First Report of Witches'-Broom Disease of Chinese Wingnut in China and its Association with a Phytoplasma of Aster Yellows Group (16SrI).

Abstract

Pterocarya stenoptera C. DC., commonly known as Chinese wingnut, is a fast-growing deciduous tree with tough bark and attractive foliage. Because of its tolerance of compact and nutritionally poor soil, drought, and heat, Chinese wingnut is an important component of the biological diversity in natural ecosystems and is a favorable shade tree in China. Chinese wingnut has also been used as a rootstock for walnuts because of its high resistance to soilborne Phytophthora spp. In the spring of 2004, a disease characterized by witches'-broom symptoms was observed affecting Chinese wingnut trees growing in suburban Taian, Shandong, China. The diseased trees developed dense clusters of highly proliferating branches with shortened internodes, leaves on the affected branches were significantly smaller, and some branches and twigs suffered dieback. Phytoplasma infection was suspected as the cause of this Chinese wingnut witches'-broom (CWWB) disease because the disease occurred in an area where phytoplasmal diseases, such as paulownia witches'-broom (PaWB) and jujube witches'-broom (JWB), are common (3). Nested polymerase chain reactions (PCR) were performed on DNA samples extracted from leaves of six diseased trees using phytoplasma-universal 16S rDNA primers (R16mF2/R16mR1 and R16F2n/ R16R2) (1,2). Results revealed that all diseased trees examined were infected by phytoplasma, whereas PCR assays of leaf samples from two nearby symptomless Chinese wingnut trees were negative. Subsequent restriction fragment length polymorphism analysis of the PCR-amplified 16S rDNA indicated that all diseased trees contained the same phytoplasma and that the CWWB phytoplasma belongs to subgroup B of the "Candidatus Phytoplasma asteris" (AY) group (16SrI). Nucleotide sequence analysis of a 16S rRNA gene cloned from CWWB phytoplasma (GenBank Accession No. AY831966) suggested that this phytoplasma is closely related to, but distinct from, PaWB phytoplasma, another member of group16SrI. To our knowledge, this is the first report of Chinese wingnut witches'-broom disease and of its association with a phytoplasma. Further work is being undertaken to examine the ecological and evolutionary relationship between CWWB phytoplasma and other phytoplasmas in the region and to assess the impact of CWWB on walnut rootstock selection.