FOV colonizes the vascular system of susceptible cotton hosts, causing vascular discoloration, wilting and sometimes death of the plant. Discrete symptoms vary with inoculum density, susceptibility of the cotton cultivar and plant age (Hao et al., 2009). In greenhouse pathogenicity tests, diagnostic symptoms of the disease were not induced at inoculum levels below 103 conidia/gram of soil (Hao et al., 2009). At lower inoculum densities, the fungus did not compromise plant health and could not be recovered from stem tissue. Although FOV colonizes the vasculature of resistant cultivars, the infection is limited and may or may not result in obvious vascular discoloration (Hao et al., 2009).
The infection of young seedlings may result in the wilting and necrosis of cotyledons and occasionally, plant death. This creates an uneven stand in the field, a pattern that may be confused with seedling diseases such as damping-off caused by Pythium spp. and Rhizoctonia spp. Internal symptoms of FOV infection include brown discoloration in the vasculature. Plants infected at a later stage of growth may be wilted and stunted, and leaves may display chlorosis and necrosis. The characteristic brown vascular discoloration is most evident in the roots and lower stem, but may be apparent in upper branches as well.