Accepted for publication 19/06/06

The Christmas cactus (Zygocactus) is a perennial succulent and is native to the South American tropics of Brazil. It is one of the most widely cultivated plants and healthy Christmas cacti in full bloom are often sold as holiday plants around the Christmas season. A pot containing a Christmas cactus with cladodes showing conspicuous malformed branches giving witches’ broom symptoms and stunted growth was found at the Agricultural University, Kunming (Fig. 1). Based on the symptomatology, a phytoplasma was suspected to be associated with the disease.

Figure 1: Witches’ broom symptoms of diseased (Left and middle) and healthy (Right) Christmas cactus plants.

Samples from the plant exhibiting witches’ broom and symptomless plants were analyzed for the presence of phytoplasmas using 16SrDNA PCR assays. Total nucleic acids were extracted from young cladodes using the DNeasy Plant Mini kit (QIAGEN) and were used as the template in a nested PCR reaction employing primers R16mF2/R16mR1 followed by R16F2/R16R2 (Gundersen & Lee, 1996). A water control was also included in all the PCR reactions to check for any contamination. A DNA fragment of the expected size (1.2 kb) was amplified by nested PCR from the diseased plant but not from symptomless plants, nor from the water control. After cloning and sequencing, the DNA sequence of 1249bp was deposited in GenBank (Accession No.AY647459). Phylogenetic analysis of the 16S rDNA sequence delineated the Christmas cactus witches’ broom phytoplasma as a Peanut witches’-broom (16SrII) group member (putative Candidatus Phytoplasma aurantifolia), most closely related to the faba bean phyllody phytoplasma (Lee et al., 1998) (Fig. 2). This is the first report of a phytoplasma disease of the Christmas cactus.

Figure 2: Phylogenetic distance tree constructed using Acholeplasma laidlawii as a root organism, comparing the partial 16S rDNA sequences of the Christmas cactus witches’ broom phytoplasma with known typical members of each phytoplasmal 16Sr group. The numbers on each branch indicate the confidence value. GenBank accession numbers for sequences are given in parentheses.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (grant no. 30260064) and the Science Fund of Yunnan Province (grant no. 2000C0014Q).

Gundersen, DE, Lee, IM, 1996. Ultrasensitive detection of phytoplasmas by nested-PCR assays using two universal primer pairs. Phytopathologia Mediterranea 35, 144-151.

Lee IM, Gundersen-Rindal DE, Davis RE, Bartoszyk IM, 1998. Revised classification scheme of phytoplasmas based on RFLP analyses of 16S rRNA and ribosomal protein gene sequences. International Journal of Systematic Bacteriology 48, 1153-1169.

The British Society for Plant Pathology