LAUSR

Aristolochia debilis is a perennial twining ornamental plant in China that is used as a food source for butterflies and occasionally sold as an ornamental plant. A. debilis was originally used in traditional Chinese medicine; however, a chemical produced by the plant, aristolochic acid, is a potent human carcinogen, and consequently treatment with A. debilis is no longer recommended (Chen et al. 2012). In September and October 2019, typical symptoms of powdery mildew were found on leaves of A. debilis plants at Henan Normal University (35.18°N; 113.52°E), Henan, China. White colonies were observed on adaxial and abaxial surfaces of leaves. Infected leaves were collected and then monitored to determine the morphological characteristics of the pathogen. Foot cells (n = 25) were 35 to 70 μm long, and conidiophores (n = 50) were 70 to 150 × 6 to 10 μm (length × width) with three to four cells. Singly formed conidia were oblong-elliptical and were 27 to 49 × 10 to 16 μm (average [n = 50], 33 × 13 μm) with a length/width ratio of 2.5. No chasmothecia were observed. These morphological features indicate that this fungus belongs to Pseudoidium (asexual morph of the genus Erysiphe) (Braun and Cook 2012). Genomic DNA was extracted from hyphae attached to leaves using the method described previously (Zhu et al. 2019). The rDNA internal transcribed spacer (ITS) regions were amplified by polymerase chain reaction (PCR) utilizing fungi universal primer pairs ITS1/ITS4 and PMITS1/PMITS2 (Cunnington et al. 2003; White et al. 1990). PCR products were sequenced by Invitrogen (Shanghai, China), and the resulting sequences of 641 bp from ITS1/ITS4 and 774 bp from PMITS1/PMITS2 were deposited in GenBank (accession nos. MN700999 and MN701000, respectively). BLASTn analysis revealed that MN700999 was 99% identical to Erysiphe takamatsui (MK478870, 639 of 641 nucleotides), E. macleayae (JQ681217, 638 of 641 nucleotides), E. sedi (MN147817, 637 of 640 nucleotides), and E. circaeae (KY660882, 633 of 637 nucleotides) and that MN701000 was 99% identical to E. catalpa (KY653194, 676 of 679 nucleotides), Pseudoidium neolycopersici (AB032484, 672 of 676 nucleotides), and P. hortensiae (MG654731, 744 of 748 nucleotides). These results suggested that this fungus belongs to the E. aquilegiae clade (Takamatsu et al. 2015). Therefore, the causal organism of the powdery mildew was confirmed by morphological characteristics and ITS sequence data as Erysiphe sp. Pathogenicity was tested by gently pressing adaxial surfaces of infested leaves onto young leaves of three healthy A. debilis plants. Three noninoculated plants were used as a control. Inoculated and control plants were separated and maintained in growth chambers (light/dark, 16 h/8 h; temperature, 18°C). White powdery mildew colonies appeared 9 to 12 days after inoculation, whereas control plants remained asymptomatic. The fungus on the inoculated leaves was morphologically identical to that observed on the original diseased leaves. Erysiphe sp. has been reported from a number of hosts in China (Zheng and Chen 1981; Zhou et al. 2015). In Australia and Korea, Erysiphe sp. was reported on Catharanthus roseus, the first reported host outside the Ranunculaceae (Cho et al. 2017; Liberato and Cunnington 2006). To our best knowledge, there has been, so far, no report of Erysiphe sp. infecting A. debilis, and this is the first report of powdery mildew on A. debilis in China, as well as worldwide. Because this powdery mildew could affect the ornamental value of A. debilis plants and reduce the populations of a critical food source for butterflies, control management and strategies should be developed and followed.