First report of Fusarium proliferatum causing leaf spot on tea plants in Jiangxi Province, China.

Tea (Camellia sinensis (L.) O. Kuntze), a perennial evergreen shrub, is one of the most important cash crops in China. In September 2021, leaf spot symptoms were observed on approximately 30% of tea plants in a 2 ha commercial field of Lushan (29°33'0" N, 115°58'48" E), Jiangxi Province, China. The symptoms initially appeared as small, gray lesions, and later became larger (10-15 mm in diameter) circular to irregular spots with light brown centers and gray borders. To isolate the pathogen, small pieces (3×3 mm) cut from the margins of lesions were sterilized with 75% ethanol for 10 s, 0.1% HgCl2 for 20 s, and then rinsed three times with sterile water. The pieces were placed onto acidified potato dextrose agar (APDA) plates, and incubated in darkness at 28℃. Pure cultures were prepared by subculturing hyphal tips. A total of 16 fungal isolates were obtained, and the colonies of 15 isolates (isolation rate 93.8%) looked identical, resembling those of the genus Fusarium. The colonies were white to pink with purple woolly mycelium. After 10 to 15 days incubation, slightly curved macroconidia with three to four septa measuring 14.0 to 34.5 × 2.0 to 3.5 µm (n = 50), and oval, unicellar microconidia measuring 4.0 to 9.0 × 1.5 to 3.5 µm (n = 50) were observed. These morphological characteristics were similar to that described for Fusarium proliferatum (Leslie and Summerell 2006). Genomic DNA of representative isolates (LSZWY, LSZWY2, LSZWY3) was extracted with the Ezup Column Fungi Genomic DNA Purification Kit (Sangon Biotech Co., Ltd, Shanghai). The translation elongation factor 1 alpha gene (EF-1ɑ) was amplified using primers EF-1H / EF-2T (O'Donnell, et al. 2015). PCR product was sequenced and the sequence was 709 bp (Accession No. OL614004, ON357634, ON595710). BLAST search results showed that it had 99.9% identity with the EF-1ɑ gene sequence of F. proliferatum (MH341215, MT371378). To test pathogenicity, nine leaves from 5-year-old healthy tea plants (Ca. Luyun 3) were wounded using a sterilized needle and inoculated with a 20µl conidial suspension (2 × 107 conidia·mL-1) on one side of the plants and the other side with sterilized distilled water as a control. All leaves were incubated in a growth chamber at 28℃ and 80% relative humidity with a 12 h light/dark photoperiod. Seven days later, all inoculated treatments showed symptoms identical to those observed in the field, while the control remained asymptomatic. The experiment was repeated three times with similar results. Koch's postulates were fulfilled by successful re-isolation and morphological and molecular identification of F. proliferatum from the inoculated leaves. This pathogen can cause diseases of many crops, e.g. tobacco, Polygonatum cyrtonema and others (Li, et al 2017; Zhou, et al. 2021). However, this is the first report of F. proliferatum causing leaf spot on tea plants in China. This new disease poses a threat to the yield and quality of tea and methods need to be developed for its control and to prevent further spread.