In Korea, the indigenous Elaeocarpus slyvestris var. ellipticus (Thunb.) H. Hara exclusive to Jeju Island, has been experiencing a decline phenomenon since 2013, and it was attributed to a phytoplasma infection in 2017. To investigate the distribution of E. slyvestris decline disease, research was conducted on 231 specimens, including street trees, parks, public facilities, apartment complexes, natural habitats, and natural monuments on Jeju Island. The primary symptoms of decline disease in E. slyvestris include yellowing and darkening of the leaves. Infected trees exhibit a progressive loss of color in the leaves, which eventually results in defoliation and, ultimately, death within a few years. Out of the randomly selected 231 E. slyvestris specimens with decline symptoms, PCR testing was conducted on 95 of them. The results revealed the presence of phytoplasma in 80 of the samples. PCR-RFLP analysis of this phytoplasma indicated the presence of two distinct lineages and a confirmed occurrence of mixed infection involving these two phytoplasmas lineages. To investigate the transmission method of E. slyvestris decline disease, vector identification, and pathogenicity transmission experiments were conducted. The results revealed that Satsumanus satsumae was most abundant in E. slyvestris. Investigations into the transmission method of E. slyvestris decline disease, involving vector identification and pathogenicity transmission experiments, revealed that S. satsumae harbors phytoplasma as detected through PCR testing. Furthermore, PCR-RFLP analysis confirmed that this leafhopper species shares identical banding patterns with Candidatus Phytoplasma malaysianum. This suggests that the leafhopper hosts the identical phytoplasma strain and might function as a potential vector in the dissemination of decline disease. Furthermore, through graft and dodder transmission experiments, the research revealed the phytoplasma's mobility and transmission velocity for each host. Substantiating the pathogenicity of the phytoplasma associated with decline disease was achieved through dodder transmission experiments, utilizing healthy periwinkle plants (Catharanthus roseus) and E. slyvestris as the inoculation host. This was affirmed by the occurrence of symptoms including phyllody, yellowing, small leaves, loss of leaves. Genetic analysis of the phytoplasma responsible for the decline disease was performed by amplifying the 16S rRNA, 16-23S internal transcribed spacer (ITS) region, 23S rRNA partial gene, and secA gene using PCR. The results indicated a 99.5% sequence identity in the 16S rRNA gene and a 98.5% sequence identity in the secA gene, aligning with the Ca. Phytoplasma malaysianum lineage. Another phytoplasma was identified as belonging to the Ca. Phytoplasma asteris lineage, with a genetic identity of 99.7% in the 16S rRNA, secA gene, rp operon, and Tuf gene. The phytoplasma responsible for decline disease was found to be divided into two distinct groups. However, the rp operon and Tuf gene of Ca. Phytoplasma malaysianum lineage showed no sequence identity with other phytoplasmas in the NCBI database. Based on the 16S rRNA sequence of the phytoplasma, a virtual-RFLP analysis was conducted, revealing the presence of a new subgroup of phytoplasma within the Ca. Phytoplasma malaysianum group. Among the 17 major restriction enzymes used, the treatment with BstUⅠ, HhaⅠ, BfaⅠ, and Sau3AⅠ enzymes resulted in distinctive band patterns that differentiated the 16SrXXXII-A, -B, -C, and -D groups, classifying them as the 16SrXXXII-E group. Another phytoplasma was classified as the 16SrI-B group, affiliated with Ca. Phytoplasma asteris. In this study, primer pairs R16(XXXII)F1/R16(XXXII)R1 were developed for the precise detection of the phytoplasma associated with the decline disease, and successfully distinguished co-infected phytoplasma strains. The PCR assay using these primers was confirmed as a group-specific primer capable of distinguishing between major phytoplasma diseases in Korea, such as jujube witches' broom disease, red sumac witches' broom disease, mulberry dwarf disease, and paulownia witches' broom disease. Using a recently developed phytoplasma diagnostic kit based on LAMP methods, successfully detected two distinct strains of phytoplasma and co-infections of phytoplasma. The detection limit of the diagnostic kit was investigated, and phytoplasma was detected in the DNA concentration of the original stock solution at a dilution factor of 1×10-1(diluted by a factor of 10), ranging from 5.0 to 6.0 ng/μL. In the case of mixed phytoplasma infections, the detection limit of the phytoplasma diagnostic kit was confirmed to be at a DNA concentration of 1×10-2 (diluted by a factor of 100), with a dilution factor ranging from 0.5 to 0.6 ng/μL. To enable early diagnosis of phytoplasma decline disease in the field, the pathogenic microorganism extraction buffer, KN5 buffer, was utilized to extract DNA from the leaves of suspected infected plant samples. The DNA extraction using KN5 buffer was rapidly completed within 1-10 minutes, and the extracted DNA was accurately detected for phytoplasma using a phytoplasma diagnostic kit. Furthermore, phytoplasma detection was possible within potential insect vectors, not limited to plants. To control Elaeocarpus sylvestris decline disease, antibiotic injections were administered twice, at 30-day intervals, in March and April. After the injections, visual inspection revealed that most of the test subjects treated with the animal antibiotic Oxytetracycline Hydrochloride injection experienced a reduction in yellowing and darkening symptoms. Moreover, healthy shoot growth resumed, accompanied by an increase in leaf density. After the injections, PCR testing results indicated that the test subjects treated with Oxytetracycline Hydrochloride showed the lowest number of phytoplasma detections compared to those treated with other antibiotics. This confirmed that Oxytetracycline Hydrochloride exhibited the most outstanding symptom alleviation and pathogen control effects among the antibiotics tested.