Elucidating the Diversity of Fusarium Species Causing Potato Dry Rot in the San Luis Valley, Colorado.

Fusarium dry rot (FDR) is a major postharvest disease of potatoes, causing significant economic losses worldwide. This study investigated the morphological and molecular diversity of Fusarium species causing dry rot in the San Luis Valley (SLV), Colorado. Potato tubers exhibiting characteristic dry rot symptoms were sampled during the 2023 and 2024 growing seasons. Fusarium species were isolated, characterized morphologically and molecularly, and identified as Fusarium oxysporum, F. sambucinum, F. clavum, and F. solani. Colony morphology, macroconidia, microconidia, and chlamydospore formation were analyzed to classify isolates into four distinct groups. Molecular characterization using ITS, rpb2, and tef markers confirmed species identity, supported by sequencing and phylogenetic analysis, which grouped isolates into well-defined clades. Radial growth assessments demonstrated significant differences among species over time, with F. sambucinum exhibiting the fastest growth. Pathogenicity assays on three potato cultivars revealed varying virulence, with F. solani causing the most severe infections in cultivars tested. This study represents the first documented occurrence of F. oxysporum, F. sambucinum, F. clavum, and F. solani as causal agents of FDR in Colorado. Furthermore, F. clavum is a novel species associated with potato dry rot, not previously reported in the United States. The integration of molecular and traditional techniques underscores the genetic and morphological diversity of Fusarium spp. in the SLV and highlights their pathogenic potential. These findings provide a foundation for future research to mitigate the economic impacts of FDR and ensure the viability of potato production in Colorado and beyond.