【Objective】Pinus sylvestris var. mongholica, a species with notable environmental adaptability, has been extensively cultivated in the Three-North regions of China. In recent years, a novel disease causing needle blight has emerged in P. sylvestris var. mongholica plantations in Hebei and Inner Mongolia. This study aimed to identify the causal pathogen, biological characteristics, and evaluate fungicide efficacy to establish scientifically grounded control measures for this disease.【Method】Isolates were obtained from both conidia and diseased tissues. Morphology combined with phylogeny identified the strains and the pathogen was determined by Koch postulates. The effects of different carbon source media, pH, photo period, and temperature conditions on mycelial growth were systematically investigated. Mycelial growth rate method was used to study the inhibitory effect of 10 fungicides on pathogen and calculated the value of EC₅₀ through virulence regression analysis.【Result】A total of 84 fungal strains were isolated from symptomatic needles, among which isolates Z-17, Z-21, Z-22 and Z-24 exhibited pathogenicity toward P. sylvestris var. mongholica, reproducing disease symptoms consistent with field observations. Conidia of these strains were subspherical, hyaline, septate, smooth, nearly cylindrical to oval, measuring (4.2-10.3) μm × (2.5-6.3) μm, aligning with the morphological features of fruiting bodies observed on necrotic needles. Phylogenetic analysis demonstrated that the four strains clustered with Sydowia polyspora with 100% bootstrap support. Based on these findings, S. polyspora was identified as the causal agent of needle blight in P. sylvestris var. mongholica. Biological characterization revealed that glucose provided the optimal carbon source for mycelial growth, whereas starch was the least utilized. Mycelial growth occurred within a pH range of 3-9, with optimal growth at pH 5. Continuous 24 h light exposure most favorably supported mycelial development, and the lethal temperature for mycelia was 55 ℃. Fungicide screening indicated that 25%(v/v) myclobutanil (EC₅₀ = 0.003 9 mg/L), 45%(v/v) prochloraz (EC₅₀ = 0.044 3 mg/L), and 10%(m/m) difenoconazole (EC₅₀ = 0.047 5 mg/L) exhibited the strongest inhibitory effects against S. polyspora. In contrast, 80%(m/m) thiram showed the poorest efficacy, with an EC₅₀ value of 16 693 mg/L.【Conclusion】This study identified the pathogen of P. sylvestris var. mongholica leaf blight as S. polyspora, the optimal carbon source for mycelial growth was glucose, pH 5, light conditions were 24 h continuous, and mycelial lethal temperature was 55 ℃. Among the tested fungicides, 25% myclobutanil, 45% prochloraz, and 10% difenoconazole demonstrated superior inhibitory effects. This study provides a theoretical foundation for developing targeted management strategies and further elucidating the pathogenic mechanisms of S. polyspora.