Elucidating the Diversity of <i>Fusarium</i> 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 <i>Fusarium</i> 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. <i>Fusarium</i> species were isolated, characterized morphologically and molecularly, and identified as <i>Fusarium oxysporum</i>, <i>F</i>. <i>sambucinum</i>, <i>F</i>. <i>clavum</i>, and <i>F</i>. <i>solani</i>. Colony morphology, macroconidia, microconidia, and chlamydospore formation were analyzed to classify isolates into four distinct groups. Molecular characterization using internal transcribed spacer, <i>rpb2</i>, and <i>tef</i> markers confirmed species identity, supported by sequencing and phylogenetic analysis that grouped isolates into well-defined clades. Radial growth assessments demonstrated significant differences among species over time, with <i>F</i>. <i>sambucinum</i> exhibiting the fastest growth. Pathogenicity assays on three potato cultivars revealed varying virulence, with <i>F</i>. <i>solani</i> causing the most severe infections in cultivars tested. This study represents the first documented occurrence of <i>F</i>. <i>oxysporum</i>, <i>F</i>. <i>sambucinum</i>, <i>F</i>. <i>clavum</i>, and <i>F</i>. <i>solani</i> as causal agents of FDR in Colorado. Furthermore, <i>F</i>. <i>clavum</i> 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 <i>Fusarium</i> 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.