First Report of Leaf Rust of Acer mono Caused by Pucciniastrum hikosanense in China.

Abstract

Acer mono Maxim, mainly distributed in China, Japan, Korea and eastern Russia (Shang et al. 2012), is a widely planted ornamental and pharmaceutical tree (Zhang et al. 2015). In September 2020, leaf samples of A. mono infected by uredinia were collected in Shaanxi Province (34°15'40.06'' N, 108°3'54.54'' E, alt. 432.35m), China. Telia development was observed in late autumn. Voucher specimens were deposited in the Herbarium Mycologicum Academiae Sinicae (no. HMAS249354), China. This led to premature defoliation and in the 90% planting wide incidence. Geospatial investigations revealed that this rust was widely distributed in local urban parks, but was nonpathogenic to A. buergerianum, A. negundo, A. oblongum, A. palmatum and A. rubrum. This fungus was morphologically characterized and most closely matched descriptions of Pucciniastrum. Uredinia were hypophyllous, subepidermal, scattered to gregarious, oval or round, 0.10-0.30 × 0.08-0.15 mm, golden yellow to orange, somewhat pulverulent. Peridia were hemispherical, erumpent with apical pores; peridial cells minute, irregularly polygonal, hyaline to pale yellow; ostiolar cells ellipsoid or roundish. Urediniospores were subglobose, ovate or ellipsoid, 20-33 × 15-21 μm, yellow to pale orange; wall 1-2 μm thick, hyaline to pale yellow, echinulate, somewhere smooth. Pedicels were deciduous, hyaline, minute, fragile. Telia were hypophyllous, subepidermal, intermixed with uredinia, irregularly polygonal, restricted by veins, 0.34-0.91 × 0.21-0.54 mm, and orange to amber brown. Teliospores were produced parallelly single-layered, and were subglobose, oblong, sometimes angular, 23-47 × 16-34 μm, colorless to pale yellow, 1-5 mediastinal, 2-6-celled; lateral wall 1-1.6 μm thick, apical wall 1-3 μm thick, smooth, hyaline. The internal transcribed spacer (ITS) and rDNA-28S regions were amplified using ITS1F/ITS4 and NL1/NL4 (Ji et al. 2019) to confirm the identification. The aligned sequences were deposited in GenBank (accession no. MW391829, MW543709, MW541916, MW541917). Phylogenetic trees were constructed based on neighbor-joining (NJ), maximum-likelihood (ML) and Bayesian methods. ML and NJ bootstrap values were calculated by bootstrap analyses of 1,000 replicates with GTR+G+I model using MEGA-X (Kumar et al. 2018), while Bayesian Markov chain Monte Carlo analyses were performed using MrBayes ver. 3.1.2 (Huelsenbeck & Ronquist 2001; Ji et al. 2019). Phylogenetic analysis revealed that HMAS249354 and Pucciniastrum hikosanense were grouped into one clade highly supported by bootstrap values of NJ, ML, and Bayesian posterior probability (Bpp) of 97%/93%/1, respectively. Koch's postulates were fulfilled with 1-year-old healthy plants of A. mono. Fresh urediniospores were collected and suspended in a 0.05% water solution of Tween 20, and 100 μl of urediniospores suspension (106 urediniospores/ml) per leaf (n=10) were sprayed, with another ten healthy leaves sprayed with sterile water as the control. The plants were placed in dark for 48 h and then moved into greenhouse at 22°C with 12 h light per day. Disease symptoms after 10-12 days' inoculation on the inoculated leaves which were identical to the original observations, while the control leaves remained healthy. Previously, P. hikosanense was reported to infect Acer rufinerve Sieb. et Zucc. in Japan (Hiratsuka 1940) and A. rubescens Hayata in Taiwan, China (Dai 1979). This is the first report of leaf rust of Acer mono caused by Pucciniastrum hikosanense Hirats. f. in China.