An unidentified pathogen caused symptoms of angular necrotic lesions on leaves, leaf chlorosis, and premature leaf drop on Japanese tree lilacs in the Upper Midwest and Idaho. We isolated the putative pathogen and confirmed pathogenicity twice through the use of Koch's postulates. Greenhouse-grown Japanese tree lilacs were inoculated with a mycelial suspension in water. Symptoms identical to those on naturally infected trees appeared 2 wk after inoculation on 1 of 8 (12.5%) of trees in the first trial and 2 of 8 (25 %) of trees in the second trial. The isolated pathogen was identified using both genetic and morphological characteristics. Amplification and sequencing of the ITS region of ribosomal DNA had high (97%) homology to several known species of Cercospora and one Septoria sp. in Genbank. Morphological characteristics included indistinctly multiseptate, hyaline spores produced within 2-5 mm necrotic lesions, delimited by leaf veins. The temperature optimum, for in vitro growth of the Cercospora sp. pathogen was determined by incubating six isolates at 10, 15, 20, 25, 30, and 37°C. Colony diameter was greatest after 18 days of incubation at 20 or 25°C. Linear regression of the results estimated the optimum temperature at 22°C. In vitro sporulation of the pathogen was studied by studying the affect of growth medium, mycelial transfer technique, and photoperiod on sporulation of six pathogen isolates. Treatments were incubated for 2 wk, after which spore counts were made by removing plugs of mycelium from each treatment, vortexing the plugs in distilled water for 30 s and using a hemacytometer to count spores. Spores were counted in only a few of the treatments, but there was some consistency in media and isolate associated with the production of spores.