Shorea assamica is an evergreen to semi-evergreen tree in the family Dipterocarpaceae. It is widely distributed in China, India, Myanmar, Malaysia, Indonesia, the Philippines, and other regions. In China, it is found in western Yunnan and southeastern Tibet. The species is valued for its aromatic white resin, which serves as an important chemical raw material (Ang and Maruyama 1997). In May 2025, S. assamica trees exhibiting symptoms of witches'-broom disease and plexus bud disease were found at Mengmao, WanDing, Nongdao, Jiegao, and Jiexiang in RuiLi County (97°31′-98°02′ E, 23°38′-24°14′ N), Dehong Prefecture, Yunnan Province, China. Following disease onset, the overall growth of infected plants was significantly slower compared to healthy individuals. Infected plants exhibited excessive branching, shortened internodes, and a clustered growth symptom. Lateral buds on diseased tree frequently developed into abnormal twigs, with many buds forming dense clusters and only a few producing leaves. The disease was designated as S. assamica witches'-broom (SAWB). Across the seven surveyed areas, over 74% of trees displayed symptoms, while the disease incidence in nursery-cultivated seedlings was 66%. The disease was found to be spreading actively. A total of 56 samples were collected from seven areas, including 42 symptomatic plants and 14 asymptomatic plants. The lateral stem tissues of 30 samples were observed under a scanning electron microscope (Hitachi S-3000N). Spherical bodies were found in the phloem sieve cells of symptomatic plants. Total DNA was extracted from 56 plant samples using the CTAB method (Porebski et al. 1997) and subjected to nested PCR testing. Double-distilled water was used as negative control, and the DNA extracted from Dodonaea viscosa affected by D. viscosa witches’-broom disease was used as a positive control. Nested PCR was employed to amplify the pathogen’s 16S rRNA gene (Lee et al. 1993; Schneider et al. 1993), and 1.2 kb segment was produced (GenBank accessions: PZ025091, PZ025092, and PZ025093). PCR specific to the ribosomal protein (rp) gene yielded a segment of approximately 1.2 kb (Lee et al. 2003) (GenBank accessions: PZ028767, PZ028768, and PZ028769). The fragment size from 31 samples was consistent with the positive control, confirming the association of phytoplasma with the disease. A BLAST analysis of the 16S rRNA sequences of S. assamica witches’-broom phytoplasma showed that it shared 99.68% identity with that of Rice orange leaf phytoplasma (GenBank accession: JQ965690). The rp sequence shared 99.83% identity with that of Salix tetradenia witches'-broom phytoplasma (GenBank accession: KC117314). An analysis with iPhyClassifier (Zhao et al. 2013) showed that the virtual RFLP pattern derived from the 16S rDNA fragment of SAWB phytoplasma was identical (similarity coefficient 0.98) to the reference pattern of 16Sr group I, subgroup B (OY-M, GenBank accession AP006628). The SAWB phytoplasma is a variant of 16SrI-B. The phylogenetic tree was reconstructed based on 16S rRNA and rp gene sequences using MEGA (Tamura et al. 2013). The analysis was conducted using the neighbor-joining method with 1,000 replicates of bootstrap analysis. The results indicated that the SAWB phytoplasmas grouped into clades including phytoplasmas belonging to 16SrI-B and rpI-B, respectively. In addition, 1-year-old S. assamica were used for grafting assays in nursery, the twigs from infected S. assamica under natural conditions were used as a scion, and the phytoplasma was detected using nested PCR after grafting for 40 days. To the best of our knowledge, S. assamica is a new host of ‘Ca. P. asteris’-related strain (16SrI-B) in China. The newly emerged disease is a threat to S. assamica.
Chen et al. (Sun,) studied this question.