First report of root rot on Carya cathayensis caused by Fusarium solani in Zhejiang Province, China

Carya cathayensis Sarg. is an economically important cultivated tree species in China, valued for its edible nuts (Li et al. 2023). Based on recent field investigations, Tuankou Town in Lin'an District has been identified as the region with the most severe incidence of root rot disease in C. cathayensis plantations across Zhejiang Province, where the plantations cover over 27 hectares with infection rates from 40% to 60%. In C. cathayensis root rot, the initial infection occurs in lateral roots, spreads to taproots causing root decay, and later progresses to stems and branches, resulting in dieback and defoliation. In June 2024, infected root tissue specimens of C. cathayensis were obtained from five sampling sites within Tuankou Town plantations (30°0′14″ N, 119°7′46″ E). Sequential surface sterilization was performed under aseptic conditions: immersion in 75% ethanol (25 s), 1% sodium hypochlorite (45 s), followed by three sterile distilled water rinses. Specimens were then plated on 2% malt extract agar (MEA) and incubated at 25°C in darkness for 3-5 days (Peng et al. 2023). Thirty single-spore isolates with colonial morphology similar to Fusarium were obtained from twenty diseased root samples. After 7 d incubation at 25°C in darkness, colonies exhibited white aerial mycelia with subsurface beige pigmentation. These cultures produced copious microconidia and macroconidia. On carnation leaf agar (CLA) medium, microconidia are oval to reniform, aseptate or 1 to occasionally 2 septa, measuring 5. 9–25. 2 × 1. 8–4. 5 μm. Macroconidia are falcate, typically 3-5 septate, with a slight curvature in the mid-region, measuring 20. 8–62. 5 × 3. 8–6. 5 μm (n = 50). The morphological characteristics of both the macroconidia and microconidia conformed to the description of Fusarium species (Leslie and Summerell 2008). Genomic DNA was extracted from three representative isolates for sequencing analysis. Genomic loci including TEF-1α, RPB1, and RPB2 were amplified and sequenced following O'Donnell et al. (2008, 2010). Sequences of the above loci for the selected isolates were deposited in the GenBank database (PX733403 to PX733411). BLASTn analysis revealed 99. 5% identity with accession OQ511089. 1TEF-1α and LT746339. 1RPB2, and 99. 75% identity with accession ON366454. 1RPB1. Phylogenetic analysis of the concatenated TEF1 -RPB1-RPB2 dataset resolved the three isolates within the F. solani species clade. To assess the pathogenicity of representative isolates (F. solani R-28), 10 healthy five-month-old C. cathayensis seedlings in total were selected. The roots of ten plants were lightly scratched with a sterilized blade and were inoculated with 10 mL of a conidial suspension (10⁶ conidia per mL) applied to the root zone soil. All plants were kept at 28°C and 85% relative humidity, with another ten plants inoculated with an equal volume of sterile water serving as controls. Within 14 days, all inoculated trees developed characteristic symptoms including root necrosis consistent with field observations, and no symptoms observed from the control. F. solani were successfully re-isolated from symptomatic root tissues, exhibiting morphological features similar with the original isolates. Identity was confirmed through sequence analysis of TEF1-α and RPB2 loci. Root rot induced by Fusarium spp. poses a serious threat to the sustainable development of the C. cathayensis industry. Previous studies have identified F. oxysporum as the main causal agents of this disease (Zhang et al. 2015). To our knowledge, this study represents the first report of F. solani causing root rot on C. cathayensis in China. Consequently, our findings provide a scientific basis for the implementation of integrated disease management strategies against C. cathayensis root rot in this region.