Jatropha curcas L. is a potential biofuel crop due to the high oil content found in its seeds (Laviola et al., 2017). Since 2016, a powdery mildew disease has been observed on J. curcas leaves in Xishuangbanna Tropical Botanical Garden (XTBG) in Yunnan province, China. Early symptoms on seedlings of J. curcas was irregular white patches, and later the powdery layers covered the whole leaves, petioles, and stems. The infection caused premature drop of leaves. Microscopic observation showed that hyphae are straight to flexuous, hyaline, thin-walled, septate, 4 to 7 μm wide. Hyphal appressoria are simply lobed to multi-lobed in opposite pairs. Conidiophores erect, 61 to 150 × 5 to 8 μm. Foot-cells erect or curved at the base, 29 to 82 × 4 to 7 μm, followed by 1 to 3 shorter cells, forming non-catenate type of conidia, the basal septum of foot-cells usually formed 3 to 19 μm above the junction with the mycelial mother cell. Conidia ellipsoid to doliiform, without fibrosin bodies, but with one big vacuole or several small vacuoles. Primary conidia obovoid-ellipsoid, with the apex rounded and the base subtruncate, 23 to 39 × 12 to 22 μm, length/width ratio 1.3 to 2.5 (mostly ≤ 2); secondary conidia doliiform when mature, ends truncate or subtruncate, 27 to 38 × 12 to 17 μm, length/width ratio 1.9 to 2.5 (mostly ≥ 2). Germ tubes lobed or longitubus pattern and produced in perihilar or apical position, short to long, without swollen tips. Chasmothecia were not found in the collected samples. The voucher specimen was deposited at the herbarium of XTBG (HITBC), Chinese Academy of Sciences under accession no. F000001. The genomic DNA from mycelium and conidia collected from J. curcas leaves was extracted and amplified the internal transcribed spacer (ITS) and 28S rDNA sequences with primer pairs ITS1/ITS4 and LSU1/LSU2, respectively (Scholin et al., 1994, White et al., 1990). A 645-bp ITS and 582-bp 28S rDNA sequences (GenBank accession No. KY865729 and MK430430) were obtained, showing ˃ 99.8% identity with both Erysiphe quercicola on Quercus mongolica (KY700706 and KY797320) and Oidium heveae on Hevea brasiliensis (KJ868176 and KJ868175), respectively. Analysis of both morphology and sequence identity suggests that the pathogen is Erysiphe quercicola S. Takam. & U. Braun, a powdery mildew fungus, which is parasitic on Q. mongolica in China (Liyanage et al., 2016). According to the Koch’s postulates (Falkow, 1988), a diseased leaf was gently pressed onto leaves of three-week-old healthy J. curcas seedlings grown in separate pots to test the pathogenicity of E. quercicola. Inoculated plants were cultured in a greenhouse at 28/25℃ (day/night) at 14/10-h light/dark photoperiod and 70% relative humidity. After six days, the inoculated leaves displayed the similar symptoms that appeared in the original diseased plants, and after 12 days, the inoculated leaves began to drop; whereas no similar symptoms were observed in the control plants. The pathogenicity test was carried out twice with similar results. Microscopic observation showed that the fungus isolated from the inoculated leaves was identical to the original pathogen. To our knowledge, this study is the first report that the powdery mildew caused by E. quercicola occurred on J. curcas in Yunnan province, China, which is also being a serious problem in J. curcas plantation worldwide.