First Report of Fusarium solani Causing Stem Rot in Cymbidium in China

Cymbidium is a genus of the family Orchidaceae. It has been an important flower in China owing to its higher ornamental value. However, stem rot in Cymbidium gradually became a severe disease in recent years. In September 2023, we isolated and purified the stem rot pathogen via the tissue isolation and culture method from a greenhouse at Nanjing (32°3′N, 118°47′E), Jiangsu Province, China. The leaf bases of diseased plants became yellow to brown water-stained, the leaf tips were yellow and gradually turned brown as they wilted, and the root system gradually dried out and darkened. Finally, the whole plant rotted under 25±2°C and 75% relative humidity condition. The infected plants was about 30%. Infected leaves were sterilized with 75% ethanol for 15s and 0.5% sodium hypochlorite (NaClO) for 4 min, and rinsed three times (Liang et al. 2023). Then, 5 × 5 mm pieces from the junction of the diseased and healthy parts were clipped, placed on potato dextrose agar (PDA) medium and incubated at 25°C. Pure cultures were obtained by transferring hyphal tips to new PDA plates. Four fungal isolates were obtained. The colonies were yellow on the back and appeared orange in the center with floccose texture and white to cream-colored aerial mycelia after incubation for 10 to 15 days. The macroconidia (n = 50) were relatively wide, straight, stout, 3 to 5 septa, and 33.5 to 56.8 × 5.2 to 7.5 µm long. The microconidia (n = 50) were oval, ellipsoid and reniform, with 0 or 1 septa, and 15.8 to 19.0 × 5.0 to 7.4 µm in size. The asci were clavate in one perithecium, and each ascus had 8 ascospores. The characteristics is similar to that of Fusarium solani (Takashina et al. 2020). For molecular identification, the CTAB protocol was used to extract total genomic DNA. The internal transcribed spacer (ITS), translation elongation factor subunit 1-alpha (EF1-α), and RNA polymerase II subunit 2 (RPB2) genes were subsequently amplified using the universal primers and then sequenced (Reeb et al. 2004; Jiang et al. 2018; O’Donnell et al. 2022). The sequences of the isolate KW04 were analyzed and submitted to GenBank (PQ530367; PQ530457; PQ530458 and PQ524514). BLAST analysis revealed that the ITS, EF1-α, RPB2 and SOL sequences all shared 98%-100% identity with those of F. solani in GenBank. A phylogenetic tree was constructed using IQ-TREE with the maximum likelihood method based on the combined sequences of KW04 and other Fusarium isolates (O‘Donnell et al., 2022), KW04 was grouped with F. solani (99% bootstrap). The inoculated plants presented water-stained spots of perithecia and mycelial layers consistent with the symptoms exhibited in greenhouse, while uninoculated plants remained healthy after 14 days of culture at 25°C. The fungi were reisolated from KW04 strain infected plants and further infected to Cymbidium plants, they exhibited the same symptoms. The result of morpho-molecular characteristics confirmed that the isolates were F. solani, thereby satisfying Koch’s postulates. To the best of our knowledge, this is the first report of stem rot in Cymbidium caused by F. solani in China.