulberry witches’ broom (PMWB) diseases were previously reported. Nested polymerase chain reactions (PCR) were carried out on the DNA samples extracted from leaves of diseased hemp fiber plants using phytoplasma-universal 16S rDNA primers. Results revealed that all diseased plants examined 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 gene fragment indicated that the phytoplasma associated with HFWB disease belongs to subgroup 16SrV-B of the elm yellows phytoplasma group. Nucleotide sequence analysis of the cloned HFWB phytoplasma partial rRNA operon 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 ribosomal protein genes rplV and rpsC was PCR-amplified and sequenced. The nucleotide sequence of the HFWB phytoplasma rplV and rpsC locus is nearly identical to that of JWB phytoplasma. To our knowledge, this is the first report of a phytoplasmal disease in Cannabis species. Since HFWB and JWB phytoplasmas share extremely high sequence identity and share the same eco-geographical 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.