The experiment carried out an pathogenic analysis of <italic>Pyropia yezoensis</italic> filaments from a laver nursery in the Lanshan alage cultivation cooperative in Rizhao City of Shandong Province. We collected <italic>P</italic>.<italic>yezoensis</italic> filaments with typical symptoms of macular disease,cultured on three mediums of 2216E,TSB and TCBS.The dominant strain PZ201809121102 was isolated and purified from the focus of <italic>P</italic>.<italic>yezoensis</italic> filaments body.The strain PZ201809121102 was identified and obtained.Through Gram staining, electron microscope scanning, negative staining observation,16S rDNA identification, physiological and biochemical analysis and drug sensitivity test, we identified and analyzed the strains and combined with regression infection experiments to confirm the pathogenicity of the strain PZ201809121102. Results showed that: 1) The colony shape of the predominant strain PZ201809121102 after separation and purification on 2216E medium was small and round, with intermediate impurities, smooth and bright yellow surface,growing in creeping status, belonging to Gram-negative bacteria,short rot-shape and with the cell size of (0.5-1.0)μm × (1.5-5.0)μm. Under transmission electron microscope, it was observed that there was a single flagellum outside the spore. 2) 16S rDNA sequencing analysis showed that the bacterium clustered with <italic>Pseudomonas</italic> in the 16S rDNA phylogenetic tree, and had the highest homology with <italic>Pseudomonas luteola</italic>, which was 100%. 3) Physiological and biochemical results showed that the strain PZ201809121102 could grow in 6% NaCl tryptone water and 10% NaCl tryptone water. Oxidase reaction was positive. Ornithine decarboxylase,lysine decarboxylase,arginine dihydrolase reaction was positive. 3% NaCl ONPG reaction was positive. It could assimilate and utilize glucose, but not polysaccharides as sucrose, lactose, denbiose and sorbitol. 4) In regression infection experiment,10<sup>3</sup>-10<sup>8</sup> CFU·mL<sup>-1</sup> of PZ201809121102 bacterial suspension was used to soak and infect healthy <italic>P</italic>.<italic>yezoensis</italic> filaments. After 13 days, 10<sup>8</sup> CFU·mL<sup>-1</sup> <italic>P</italic>.<italic>yezoensis</italic> filaments appeared in different sizes in the experimental group. The yellow spots were consistent with the symptoms of naturally-occurring filaments macular disease, and the bacteria was isolated again to obtain the same domain isolate,which proved that PZ201809121102 was a pathogen of <italic>P</italic>.<italic>yezoensis</italic> filaments macular disease. However in treatments of <italic>P</italic>.<italic>yezoensis</italic> suspension concentrations lower than 10<sup>8</sup> CFU·mL<sup>-1</sup>, there was no obvious symptom of macular disease. 5) Results of drug sensitivity experiments showed that PZ201809121102 was highly sensitive to two drugs, romefloxacin and fleroxacin ,but not sensitive to cefazolin, cephalexin, and cefraditin. In this study, a species of <italic>P</italic>. <italic>yezoensis</italic> filamentous macular disease caused by <italic>P</italic>. <italic>luteola</italic> in seedling stage was discovered, providing a useful reference for the prevention and control of <italic>Pyropia yezoensis</italic> disease.