Epicoccum sorghinum Causing Leaf Spot on Polygonatum cyrtonema in China.

ABSTRACT

Polygonatum cyrtonema Hua (family Asparagaceae) is a traditional Chinese medicinal plant that is widely cultivated in various parts of China, including Hunan Province. In summer 2022, a leaf spot disease was observed in 10% of the P. cyrtonema plants (Huang jing) in 18 hectares of this crop in the Hongjiang District (27°18'4″N, 110°11'1″E) of Hunan Province. The initial symptoms of the disease were brown spots on young leaves, and adjacent tissues gradually changed from green to yellow. The entire leaf then became yellow, withered, and eventually exhibited a thn and black appearance. In total, 12 diseased plants from four sampling sites (three plants per site) were collected for laboratory analysis to address the concerns of P. cyrtonema growers. Symptomatic leaf samples were selected, and the leaf fragments containing infected parts of the plants were disinfected with 75% ethanol for 1 min, then immersed in 2.5% hypochlorite for 45 s. After disinfection, symptomatic leaf samples were rinsed three times with sterile water, placed on potato saccharose agar containing 50 µg/ml kanamycin and incubated at 25°C for 2 days. Subsequently, 12 fungal isolates were isolated from various leaf samples through hyphal tip transferring. Ten of the 12 fungal isolates had similar morphological features, and one of them (isolate hjh) was used as the representative isolate for the study. With a growth rate of 6.3 mm per day, its white colonies transformed into red concentric rings in five days; they gradually became black after 10 days of growth. The chlamydospores were round (4.0-9.9 × 3.1-9.3 µm, n = 30), whereas the conidia were ovate (8.0-12.1 × 3.2-6.5 µm, n = 30). The morphological features of the isolate hjh were similar to the features of Epicoccum spp. (Aveskamp et al. 2010). The internal transcribed spacer (ITS) region (including the partial ITS1 sequence and the 5.8S and ITS2 complete sequences), ß-tubulin (tub) gene, and large subunit (LSU) rRNA gene, were amplified from the isolate hjh using the primer pairs ITS5/ITS4, Bt2a/Bt2b, and LROR/LR5, respectively (Taguiam et al. 2021). BLASTn analysis showed that the ITS (OR253745), tub (OR253764), and LSU (OR253746) sequences generated from the isolate hjh were 98-99% similar to the sequences of E. sorghinum strains CBS 179.80 and CBS 627.68. Subsequently, the ITS, tub, and LSU sequences were combined using Sequence Matrix software; phylogenetic analysis via Bayesian and maximum likelihood methods (Vaidya et al. 2011; Li et al. 2021) classified the isolate hjh into the E. sorghinum clade. To fulfill Koch's postulates, pathogenicity tests were conducted on healthy (lesion-free and disease-free) 2-year-old P. cyrtonema plants. Three healthy plants were inoculated by spraying whole plant until run-off with a spore suspension of the isolate hjh (1 × 106 conidia/ml); Three other healthy plants were sprayed with sterile water as controls. The inoculated plants were incubated in a growth chamber at 25 ± 2°C with 85% humidity for 28 days(Chen et al. 2021). Leaves from the inoculated plants gradually became brown within 15 days. Finally, the plants died 28 days after inoculation. The control plants showed no symptoms throughout the experimental period. Isolates (isolate hjh1, hjh2 and hjh3) that were reisolated from the inoculated plants exhibited morphologically similar characteristics and molecularly identical to the original isolate hjh. To our knowledge, this is the first report of E. sorghinum causing leaf spot disease on P. cyrtonema. The results of this study may facilitate the production of P. cyrtonema in China.