Identification and Pathogenicity of <i>Fusarium</i> Species Associated with Fusarium Wilt of Flue-Cured Tobacco

In 2022, an outbreak of apparent Fusarium wilt raised concern because the disease has historically been sporadic and infrequent. Previous studies reported that the primary causal agent of this disease in the United States is <i>Fusarium oxysporum</i> f. sp. <i>nicotianae</i>, but the identity of other <i>Fusarium</i> spp. responsible for Fusarium wilt and root rot and their pathogenicity have not been thoroughly investigated. To identify <i>Fusarium</i> species associated with the recent disease outbreak in flue-cured tobacco, 36 symptomatic plants with apparent one-sided wilting were sampled from 14 fields in Virginia and North Carolina. Forty-seven obtained isolates showed colony morphology typical of <i>Fusarium</i> spp. on potato dextrose agar (PDA) medium. Fifteen isolates representing colony variations on PDA were further selected for microscopic examination and molecular identification using the translation elongation factor 1-alpha (<i>tef1-α</i>) gene. Phylogenetic analysis of <i>tef1-α</i> nucleotide sequences identified nine <i>Fusarium</i> species representing five distinct species complexes: <i>Fusarium incarnatum-equiseti</i> (six isolates), <i>F</i>. <i>oxysporum</i> (three isolates), <i>F</i>. <i>solani</i> (three isolates), <i>F</i>. <i>fujikuroi</i> (two isolates), and <i>F</i>. <i>tricinctum</i> (one isolate). The pathogenicity of each isolate was assessed on flue-cured tobacco cultivar K326 seedlings at the six-leaf stage in greenhouse inoculation experiments with or without wounding. All 15 <i>Fusarium</i> isolates were pathogenic, causing root rot, leaf necrosis, and reduced growth parameters compared with the control treatment. The diversity of <i>Fusarium</i> species associated with Fusarium wilt of tobacco and the variability in aggressiveness among <i>Fusarium</i> species within the same species complex or belonging to different <i>Fusarium</i> species complexes provide a theoretical basis for future research on the epidemiology of Fusarium disease in tobacco and integrated disease management strategies to reduce disease development and severity.