First Report of Didymella uniseptata Causing Leaf Spot on Taraxacum mongolicum in China.

ABSTRACT

Dandelion (Taraxacum mongolicum), belonging to the Asteraceae family, is one of the main associated species in the alpine meadow, and is famous for its both feeding and medicinal values (Lin et al. 2022; Wang et al. 2022). In September 2021 (vigorous growth period), a newly emerging leaf spot disease of T. mongolicum were observed on natural grassland in Ruoergai County, Aba (Ngawa) Tibetan and Qiang Autonomous Prefect, China (33°59'51'' N, 102°44'57'' E, alt. 3414 m). Leaf disease incidence was ranged from 10% to 15%. The symptoms appeared as brown to dark brown, circular or irregular, sunken spots; eventually, the infected sites of leaves formed a hole in the middle position of lesions. For isolation, 21 tissue pieces (5mm × 5mm) from 7 symptomatic leaf samples of 4 different plants were surface sterilized with 70% ethanol for 30 s and rinsed three times with sterilized distilled water. Then, these tissues were placed on potato dextrose agar (PDA) at 25°C and incubated in the dark for 2 to 7 days. Finally, six pure Didymella strains with consistent colony characteristics were obtained from hyphal tips as described by Xue et al. (2023). Colonies on PDA were brown to black with concentric circles, and abundant black pycnidia were visible; reverse similar in color. Conidia were ellipsoidal, ovoid, hyaline, 0 to 1-septate, 4.07 to 8.67 × 2.74 to 5.35 µm (average 3.60 × 6.55 µm; n = 50). Seven-week-old healthy plants were obtained by growing T. mongolicum seeds in pots (two plants per pot). The six pure Didymella strains were subsequently used to inoculate healthy plants as follows for each strain, eight pots were spray inoculated with a mycelial suspension of about 4 × 104 CFU/ml, referring to our previous method (Xue et al. 2023). In addition, eight pots considered as non-inoculated controls were sprayed with sterilized distilled water. All pots were individually covered with transparent polyethylene bags for 5 days to maintain high relative humidity and placed in a greenhouse at 23 to 29°C. After incubation for 10 days, the typical symptoms consisted of brown to dark brown, sunken spots, which were similar to those previously observed in nature grassland; however, symptoms were not observed on the non-inoculated plants (controls). The same fungus was reisolated from the lesions and confirmed by the morphological and molecular methods described in this note, thus fulfilling Koch's postulates. To further identify this fungal pathogen, ITS-rDNA, and two other protein-coding genes (rpb2 and tub2) of the representative strain REG28 were amplified with primers described by Chen et al. (2022). Sequences were deposited in GenBank (PP385777 for ITS, PP781948 for rpb2, and PP781947 for tub2). A maximum likelihood (RAxML) phylogenetic tree based on the combined ITS, rpb2, and tub2 alignments showed REG28, and ex-type CGMCC 3.20069 of D. uniseptata (Chen et al. 2022) formed a subclade with 100% bootstrap support (Fig. S1). The causal agent of this disease was confirmed as D. uniseptata by the morphological, molecular, and pathogenic features described above. Recently, D. macrophylla has been reported as the first record on T. officinale in Russia (Gomzhina et al. 2020). To our knowledge, this is the first report of D. uniseptata causing leaf spots on T. mongolicum worldwide. This information will be useful for the diagnosis, detection, pathogen identification and future control of this disease on T. mongolicum in natural grassland.