First report of a phytoplasma associated with Christmas cactus witches' broom

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.

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.