First Report of Powdery Mildew Caused by Microidium phyllanthi on Chamber Bitter (Phyllanthus urinaria) in China

Abstract

Chamber bitter (Phyllanthus urinaria L.), a species of the Euphorbiaceae family, is an annual perennial herbal species distributed in tropical Asia, America, China, and the Indian Ocean islands. In China, this herb is known to contain various bioactive compounds and is commonly used in traditional Chinese herbal medicine (Geethangili and Ding 2018). In December 2018, typical symptoms of powdery mildew were observed on approximately 80% of P. urinaria plants on the Hainan University campus in the city of Haikou, Hainan Province, China. Powdery mildew colonies covered the leaf surfaces of affected plants, causing discoloration and defoliation. Conidiophores were unbranched, 83 to 153 × 5 to 7 µm. Foot cells were cylindric, straight, occasionally slightly curved at the base, 41 to 105 µm long, and produced three to nine conidia in chains. Conidia were short, cylindrical or ovoid- cylindrical, 15 to 25 ×7 to 11 µm (length/width ratio = 1.7 to 2.6), without fibrosin bodies, showed angular/rectangular wrinkling of outer walls, and produced Microidium-type germ tubes on the shoulder. Two small germ tubes were broad club-shaped, with nipple-shaped appressoria. Hyphal appressoria were nipple-shaped, in pairs or single. Based on these morphological characteristics, this pathogen resembles Microidium phyllanthi (Braun and Cook 2012). The teleomorph was not found. A specimen was deposited in the Hainan University Plant Pathology Herbarium as HNPU-18. To confirm its identification, genomic DNA was extracted from mycelium, conidiophores, and conidia using a Fungal DNA kit (Omega Bio-Tek, U.S.A.). The rDNA internal transcribed spacer (ITS) region was amplified with the primers ITS1/ITS4 (White et al. 1990) and sequenced directly. The resulting 607-bp sequence was deposited in GenBank (accession no. MN186773). A BLASTn search in GenBank of this sequence showed 100% similarity with the ITS sequences of M. phyllanthi on plants of P. urinaria from China (MH359093) and Vietnam (KM260738 and KM260739) on and P. acidus from Japan (AB719943 and LC259488). To confirm pathogenicity, five healthy potted chamber bitter plants were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five noninoculated plants served as controls. Plants were maintained in a greenhouse at 24 to 30°C, 70% relative humidity, and a 16-h photoperiod. After 7 days, inoculated leaves showed powdery mildew symptoms, whereas no symptoms were observed on control plants. The fungus observed on inoculated plants was morphologically identical to that on the original infected leaves, fulfilling Koch’s postulates. Previous studies reported that M. phyllanthi can infect other Phyllanthus species, including P. acidus, P. amarus, P. niruri, P. reticulatus, P. rheedii, and P. urinaria (Tam et al. 2015). However, the morphological characteristics of powdery mildew fungus on different Phyllanthus species are slightly different (To-anun et al. 2005). Our study showed that the morphological characteristics of the pathogen are different from what have been reported (Tam et al. 2015). To our knowledge, this is the first record of M. phyllanthi infecting P. urinaria in China. The pathogen may pose a threat to the production of P. urinaria in the future.