First Report of Powdery Mildew Caused by Erysiphe trifoliorum on Aeschynomene indica in China.

Abstract

Aeschynomene indica is a semiaquatic legume that can be used as field green manure, forage grass, and medicinal plant (Zhang et al., 2019). It is also an "amphibious" plant commonly used as a wetland park green plant. In October 2020, typical powdery mildew symptoms were found on A. indica plants in Fanglan Lake Wetland Park, Jiujiang City, Jiangxi Province, China. More than 90% of A. indica plants were infected, and powdery mildew covered both sides of infected leaves, as well as stems and pods, forming circular to irregular patches. The infected leaves were collected to identify the pathogen, and microscopic examination revealed that there were abundant conidia and hyphae on the surface of the leaves. The conidia were solitary, elliptical to cylindrical, 23.23-38.03 μm in length and 9.14-17.56 μm in width (n = 100). The conidiophores were straight, cylindrical, unbranched, and ranged from 43.80 to 76.16 μm in length (n = 100). The foot cells were cylindrical or, rarely, somewhat curved at the base, 23.20-47.07 × 4.23-8.87μm (n = 100) and were followed by 1-2 shorter cells. Appressoria and chasmothecia were not observed on the collected samples. Based on these morphological characteristics, the powdery mildew fungus was tentatively identified as Erysiphe trifoliorum (Lee and Thuong, 2015). To further confirm this identification, the ribosomal DNA internal transcribed spacer (ITS) region and the partial sequence of the 28S large subunit (LSU) were amplified using primer pairs ITS1/ITS4 (White et al., 1990) and NL1/NLP2 (Mori et al., 2000), respectively. The obtained 662-bp ITS sequence (GenBank accession no. OK021589) was 99.55% identical to the ITS sequences of E. trifoliorum on Medicago littoralis in Azerbaijan (LC270860). The obtained 656-bp LSU sequence (GenBank accession no. PQ475931) was 99.69% identical to the ITS sequences of E. trifoliorum on Mimosa pudica in China (PQ184869). Additionally, a phylogenetic tree analysis was conducted based on the combined ITS and LSU sequences, which indicated that the isolate was grouped in the same clade as E. trifoliorum. Therefore, this fungus was identified as E. trifoliorum based on the morphological and molecular characteristics. Pathogenicity tests were performed by gently pressing infected leaves onto the young leaves of six healthy A. indica plants, and non-inoculated plants were used as controls. All plants were maintained in a greenhouse (25 ± 2 °C, 80% relative humidity, and a 16-hour photoperiod). After 14 days, typical powdery mildew features were observed on the inoculated plants, whereas no such features appeared on the control plants. The fungus on the inoculated leaves was morphologically and molecularly identical to that found on naturally infected plants collected from the Jiangxi Province. Powdery mildew caused by E. trifoliorum has been reported to affect some leguminous plants, including Indigofera amblyantha, Trifolium hybridum, Trifolium repens, and Melilotus indicus (Cho et al., 2016; Hong et al., 2020; Lee et al., 2015; Mukhtar et al., 2017). To our knowledge, this is the first report of powdery mildew caused by E. trifoliorum on A. indica in China, which seriously threatens the utilization of A. indica. The identification of E. trifoliorum as the causative agent of powdery mildew on A. indica is critical for the prevention and management of this disease in the future.