Broussonetia papyrifera, commonly known as paper mulberry, is an ornamental tree that is native to northeastern Asia. Because of its fast-growing nature and tolerance of dust, smoke, and high temperatures, paper mulberry is an important component of the biological diversity in natural ecosystems as well as a favorable shade tree in the region. In September of 2003, a disease characterized by pronounced witches'-broom symptoms was observed in paper mulberry trees growing near a jujube (Ziziphus jujuba) orchard and in home gardens located in Taian, Shandong, China. The diseased trees developed dense clusters of highly proliferating branches with shortened internodes. Leaves on the affected branches were chlorotic and greatly reduced in size. Phytoplasma infection was first suspected in this paper mulberry witches'-broom (PMWB) disease because the disease occurred in an area where other phytoplasmal diseases, including jujube witches'-broom (JWB) disease and paulownia witches'-broom (PaWB) disease, are common (4). Results from nested polymerase chain reactions (PCR), performed using phytoplasma-universal 16S rDNA primers (P1/P7 and R16F2n/R16R2) (1,2,3), revealed that all seven diseased trees tested contained phytoplasma, whereas PCR assay of comparable leaf samples from three nearby symptomless paper mulberry trees were negative. Subsequent restriction fragment length polymorphism (RFLP) analysis of the PCR-amplified 16S rDNA indicated that all diseased trees contained the same phytoplasma and that the PMWB phytoplasma belongs to the subgroup B of the elm yellows (EY) phytoplasma group (16SrV-B). Nucleotide sequence analysis of the cloned PMWB phytoplasma partial rRNA operon (GenBank Accession No. AY576685), spanning a near full-length 16S rRNA gene, a 16S--23S rRNA intergenic spacer, a tRNA-Ile gene, and a partial 23S rRNA gene, suggested that PMWB phytoplasma is most closely related to JWB phytoplasma, a member of the subgroup16SrV-B. To our knowledge, this is the first report of a paper mulberry witches'-broom disease and the first report of its association with a phytoplasma. Further work is underway to determine whether the PMWB phytoplasma is distinct from previously characterized phytoplasmas included in group 16SrV and to assess impacts of the phytoplasma on the ecosystems in the region.
References: (1) S. Deng and C. Hiruki. J. Microbiol. Methods 14:53, 1991. (2) D. E. Gundersen and I.-M. Lee. Phytopathol. Mediterr. 35:144, 1996. (3) C. D. Smart et al. Appl. Environ. Microbiol. 62:2988, 1996. (4) S. Zhu et al. Acta Hortic. 472:701, 1998.
