First report of Fusarium proliferatum causing rhizome rot on Polygonatum cyrtonema in China.

Polygonatum cyrtonema Hua (Asparagaceae) is a perennial herb widely distributed in China (Chen et al. 2021). The rhizome has antioxidant, anti-inflammatory, and immunomodulatory properties and is traditionally used to treat dizziness, diabetes, and asthma (Lu et al. 2023; Pang et al. 2022). About 3000 P. cyrtonema plants were planted in five fields at the Institute of Medicinal Plant Development (40.04°N, 116.28°E), Beijing, China, where rhizome rot was observed from June to October 2023, with an incidence rate of about 10%. Most cases occurred in low-lying, waterlogged fields. Initially, infected plants had no obvious aboveground symptoms but had brown spots on their rhizomes. In severe cases, plants exhibited aboveground withering and brown rot in rhizomes. A fungus was isolated from symptomatic plants (isolation rate 73%), and single spores were used to grow pure colonies on potato-dextrose-agar (PDA) and incubated for 5 d at 25°C in the dark. Eleven isolates with the same morphology were obtained. The upper surface of the colonies was white, with cottony mycelium and a light purple center. The lower surface of the colonies was hazel in the center. Two representative isolates, C76 and C78, were cultivated on carnation leaf agar medium. Microconidia of the isolates were aseptate, oval, elliptic or clavate, and their dimensions were 4.9 to 12.1 × 1.5 to 4.5 μm (n = 50). Macroconidia were long, slender and thin, rod-shaped or slightly curved, with one to five septa, and their dimensions were 15.3 to 44.9 × 2.7 to 4.5 μm (n = 50). The isolates were classified as Fusarium based on morphology (Leslie and Summerell 2006). To determine the species identity, we sequenced the translation elongation factor (TEF-1α) and partial RNA polymerase second largest subunit (RPB2) of two isolates (i.e., C76 and C78, GenBank access numbers: TEF-1α, PQ550044, PQ285402; RBP2, PQ550045, PQ285403, respectively) (Crous et al 2009) and compared them to other Fusarium species found at Fusarium-ID and GenBank databases. Both isolates exhibited 99.84% (TEF-1α, MT305203) and 100% (RBP2, LT841252) similarity with Fusarium proliferatum. The phylogenetic tree was constructed by combining TEF-1α, and RBP2 using MEGA6, and the two isolates clustered with F. proliferatum. To demonstrate pathogenicity, 2 mL of conidial suspension (1 × 106 conidia/mL) of the isolate of F. proliferatum designed C76 were dropped on the surface of four rhizomes without wounding. For the control, sterile water was applied on two control rhizomes. All treatments were repeated three times. Seedlings were grown at 25°C in moist soil. After 7 d, inoculated roots exhibited similar symptoms to those in the field, while control roots showed no symptoms. The same isolate was reisolated from diseased roots and was identified based on morphological characteristics, fulfilling Koch's postulates. Based on morphology and molecular biology, the isolates were identified as F. proliferatum, a ubiquitous pathogen (Li, et al 2017) that was reported to cause leaf spot on P. cyrtonema (Zhou, et al. 2021). This is the first report of F. proliferatum causing rhizome rot on P. cyrtonema. This research will help develop strategies to manage rhizome rot incited by F. proliferatum.