A leaf blight disease was found on Ailanthus altissiman growing in the Manchon Mountain Park in Daegu city. When isolated, the causal fungus readily formed sex organs, being homothallic. Oogonia were spherical, 19.5-42.9 μm in diameter with an average of 29.4 ± 4.2 μm. Antheridia were amphigynous, round to ovoid, and measured 11.3-15.0 μm long and 12.5-14.5 μm wide. Oospores in the oogonia were spherical, 26.1-29.0 μm in diameter. Sporangia that formed in water were spherical to sub-spherical with a conspicuous papilla and measured 19.5-56.6 × 15.6-44.9 μm with an average of 44.0 ± 8.7 × 32.7 ± 6.3 μm. The mean length/breadth (l/b) ratio was 1.35. Papillae were 3.9-11.7 μm high and 3.9-9.8 μm wide. Sporangia formed slowly on V8 juice agar medium when cultured under fluorescent light at 12-hour alternation. The sporangia that formed on the agar medium were more spherical and measured 26.5-39.0 × 23.4-35.1 μm with an average of 33.6 ± 3.4 × 28.2 ± 3.2 μm and l/b ratio of 1.19. Disease symptom was reproduced by artificial inoculation of the healthy plants with the isolate. The causal organism was identified as Phytophthora boehmeriae on the basis of its morphological characteristics.