First report of seedling blight caused by Fusarium tricinctum on Cynanchum stauntonii in China

Cynanchum stauntonii is a perennial herb plant of the Asclepiadaceae family. The dried roots and rhizomes have been used as medicine in China for 1500 years and are considered a remedy for cough and phlegm. In recent years, the wild C. stauntonii resources have not been sufficient for market demand, therefore, a large artificial cultivation area was established in Xinzhou, Tuanfeng and Macheng in Hubei province. In March and April 2022, serious outbreaks of seedling blight were observed in C. stauntonii in Xinzhou county (N30°48′12″, E114°49′24″), and the disease occurred on 10 to 15% of plants in five C. stauntonii nursery beds. Early symptoms included withered tips, chlorosis, stunting, yellow leaves and leaf drop, and later, seedlings die in patches. To determine the causal agent of disease, pieces (5 mm × 5 mm) of diseased tissue at the junction of disease and healthy tissue were surface disinfected by soaking in 75% ethanol for 3 min, rinsed three times with sterilized water, and pieces were placed on PDA at 25°C. Fungal isolates obtained were yellow-brown at the center and pink to white toward the periphery, and dark red pigments were observed in the agar. Isolates were cultured in synthetic low nutrient agar (SNA) and carnation leaf agar to observe the spore morphology. The macroconidia were sickle-shaped with 3-4 septate, with sizes of 30.26±2.36×3.77±0.53 μm on SNA and 33.52±2.20×3.81±0.48 μm on carnation leaf agar (n=30). Morphological characteristics of the isolates were consistent with those of Fusarium sp in the Fusarium Laboratory Manual (Leslie et al. 2006). Furthermore, the genomic DNA from a representative isolate BQ-2 was extracted, the ITS, TEF-1α, RPB1 and RPB2 genes were amplified with primers ITS1/ITS4, EF1/EF2, Fa/G2R and 5F2/7cr, respectively (Zhang et al. 2022). BLAST analysis showed that the ITS (ON935780.1), TEF-1α (OP985126.1), RPB1 (OP985125.1) and RPB2 (OP985124.1) amplicon sequence were 99.44%, 98.94%, 99.88% and 100% identical to the sequences of F. tricinctum strain (KU350724.1, AB674264.1, LC701712.1, MW474678.1), respectively. A phylogenetic tree constructed based on a concatenated sequence (ITS, TEF-1α, RPB1, RPB2) using the neighbor-joining and maximum likelihood method in MEGA7 revealed that BQ-2 grouped with concatenated sequences from four representative F. tricinctum isolates in GenBank. Based on the morphological characteristics and molecular identification, the strain BQ-2 was identified as F. tricinctum. For pathogenicity tests, 5 mm pieces of a BQ-2 colony on PDA were placed on excised leaves of healthy C. stauntonii wounded with a needle (n=5) and kept at 25±2℃. Leaves treated PDA were used as a control (Li et al.2020). After three days inoculation, the mycelia proliferated and began to infect leaf tissues. Ten days later, large parts of the detached leaves were extensively infested with the pathogen and brown. For live plant inoculation, stem bases of five healthy seedlings were punctured with sterile needle and then inoculated with BQ-2 mycelia from PDA. Controls were treated with only PDA. The seedlings began wilting after three days and at five days showed typical disease symptoms, similar to those observed in the field. The controls were asymptomatic. The pathogen was reisolated from the diseased tissues, and the colonies and microscopic characteristics were similar to those of BQ-2. To the best of our knowledge, this is the first report of F. tricinctum causing seedling blight on C. stauntonii in China. This report will provide resources and reference for controlling the increased incidence and economic losses of seedling blight on C. stauntonii.