First Report of Fusarium avenaceum Causing Root Rot of Raspberry (Rubus corchorifolius) in China

Raspberry (Rubus corchorifolius) plants hold historical, economic, and medicinal importance in China (Yang et al. 2022). Raspberries are cultivated to generate income for local farmers in Lintao County, Dingxi City, Gansu Province. However, farmers encountered challenges due to raspberry plants exhibiting root rot disease, resulting in plant death. During a thorough field survey conducted in June 2022, symptoms ranging from leaf yellowing and wilting to necrotic lesions and root rots were observed, where approximately 30% of raspberry plants were affected. Five diseased and healthy plants were collected from the farmers' fields in Lintao (35.53oN, 103.84oE) for pathogen identification. Symptomatic and asymptomatic root tissues were surface sterilized with 75% ethanol for 30 s and 3% NaOCl for 5 min, followed by three rinses in sterile water. Small pieces (0.5 × 0.5 cm) were cut and incubated on potato dextrose agar (PDA) plates at 25°C for 7-10 days. Twenty-two pure Fusarium isolates, which displayed four distinct colony groups morphologically, were obtained. Pathogenicity tests on isolates RB10, RB1, RB30, and RB23, representing each colony group, revealed that RB10 exhibited symptoms similar to those observed in the field. The RB10 strain produced yellowish-white to greyish-white colonies on PDA and was then cultured in a carboxymethylcellulose (CMC) broth for enhanced conidia production (Zhang et al. 2020). Macroconidia were sickle-shaped or slightly curved, with three to five septa (19.2 to 38.5 x 3.1 to 5.8 µm, n =40). Microconidia were oval to ellipsoidal, non-septate or featuring 1 to 2 septa (4.8 to 10.5 x 2.1 to 5.2 µm, n=20). These morphological features indicated the isolate was similar to Fusarium avenaceum (Leslie and Summerell, 2006). For further identification of the strains, genomic regions (ITS-rDNA, TEF-1α, and RPB2) were amplified and sequenced using specific primers ITS1/ITS4, EF-1/EF-2, and 5f2/7cr, respectively (O'Donnell et al. 2010; Uwaremwe et al. 2021; Zarrin et al. 2016). PCR BLASTn queries of NCBI GenBank revealed a 99.8% (522 bp), 99.4% (355 bp) and 99.6% (985 bp) homology with F. avenaceum (MZ724839.1, MN271631.1, and MK185026.1), respectively. Sequences were deposited in GenBank (ITS, OR735571; TEF-1α, PP216660; RPB2, PP857820). One-year-old raspberry seedlings were planted in pots with a sterile soil mix (2:2:1 v/v ratio of soil, peat, and vermiculite) under controlled greenhouse conditions (23-26°C, 16h light/8h dark). A month post-planting, taproots were wounded in six pots and inoculated with 20 ml of conidia suspension (106 conidia/ml), while the other six pots were maintained as controls. After 14 days, RB10-infected plants showed symptoms similar to field observations, while controls remained healthy. The experiment was conducted twice, and re-isolation confirmed both the pathogenicity and identity of the pathogen. In the concatenated phylogenetic tree of ITS, TEF-1α and RPB2, strain RB10 was clustered with the F. avenaceum representative strains KG502, KG431 and F094. Studies revealed F. avenaceum varied pathogenicity across plants (Bugingo, 2022; Moparthi et al. 2020& 2024; Yli-Mattila et al. 2018), and it has been reported to induce raspberry fruit rot (Wang et al. 2017). However, no previous reports linked this fungus to raspberry root rot. This report is crucial for understanding the impact of root rot disease on raspberry cultivation and developing effective management strategies.