The tea plant (Camellia sinensis L.) is an economically important woody crop worldwide (Shen et al., 2018). In April 2020, a field survey was conducted in Tai’an City, P. R. China (GPS: 116.94007, 36.21326), which revealed some symptoms on tea plants, including stunted growth, wilting, partial defoliation, and stem necrosis. The incidence of these symptoms was approximately 5%. Thirty-three aboveground tea plants, cv. Fudingdabai, with these symptoms, were randomly collected and tested for pathogen presence, whose roots were totally rotted. The tissues of the stem crown between healthy and diseased were removed, sterilized using 75% ethanol solution for 60 s, rinsed with sterile distilled water five times, placed onto potato dextrose agar (PDA), and then incubated at 28°C for 72 h; forty-nine single purified isolates by single hyphae isolation were obtained, which were divided into three types: 25 isolates with light-salmon-colored colonies, three isolates with light gray-colored colonies, and 21 isolates with white-colored colonies. The 21 isolates in white with cottony appearances and abundant aerial hyphae, whose primary hyphae were 4 ± 2 μm; hyphal swellings were terminal or intercalary, and 14 ± 7 μm in diam; oogonia were terminal or intercalary and globular, and 18 ± 3 μm in diam; oospores were aplerotic, globose, and 15 ± 5 μm in diam, and the temperature range for optimal growth was 25 to 30°C. These morphological characteristics were observed using an Olympus IX73 microscope with cellSens Standard software. Further PCR analysis with specific primers ITS1/ITS4 identified 25 isolates in light salmon-colored as Colletotrichum gloeosporioides and three isolates in light gray-colored as C. fructicola, which have been reported as the pathogen on tea plant (Wan et al., 2021; Zhang et al., 2021). The remaining 21 isolates were identified through DNA sequence analysis using BLAST. The confirmation of identity was based on comparing DNA sequences of the rDNA internal transcribed spacer region (ITS), cytochrome c oxidase I and II (coxI and coxII) genes, which were amplified using specific primers ITS1/ITS4 (White et al. 1990), Cox1-OomCOI-Lev-up/FM85-mod (Bala et al. 2010), and Cox2-F/RC4 (Choi et al. 2015). These selected 21 isolates for sequence analysis showed complete homology; hence, only one isolate, TPSD, was submitted to GenBank (Accession nos.: ITS, OM432004; coxI, OM371060; coxII, OM371061). The sequences of the ITS, coxI, and coxII of the TPSD exhibited 99% homology (944/950 bp), 98% homology (719/731 bp), and 99% homology (550/556 bp) with KU210955, MT222552, and MW006635 of Globisporangium irregulare. Based on morphological and sequence analysis, the isolate was identified as G. irregulare. Nine out of total plant samples contain both G. irregulare and Colletotrichum gloeosporioides. The pathogenicity of TPSD was tested by inoculating 15 one-year-old tea plants, cv. Fundingdabai, with 3 to 4 mm agar disks of TPSD. Five tea plants were inoculated with PDA disks only as a control. The inoculated plants were placed in a growth chamber at 25°C, 80% relative humidity, 16 h/8 h light/dark cycle. Fourteen days post-inoculation, the symptoms, such as stunted growth, wilting, partial defoliation, and stem necrosis, on inoculated plants were the same as those observed in the field. The experiments were conducted three times, and the fungus was recovered and identified as G. irregulare isolate TPSD by both morphological characteristics and molecular analysis, satisfying Koch's Postulates. The control remained asymptomatic, and G. irregulare was not detected. This is the first report of G. irregulare as a pathogen on tea plants in China. Henceforth, G. irregulare could be a candidate oomycete pathogen for tea stem and root rot, because its control methods would differ from fungi pathogens, such as Colletotrichum gloeosporioides and C. fructicola.
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