Cotton (Gossypium hirsutum L.), an annual plant of the genus Gossypium in the Malvaceae family, is an important economic crop widely cultivated worldwide. In August 2025, during a field disease survey at a cotton farm in Nanjing, Jiangsu Province, China (31.89°N, 118.90°E), some cotton plants exhibited typical symptoms of root rot, with an incidence rate ranging from 15% to 25% (surveyed 200 plants). The initial symptoms included partial browning and rotting of the root system. As the disease progressed, the symptoms of rot spread throughout the entire root system, and the above-ground parts displayed yellowing and wilting of the leaves. To identify the pathogen causing the disease, 15 cotton seedlings with typical symptoms were collected. Lesion margins (3–5 mm in length) were excised from the diseased tissue, surface sterilized sequentially in 75% ethanol for 30 s and 2% sodium hypochlorite for 5 min, rinsed 7 to 8 times with sterile distilled water, dried with sterile filter paper towels, placed on potato dextrose agar and incubated in a biochemical incubator for 5–7 days at 25°C. Ten pure culture strains were obtained via the single-spore isolation method. After 5 days of incubation, the colonies presented a grayish-white color, dense and vigorous aerial mycelia in a velvety form, and uniform expansion. The macroconidia produced were sickle shaped or elliptical, measuring 4.6 to 16.6 × 1.6 to 4 µm (n=50 conidia). Morphological observation and molecular biological identification were conducted, and the strain was preliminarily identified as Fusarium incarnatum. Genomic DNA was extracted from the representative strain NXU-MHG1. Four genetic loci were amplified and sequenced: the internal transcribed spacer (ITS) gene, calmodulin (CAL) gene, translation elongation factor 1 (TEF1) gene and RNA polymerase second largest subunit (RPB2) gene, which were amplified and sequenced via the ITS1/ITS4, CL1/CL2A, EF1-728F/EF1-986R, and fRPB2-5F/fRPB2-7Cr primers (Xia et al. 2019). The ITS, CAL, TEF1, and RPB2 sequences of the strain have been deposited in GenBank with accession numbers PX495956, PX911615, PX911616, and PX911617, respectively. BLASTn analysis revealed high sequence similarity with F. incarnatum references: 99.8% (ITS: PV776307.1), 100% (CAL: GQ505503.1), 99.85% (TEF1: GQ505615.1), and 100% (RPB2: GQ505794.1). A maximum likelihood phylogenetic tree was constructed on the basis of concatenated sequences of ITS, CAL, TEF1, and RPB2 with MEGA5 software. On the basis of the phylogenetic and morphological analysis, the strain was within the same clade as F. incarnatum, confirming the identification of the strain as F. incarnatum. Pathogenicity tests were conducted on 2-leaf-stage cotton seedlings to fulfill Koch’s postulates. The inoculation procedure was as follows: healthy cotton seedlings were selected, and a shallow wound was made at the base of the stem with a sterile blade. A mycelial plug of NXU-MHG1 (0.5×1.0 cm) was inoculated, and the wound was kept moist with a cotton ball. The control plants were mock inoculated with a PDA plug. Since the isolation of pathogens primarily focuses on pure culture growth, while pathogenicity testing simulates the field environment, therefore in the pathogenicity test, all the plants were maintained at 28°C with 90% relative humidity under a 12-h light/ dark cycle. Five days later, the inoculated plants presented the same root rot symptoms as those observed in the field, whereas the control plants presented no symptoms at the wound site and continued to grow normally. The experiment consisted of two independent trials, with six seedlings in each trial. F. incarnatum was successfully reisolated from symptomatic roots and confirmed by morphological and molecular methods, satisfying Koch’s postulates and verifying its pathogenicity. F. incarnatum has been reported to cause root and stem rot in various plants worldwide, including muskmelon (Wonglom and Sunpapao 2020) and Bambusa multiplex (Lin et al. 2019). To our knowledge, this is the first report of F. incarnatum causing root rot in cotton in China, providing basic information for the diagnosis and control of this disease.
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