First report of leaf blight on Magnolia coco caused by Nothophoma quercina in China.

Abstract

Magnolia coco (Lour.) DC. is an ornamental shrub and widely cultivated in southern China (Nana et al. 2017). In April 2020, leaf blight symptoms were observed on the leaves of M. coco in the Chengdu campus of Sichuan Agricultural University (30°42'19.92″N, 103°51'30.61″E, 493 m) where didn't have great protection, with roughly 70% leaves per plant were diseased. The initial symptoms presented on the leaf apex, which was manifested as dark brown spots surrounded with obvious yellowish halo (Fig. 1). As the disease progressed, spots gradually enlarged and coalesced covering the leaf, and severe infection finally caused leaf necrosis and plant decline. Four specimens from different diseased plants were used for pathogen isolation and morphological observation. Four fungal isolates were obtained from four specimens, following Chomnunti et al. (2014). Colonies on potato dextrose agar (PDA) medium were initially white and then light brown to dark brown. Pycnidia measured 284-427 × 326-554 μm (x=372.8 μm × 476.1 μm, n=20), and were brownish-black to black, broadly globose to irregular. The pycnidial wall measured 16-27 μm wide (n=20) and was composed of hyaline to brown cells of textura angularis. Conidiophores were absent, and the conidiogenous cells are pear-shaped, colorless, and smooth. Conidia measured 5-8 × 4-6 μm (x=6.5 μm × 4.6 μm, n=50), and were elliptical or subglobose, thick-walled, aseptate, hyaline, smooth, brown. These asexual structures were similar to Nothophoma quercina (Syd. & P. Syd.) Qian Chen & L. Cai described by Chen et al. (2017). The genomic DNA of representative isolate SICAUCC 21-0011 was extracted, and the internal transcribed spacers (ITS), 28S large subunit rDNA (LSU), RNA polymerase II large subunit 2 (RPB2), and beta-tubulin (TUB2) regions were amplified using the primer pairs ITS5/ITS4, LR0R/LR5, FRPB2-5F/FRPB2-7cR, and T1/BT4R, respectively. The accession numbers deposited in GenBank were MW541930 (ITS), MW541934 (LSU), MW883395 (RPB2), and MW883394 (TUB2). Nucleotide BLAST showed high homology with the sequences of N. quercina, viz. GU237900 (ITS, 485/486, 99.79%), EU754127 (LSU, 862/862, 100%), KT389657 (RPB2, 593/596, 99.49%), and GU237609 (TUB2, 333/335, 99.40%). Phylogenetic analyses based on a combined dataset showed 100% bootstrap support values in a clade with N. quercina complexes (Fig. 2). Four healthy potted plants (2-years-old) with 15 to 20 leaves per plant were sprayed with conidial suspension (105 conidia/mL) prepared from 4-week-old cultures of SICAUCC 21-0011, which incubated on PDA at 25℃, onto the wounded sites via pin-prick inoculation described by Desai et al. (2019). Another four plants were sprayed with sterilely distilled water as controls. Inoculated plants were cultured in a growth chamber (25℃, 95% relative humidity, and 12-h photoperiod). About 30 days later, brown spots were found on the inoculated leaves, which were similar to those observed in the field. There were no symptoms on the control plants, and the pathogen was re-isolated from the diseased leaves and characterized morphologically. N. quercina has been reported on Photinia × fraseri Dress, Aucuba japonica, Malus micromalus, and Chaenomeles sinensis (Mohamed et al. 2019, Lv et al. 2020, Zou et al. 2021). To our knowledge, this is the first report of leaf blight on M. coco caused by N. quercina. M. coco is one of the important ornaments in the courtyard, street, and park in China, and the risk of this pathogen needs further exploration and effective control measures should be made. Qian Zeng, Yicong Lv, and Xinyue Li contributed equally to this work.