First Report of Fusarium incarnatum Causing Root Rot on Apocynum venetum (Luobuma) in China

Apocynum venetum, also known as "Luobuma", is a perennial herbaceous plant with drought and salt-alkali tolerance (Xu et al., 2018). With expanding cultivation in Bayingolin Mongol Autonomous Prefecture, Xinjiang, root rot has emerged as a serious threat. In April 2024, severe root rot was observed in Apocynum venetum seedling nurseries and detected across more than 1,000 hectares of two-year-old mature plants in Yuli County (41.3594°N, 86.2838°E), with disease incidence ranging from 5% to 21%. At the seedling stage, the disease caused wilting of the aboveground parts, root browning, and poor root development. In mature plants, infection caused complete leaf yellowing, discoloration of the root epidermis with lesions expanding inward, and decay of the internal pith cavity. To isolate the causal agent, symptomatic root tissue segments (~5×5×5 mm) were surface-sterilized with 10% sodium hypochlorite, plated on PDA, and incubated at 25 ℃. Pure cultures were obtained through two rounds of hyphal-tip subculturing. Two isolates (YLTZRR607-2-2wuP and YLTZRR607-1-1P) were selected for characterization. Colonies initially exhibited white mycelia, forming circular shapes with smooth margins and dense floccose aerial hyphae. Following two days of incubation, a pale-yellow coloration developed at the center, progressing outward and intensifying to orange-red toward the periphery. Isolates cultured in liquid medium (150 rpm, 26°C, 7 days) produced falcate macroconidia, predominantly 3- to 5-septate (most commonly 3-septate), measuring (23.21-46.76) × (2.85-5.84) µm. Microconidia were 0- to 2-septate, measuring (12.31-20.31) × (2.77-4.61) µm. The pathogen was preliminarily identified as belonging to the genus Fusarium. For molecular identification, Tef-1α and RPB2 genes were amplified and sequenced (Chehri et al. 2015; O'Donnell et al. 2013). Sequences were deposited in GenBank (PX849487-PX849488, Tef-1α; PX849489-PX849490, RPB2). Phylogenetic analysis clustered both isolates within F. incarnatum, confirming identification based on morphology and molecular phylogeny. Pathogenicity was confirmed via root-drench inoculation of A. venetum seedlings (1-2 cotyledon stage) with 25 mL conidial suspension (1 × 106 conidia/mL; six replicates per isolate). All inoculated seedlings developed symptoms identical to those observed in the nursery, including aboveground wilting and root browning. No symptoms were observed in control plants. The pathogen was reisolated from all inoculated roots only and confirmed as F. incarnatum based on morphology and sequence analysis. To our knowledge, this is the first report of F. incarnatum causing root rot of A. venetum in China. These findings may be helpful to the control and prevention of root rot disease of A. venetum.