First Report of Powdery Mildew Caused by Podosphaera xanthii on Phasey Bean (Macroptilium lathyroides) in China

The forage legume phasey bean Macroptilium lathyroides (L.) Urb. is an annual or short-lived perennial of the family Fabaceae. It is native to the tropical and subtropical areas from North to South America, and is naturalized throughout the tropics and subtropics of the world (Tobisa and Nakano 2019). It is mainly used for forage, green manure, and slope protection (Silva et al. 2018). In addition, the nitrogen fixation ability of this plant can also improve the soil. In February 2022 and January 2023, powdery mildew symptoms were observed on 70% of M. lathyroides plants on the Hainan Medical University campus (19° 58′ 53″ N; 110° 19′ 47″ E) in Haikou, Hainan Province, China. Powdery mildew colonies covered the leaf surfaces and stems of affected plants, causing discoloration and defoliation. Mycelia were superficial and hyphal appressoria were nipple-shaped. Conidiophores (n =30) were unbranched, cylindrical, 100 to 233 × 8 to 15 µm, and produced three to five immature conidia in chains with a crenate outline. Foot cells (n =30) were cylindrical, straight or sometimes curved at the base, and 36 to 56 µm long. Conidia (n =100) were ellipsoid-ovoid to doliiform, 24 to 34 ×13 to 20 m (length/width ratio = 1.5 to 2.3), with well-developed fibrosin bodies, and produced germ tubes from the lateral position. Based on these morphological characteristics, the pathogen was provisionally identified as Podosphaera xanthii (Braun and Cook 2012). The teleomorph was not observed. A specimen was deposited in the Hainan Medical University Plant Pathology Herbarium as HMML-23. To confirm the identification, genomic DNA was extracted from mycelium, conidiophores, and conidia using a fungal DNA kit (Omega Bio-Tek, USA). The rDNA internal transcribed spacer (ITS) region was amplified with primers ITS1/ITS4 (White et al. 1990) and sequenced directly. The resulting 575-bp sequence was deposited in GenBank (accession no. OR240256). A BLASTn search in GenBank of this sequence showed 100% similarity with the ITS sequences of P. xanthii isolates from China (MT242593, MK439611 and MH143483), Korea (MG754404), Vietnam (KM260704), Japan (MZ604267), and Puerto Rico (OP882310). Additionally, the 28S rDNA region was amplified using the primer pairs NL1 and NL4 (O´Donnell 1993; accession no. OR240255). This region shared 100% similarity with P. xanthii isolates (MK357436, LC371333, OP765401, and MZ604267) as well. To confirm pathogenicity, five healthy potted plants of M. lathyroides were inoculated by gently pressing a powdery mildew-infected leaf onto 15 young leaves. Five non-inoculated plants served as controls. All plants were maintained in a greenhouse at 24 to 30°C, 70% relative humidity, with a 16-h photoperiod. After 7 days, inoculated leaves showed powdery mildew symptoms whereas no symptoms were observed on control plants. The fungal colonies observed on inoculated plants were morphologically identical to those found on the originally infected leaves collected from Hainan Province. Based on the morphological characteristics and molecular identification, the fungus was identified as P. xanthii. This fungus has been reported causing powdery mildew on M. atropurpureum in Thailand (Meeboon et al. 2016). In the United States phasey bean powdery mildew caused by Erysiphe fallax has been previously reported (Poudel and Zhang 2019). To our knowledge, this is the first record of P. xanthii infecting M. lathyroides in China. Over the past 50 years of introduction, phasey bean has become one of the main leguminous forages for establishing artificial mixed seeding grasslands in southern China. We are concerned that the pathogen could become a threat to the widespread planting of M. lathyroides in the future.