Cuphea hyssopifolia (Mexican heather) is a popular evergreen perennial shrub used for ornamental and medicinal purposes. Due to its high ornamental value, it is often used as a ground cover in parks and gardens in China. During February and March 2019 & 2020, powdery mildew was observed on C. hyssopifolia in the districts of Minhou and Jinshan of Fuzhou, China. Disease incidence was 70% but of low severity with only a few older leaves showing yellowing and wilting. Sparse irregular patches of white superficial powdery mildew observed on both sides of mature and young leaves. The powdery mildew fungal appressoria that occurred on epigenous hyphae, were indistinct to nipple-shaped, hyaline, and smooth. Conidiophores were erect, smooth, 80 to 210 × 10 to 12 µm, and produced two to eight crenate-shaped conidia in chains. Foot-cells of conidiophores were straight, cylindric, and 30 to 65 × 10 to12 µm. Conidia were hyaline, smooth, ellipsoid-ovoid to barrel-shaped, 25 to 38 × 16 to 20 µm with distinct fibrosin bodies. Germ tubes were simple to forked and produced from the lateral position of the germinating conidia. No chasmothecia were observed on the surface of infected leaves. Based on the morphology of the imperfect state, the powdery mildew fungus was identified as Podosphaera xanthii (Castagne) U. Braun & N. Shishkoff (Braun and Cook 2012). To confirm fungal identification, total DNA was extracted (Mukhtar et al., 2018) directly from epiphytic mycelia on infected leaves collected from both districts. Internal transcribed spacer (ITS) regions and the partial large subunit (LSU) rDNA were amplified using primers ITS1/ITS4 and LSU1/LSU2 (Scholin et al. 1994, White et al. 1990), respectively. The sequences were deposited in GenBank (ITS: MW692364, MW692365; LSU: MW699924, MW699925). The ITS and LSU sequences were 99 to 100 % identical to those of P. xanthii in GenBank, (ITS: MT568609, MT472035, MT250855, and AB462800; LSU: AB936276, JX896687, AB936277, and AB936274). Koch's postulates were completed by gently pressing diseased leaves onto leaves of five healthy potted C. hyssopifolia plants that were held in a greenhouse at 24 to 30°C without humidity control. Five non-inoculated plants served as controls. Inoculated plants developed symptoms after 6 to 10 days, whereas the controls remained symptomless. The morphology of the fungus on the inoculated leaves was identical to that observed on the originally diseased leaves. Previously, Podosphaera sp. has been reported on C. rosea in the United Kingdom (Beales & Cook 2008) and P. xanthii on C. hyssopifolia in Taiwan (Yeh et al. 2021). To our knowledge, this is the first report of powdery mildew caused by P. xanthii on C. hyssopifolia in mainland China. Our field observations suggest that the P. xanthii infections would be a potential threat to the health of C. hyssopifolia in China. References: Beales, P. A., and Cook, R. T. A. 2008. Plant Pathol. 57:778. Braun, U., Cook, R. T. A. 2012. The Taxonomic Manual of the Erysiphales (Powdery Mildews). CBS Biodiversity Series 11: CBS. Utrecht, The Netherlands. Mukhtar, I., et al. 2018. Sydowia.70:155. Scholin, C. A., et al. 1994. J. Phycol. 30:999. White, T. J., et al. 1990. Page 315 in: PCR Protocols: A Guide to Methods and Applications. Academic Press, San Diego, CA. Yeh, Y. W., et al. 2021. Trop. Plant Pathol. 46:44.

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