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Title: First report of a natural infection of Opuntia sp. by a 'Candidatus Phytoplasma asteris'-related phytoplasma in China

Author
item WEI, WEI - LIAONING CHINA
item HONG, CAI - YUNAN CHINA
item CHEN, HAIRU - YUNAN CHINA
item Davis, Robert
item Zhao, Yan

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/22/2007
Publication Date: 4/1/2007
Citation: Wei, W., Hong, C., Chen, H., Davis, R.E., Zhao, Y. 2007. First report of a natural infection of Opuntia sp. by a 'Candidatus Phytoplasma asteris'-related phytoplasma in China. Plant Disease. 91:461.

Interpretive Summary: Cacti are juicy, fleshy plants native to the Americas. Because of their pleasing appearance and remarkable blossoms, and their extreme tolerance to dry climate and poor soil conditions, many cacti have been cultivated and introduced into the international market as landscaping and potted house plants. Cacti are vulnerable to infection by phytoplasmas -- a group of small bacteria that lack a cell wall and cause numerous diseases in economically important crops worldwide. Infection of cacti by phytoplasmas can significantly change the shape of the infected plants, impacting their commercial value. In the present study, through DNA fingerprinting analysis on diseased cactus plants collected in Yunnan Province, China, we discovered that cacti can be infected by two distinctly different groups of phytoplasmas, one of which had never been reported in association with cactus plants before. Results of this study suggested that two different groups of phytoplasmas share a common territory in that region. Such findings have important implications for managing phytoplasma diseases in cacti and other crops. This information will be of interest to scientists and extension personnel who are concerned with phytoplasma diseases and to regulatory agencies that are concerned with implementing new quarantine regulations.

Technical Abstract: Cacti (Opuntia spp.) are perennial, evergreen, succulent plants native to arid areas of the Americas and West Indies. Because of their aesthetic appearance, many cacti have been cultivated and introduced into the international market as ornamental landscaping and house plants. Cacti are susceptible to phytoplasma infections and develop witches’-broom (WB) disease, impacting their commercial value. To date, all reported cactus WB cases were associated with infections by phytoplasmas in the peanut witches’-broom group (16SrII). During a phytoplasma diversity survey carried out in 2004 in Yunnan, China, we collected 29 malformed cactus plants from 14 locations. Each of the 29 malformed plants exhibited stunted growth and possessed clusters of highly proliferating cladodia, typical symptoms of cactus WB disease. Nested polymerase chain reactions (PCR) were carried out on the DNA samples extracted from young cladodia of diseased plants using phytoplasma-universal 16S rDNA primers. Results revealed that all diseased plants examined were infected by phytoplasmas. Subsequent restriction fragment length polymorphism (RFLP) analysis of the PCR-amplified 16S rDNA gene fragment indicated that 28 diseased plants were infected by a phytoplasma of group 16SrII, whereas the remaining one was infected by a ‘Candidatus P. asteris’-related (group 16SrI) phytoplasma. Nucleotide sequence analysis of the cloned 16SrI-cactus WB phytoplasma partial rRNA operon suggested that this phytoplasma is most closely related to ash WB phytoplasma and epilobium phyllody phytoplasma, both members of subgroup 16SrI-B. This 16SrI phytoplasma-infected cactus plant was transferred to a greenhouse and maintained for over two years, during which time the cactus plant remained infected and symptoms were unchanged. To our knowledge, this is the first report of a natural infection of a cactus species by a group 16SrI phytoplasma. Since this 16SrI-cactus WB phytoplasma was found in the same geographic location where 16SrII-cactus WB phytoplasma had been reported, the findings suggested that 16SrI- and 16SrII-cactus WB phytoplasmas may have overlapping geo- and bio-ecological niches. Further investigations are being carried out to determine whether mixed infections can actually occur in a single cactus plant, an issue having important implications in managing phytoplasma diseases of cacti and other economically important plants in the region.