First report of Trichoderma roseum causing tea leaf rot in China.

Abstract

Tea (Camellia sinensis) is consumed worldwide for its numerous benefits and China lead the world production. In March 2023, leaf spots were observed on approximately 10% of tea plants in a 50-ha commercial tea plantation in Menghai (21°46'13"N, 100°30'6"E), Yunnan, China. Initial symptoms appeared as small spots, which progressively expanded and spread over the entire leaf surface. Subsequently, pale pink mold layers developed from the lesions (Fig. S1). To isolate the pathogen, small leaf pieces (3 × 3 mm) were cut from the margins of the lesions, sterilized with 75% ethanol for 30 sec and 0.5% NaClO for another 30 sec, and rinsed three times with sterile water. The pieces were placed on acidified potato dextrose agar (PDA) plates and incubated in darkness at 28°C. A total of 15 fungal isolates with identical morphologies were collected. The colonies appeared pale pink with white mycelia initially then turned orange-pink at the center and light white at the edges. After 10-15 days, exhibiting a powdery texture and concentric rings with uniform edges. Conidia were found at the apex peduncle and were inverted pear-shaped or oval, either non-septate (15.3 ± 2 × 7.8 ± 1.8 μm in size, n = 60) or septate, with a slightly constricted spore base featuring papillary projecvtions (14.8 ± 1.5 × 7.4 ± 0.7 μm in size, n = 60). The morphology closely resembled Trichoderma roseum (Oh et al. 2014). To confirm the species, the strain CYB5 was selected for identification by sequencing the ribosomal internal transcribed spacer (ITS) and large subunit (LSU) genes using polymerase chain reaction (PCR) (White et al.1990). The ITS (GenBank accession OR889657) and LSU (PQ270526) gene sequences exhibited 98% similarity with the Trichoderma roseum sequence KP317992 from NCBI database. A phylogenetic tree was constructed using MEGA 11 (Felsenstein 1981) based on the concatenated sequences (ITS and LSU) of the strain CYB5 and reference strains (Fig. S2). The analysis confirmed that CYB5 is T. roseum (Inácio et al. 2011). Pathogenicity tests were conducted on detached healthy tea leaves placed on wet filter paper in petri dishes. Micro-wounds were made on leaves using a sterilized needle, followed by inoculation with a 6-mm plug of CYB5. Control leaves were inoculates with fungus-free agar disks. The dishes were incubated at 25°C in the dark for 7 days. The leaves inoculated with CYB5 developed reddish brown to dark brown lesions around the inoculated sites, while control leaves remained asymptomatic. The fungus was reisolated from the lesion, and the isolates were morphologically identical to the original cultures. A second pathogenicity test was conducted on potted tea plants of the cultivar 'Yunkang No. 10.' Three plants scratched with a needle and three non-wounded plants were inoculated by spraying 20 ml of a spore suspension (105 spores/ml) of CYB5. Plants sprayed with sterile water served as controls. All plants were maintained in a growth chamber at 28°C, and 70% relative humidity. The lesions developed three days post-inoculation, and typical symptoms appeared after 10 days on spore-inoculated plants only. T. roseum was reisolated and reidentified based on the morphology and molecular analyses, thus fulfilling Koch's postulates. To our best knowledge, this is the first report of T. roseum causing tea leaf rot in China.