First Report of Powdery Mildew on Vigna unguiculata Caused by Podosphaera xanthii in China

Abstract

Vigna unguiculata belongs to the Legume family, and is an annual twining, herbaceous vine plant, which is native to Africa. V. unguiculata is the most economically beneficial type of off-season vegetables in Hainan, China because of its rich in nutrients such as protein, minerals, dietary fiber, and vitamins (Jayathilake et al. 2018). In April 2022, typical powdery mildew infection was observed on V. unguiculata leaves in Haikou, Hainan Province, China (20°3'40.428"N, 110°19'45.217"E). More than 70% leaves of 13 V. unguiculata plants displayed severe powdery mildew disease. The diseased leaves at first exhibited white rounded irregular patches, which gradually enlarged, fused and covered all the leaf as well as stems. Edges of the infected leaves crinkled upwards, and the leaves often fell off the plants at the late infection stage. On the infected leaves, many conidiophores and dense mycelium were observed by microscopic analysis. Hyphae were septate, branched, epigenous, and flexuous to straight. Hyphal appressoria were indistinct or slightly nipple-shaped, and the haustoria developed were in the shape of oval-sphere and 9 to 11μm long. Foot cells of conidiophores were straight, cylindrical, and measured 43 to 70 × 10 to 12 μm. The conidiophores arising straightly from a hyphal cell, were measured 168 to 252 (mean = 204) μm in length and produced 6 to 9 immature conidia in each chain. Conidia were ovate, pyriform or barrel-shaped, with fibrosin bodies, and measured 26 to 32 (mean = 29.4) × 16 to 20 (mean = 18.6) μm. The chasmothecia was not found in all samples. These morphological characteristics were typical of the conidial stage of the powdery mildew Podosphaera xanthii of genus Podosphaera (Braun and Cook 2012). To further confirm the identity of this causal fungus, the internal transcribed spacer (ITS) region, and the partial sequence of large subunit ribosomal RNA gene (28S rRNA), were amplified with primer pairs ITS1/ ITS4 (White et al. 1990) and NL1/NLP2 (Mori et al. 2000) from extracted genomic DNA. The obtained 563-bp ITS region and 715-bp 28S rRNA gene sequences were deposited in GenBank (ITS, OQ415534; 28S rRNA, OQ415545.1), and were compared with BLAST analysis in the GenBank nr database. The results revealed that the ITS region sequence was 99.82% identity with P. xanthii isolate HUVU-08 (MH143485.1), and the 28S rRNA gene partial sequence was 100% identity with P. xanthii isolate XHL1 (MK357442.1). On the basis of the morphological characteristics and sequence analysis, this fungus was identified as P. xanthii. Pathogenicity tests were performed by gently brushing conidia onto the leaves of six healthy potted V. unguiculata plants. Six non-inoculated plants were used as control. All plants were maintained in a greenhouse at 26 ± 2°C. After 2 weeks inoculation, similar symptoms were observed in the inoculated plants, whereas no symptoms occurred on the control plants. By microscopic observation, the fungus present on the inoculated plants was morphologically identical to those on originally diseased plants. Furthermore, ITS and 28S rRNA sequences of the re-isolated fungus individually displayed 100% identity with OQ415534 and OQ415545.1. So far, although powdery mildew disease caused by P. xanthii on different plants including Sigesbeckia orientalis (Mukhtar et al. 2022), Vigna radiata (Sheu et al. 2021), Cosmos bipinnatus (Kong et al. 2023), Verbena brasiliensis (Luecke et al. 2020), Cucurbita ficifolia (Choi et al. 2022), Glandularia tenera (Pei et al. 2023) and Verbena bonariensis (Choi et al. 2023) have been reported, to our knowledge, this is the first report of powdery mildew caused by P. xanthii on V. unguiculata in Hainan, China, which seriously threatens the utilization of V. unguiculata on off-season vegetables industry.