Erysiphe russellii Causing Powdery Mildew on Oxalis corniculata L. in Central China.

Abstract

Oxalis corniculata L. (Creeping woodsorrel) is a perennial plant of the genus Oxalis in Oxalidaceae family, which has the high ornamental and medicinal value. Extracts of creeping woodsorrel are used as antioxidants and cholinesterase inhibitors and for anti-inflammatory, antiseptic, analgesic, antirheumatic ache, and antimicrobial purposes (Leporatti et al. 2003). In June 2024, powdery mildew was found on the leaves of Creeping woodsorrel in Xinxiang City, Henan Province, China (113.925°E, 35.294°N). About 100 plants were examined and 80 % were infected with disease symptoms, i.e. curling and senescence. The white masses were on the both sides of plant leaves covering up to 90% of the leaves area. The slightly or straight curved conidiophores (n = 50) were 68 to 99× 6 to 12 μm in size and consisted of foot cells, shorter cells and singly conidia. The ellipsoidal to oval conidia (n =50), were 26 to 10 ×11 to 5 μm in size and had a length/width ratio of 1.6 to 2.2. These morphological characteristics were similar to the previously reported Creeping woodsorrel powdery mildew fungus, Erysiphe russellii (Thuong et al. 2017; Takamatsu et al. 2015). Following previously described methods (White et al. 1990; Bradshaw et al. 2022; Zhu et al. 2022), the ITS (ribosomal transcribed spacer), GAPDH (glyceraldehyde-3-phosphate dehydrogenase), RPB2 (RNA polymerase II), GS (glutamine synthetase) and CAM (calmodulin) gene regions of three isolates were amplified with specific primers ITS1/ITS4 (ITS1 5'-TCCGTAGGTGAACCTGCGG-3'; ITS4 5'-TCCTCCGCTTATTGATATGC-3'), PMGAPDH1/PMGAPDH3R (PMGAPDH1 5'-GGAATGGCTATGCGTGTACC-3'; PMGAPDH3R 5'-CCCCATTCGTTGTCGTACCATG-3'), CAM1/CAM4R (CAM1 5'-CTTTGCATCATGAGTTGGAC-3'; CAM4R 5'-GGCTCGAAAAATGAAAGATACCG-3'), Rpb2_4/Rpb2_6R (Rpb2_4 5'-GCAAGCTCAACTGCTGGTG-3'; Rpb2_6R 5'-TCCAGCGATGTGCTGTTGG-3'), GSPM2/GSPM3R (GSPM2 5'-CCAATCAGTTACTGTTTGTTCCC-3'; GSPM3R 5'-GGACTTCCTGATATTATGCC3'). Sanger sequencing results showed that each sequence of the three isolates were the same. Then sequences of one isolate were uploaded in GenBank (Accession No. PQ044579, PQ149219, PQ149220, PQ149221and PQ149222, respectively). The sequences were 100% identical to those of a previously reported E russellii on O. corniculata (Thuong et al. 2017; Takamatsu et al. 2015). The pathogen and the previously reported E russellii are clustered in the same branch in the phylogenetic tree (Thuong et al. 2017; Takamatsu et al. 2015). The pathogenicity was tested according to the method previously described (Zhu et al. 2021). By blowing conidia on infected leaves with pressurized air, the fungus was inoculated onto the leaves of three healthy plants (three-month-old), with three uninoculated plants (three-month-old) treated as controls. The infected plants and the control plants were placed in the culture room with the temperature 23 °C, humidity 50% and light/Dark 16/8 h, respectively. 10-12 days post inoculation, the leaves of the inoculated plants showed signs and symptoms of powdery mildew, while the control group was unaffected. The pathogen of the infected plant disease was re-examined by morphological characteristics and was similar to the original fungus. The pathogenicity tests were repeated three times and same results were obtained. Therefore, the pathogen was identified and confirmed as E russellii (isolate ER-ZM2024). Previously, E russellii was reported on Oxalis corniculata L. in Japan and Korea (Thuong et al. 2017; Takamatsu et al. 2015). To the best our knowledge, this is the first report of powdery mildew caused by E russellii on O. corniculata L. in central China. This identification of E russellii on O. corniculata L. provides a new perspective for the study of the disease. The results of this study provide the sequences of E russellii for further phylogenetic analysis.