First Report of <i>Fusarium culmorum</i> Causing Wheat Crown Rot in Ningxia, China

Fusarium crown rot (FCR) of wheat, primarily caused by Fusarium pseudograminearum, is a globally significant soil-borne disease. Since its initial detection in Henan province, China (Li et al. 2012), FCR has become an increasingly prevalent and yield-limiting issue in China's winter wheat regions. While F. culmorum serves as another major pathogen responsible for FCR in Europe, West Asia, and North America, its presence in China was initially limited to winter wheat regions, including Hebei Province (Li et al. 2016) and Xinjiang Uygur Autonomous Region (Gao et al. 2024), but no reports from China's spring wheat regions. FCR was observed in the spring wheat region of Ningxia, China, with an affected area of about 13.6 kha. While the overall epidemic was mild, some fields experienced severe infections. In June 2024, wheat plants showing typical crown rot symptoms were collected from a severely infected area in Qingtongxia, Wuzhong City (38.167757°N, 106.026256°E), where the white head rate reached 30%. Tissue fragments (5 mm) were surface sterilized sequentially with 75% ethanol (30 s) and 1% sodium hypochlorite (3-5 min), followed by three rinses with sterile distilled water and drying on sterile filter paper. The fragments were then cultured on potato dextrose agar (PDA) supplemented with 50 μg/mL ampicillin at 25°C for 2-5 days. Pure cultures were obtained through monoconidial and single hyphal tip isolation methods and preserved at -80°C. Isolate WZ2415-2A exhibited rapid growth on PDA, forming fluffy white to pale-yellow mycelia and producing a pale-red to dark-red pigment in the medium (Figure 1A). On carnation leaf agar (CLA), abundant macroconidia formed orange-colored colonies. The macroconidia were falcate, typically 3-5-septate, measuring 25-50 × 5-7 μm, with no observed microconidia (Figure 1B). These morphological characteristics matched the description of F. culmorum (Leslie and Summerell 2006). Genomic DNA was extracted using the CTAB method. Three genetic markers were amplified: the internal transcribed spacer (ITS) region of rDNA, partial translation elongation factor 1-alpha (Tef-1α), and β-tubulin 2 (Tub2) genes, using primer pairs ITS1/ITS4 (White et al., 1990), EF1/EF2 (O'Donnell et al. 1998), and T1/T22 (O'Donnell and Cigelnik 1997), respectively. Sequencing of the PCR products (approximately 530 bp, 670 bp, and 1300 bp) revealed 99.81%, 99.86%, and 99.92% identity with published F. culmorum sequences (e.g., MH681147, KP065682, and GQ915440) in GenBank (accession nos. PV488304, PV867328, and PV867329). Species-specific PCR amplification using primers Fc01F/Fc01R (Nicholson et al. 1998) yielded the expected ~570 bp product, further confirming the isolate as F. culmorum. Pathogenicity tests were conducted on spring wheat cultivar Ningchun 4. Sterilized millet was inoculated with strain WZ2415-2A and incubated at 25°C in darkness for 5-7 days. Fifteen germinating seeds were planted in sterile soil (150 g) in 7 cm × 7 cm × 5 cm pots. For the treatment group, 0.25 g of inoculum was mixed with 50 g sterile soil as a covering layer, while controls received sterile soil only. After two weeks, inoculated plants displayed characteristic FCR symptoms: stunted growth, chlorosis, and stem base necrosis (Figure 2). Control plants remained asymptomatic. The pathogen was successfully re-isolated from infected tissues, fulfilling Koch's postulates. This study represents the first documented occurrence of F. culmorum-induced wheat crown rot in Ningxia, one of the China's spring wheat regions, where experiences cooler temperatures compared to other wheat-growing areas in the country. These findings are critical for monitoring FCR epidemiology and pathogen population dynamics in China. Moreover, this discovery will facilitate the development of effective management strategies, including the breeding of resistant cultivars and optimization of fungicide applications, to safeguard wheat production in the region.