First Report of Scab on Aralia elata Caused by Elsinoë araliae in China
TL;DR
The pathogen was successfully re-isolated from the symptomatic tissues of the inoculated plants, and its identity was confirmed through morphological observation, fulfilling Koch’s postulates.
Abstract
Aralia elata (Miq.) Seem, is an important medicinal and edible woody plant widely cultivated in Northeast China. In June 2024, typical scab symptoms were observed on the leaves, petioles, and young stems of A. elata in commercial plantations in Qingyuan County (42.32 °N, 125.07 °E), Liaoning Province, China. The disease incidence was approximately 70% across the surveyed fields. On stems and petioles, lesions appeared as small, raised, corky brown spots that coalesced into irregular necrotic patches, restricting shoot growth. On leaves, symptoms consisted of circular to irregular necrotic spots (0.5 to 3.0 mm in diameter.) with dark brown, slightly raised margins and light brown centers. Lesions also developed along leaf veins and petioles, leading to leaf distortion, premature yellowing, and defoliation in severe cases. To isolate the pathogen, small pieces (3 × 3 mm) of symptomatic tissue from lesion margins were surface-disninfested with 75% ethanol for 30 s and 2% sodium hypochlorite for 2 min. The tissues were rinsed twice with sterile distilled water, plated on potato dextrose agar (PDA), and incubated at 25°C. The resulting isolates grew very slowly on PDA, reaching 15 to 20 mm in diameter after 30 days. Colonies were initially white to grayish-white with sparse, velvety aerial mycelium, later becoming raised, cerebriform, and wrinkled at the center. A reddish-brown pigment diffused into the medium, and the reverse side exhibited a dark tawny discoloration. Conidia were hyaline, aseptate, ellipsoid to oval, with obtuse apices and smooth walls, sized 6.48 (5.54 to 8.82) μm × 3.04 (2.29 to 3.98) μm. For molecular identification, the internal transcribed spacer (ITS) region, large subunit (LSU) ribosomal DNA, and second largest subunit of RNA polymerase II (RPB2) genes were amplified using the respective primer pairs ITS1/ITS4 (GARDES and BRUNS 1993), LR0R/LR7 (Zheng et al. 2024), and RPB2-5F/RPB2-7CR (Reeb et al. 2004). A BLASTn search of the ITS, LSU, and RPB2 sequences revealed 99.68% to 99.87% sequence identity with Elsinoë araliae reference sequences (GenBank accession nos. JX462672, PP719618 and OQ472905, respectively). The sequences have been deposited in GenBank with the accession numbers PX698841 (ITS), PX697238 (LSU), and PX711174 (RPB2). A maximum-likelihood phylogenetic tree constructed using PhyloSuite v 1.2.3 (Zhang et al. 2020) clustered the isolate (designated as CJ-1) within the E. araliae clade. Pathogenicity was verified by spraying a conidial suspension (1×106 conidia/mL) of isolate CJ-1 onto leaves of 2-year-old healthy A. elata seedlings (n=10), control plants (n=10) were sprayed with sterile distilled water. All plants were maintained in a greenhouse at 25°C with 90% relative humidity. After 20 days, typical scab symptoms identical to those observed in the field developed on all inoculated plants, whereas control plants remained symptomless. The pathogen was successfully re-isolated from the symptomatic tissues of the inoculated plants, and its identity was confirmed through morphological observation, fulfilling Koch’s postulates. E. araliae was previously reported causing scab disease on A. elata in Korea (Choi et al. 1998). To our knowledge, this is the first report of scab disease on A. elata caused by E. araliae in China. The disease poses a potential threat to the commercial production of A. elata in Northeast China, and further studies on its epidemiology and management are warranted.