Sclerotinia stem rot, caused by Sclerotinia minor, has not been reported in sunflower (Helianthus annuus) during vegetative growth. During an investigation of disease development in the highly susceptible inbred line, PacF2582, the disease was first observed in the field at bud development. In glasshouse studies with inoculated soil, onset of disease was delayed until late vegetative growth. Direct inoculation of roots with disks of mycelium or with pregerminated sclerotia confirmed that sunflowers are susceptible to infection during the vegetative growth stages. Root measurements found that the rate of increase in root dry weight was maximum between the early and midbud stages in field-grown plants (230%/week) and between the 6- and 10-leaf stages in glasshouse plants (240%/week). Synchronisation of disease onset with proliferation of lateral roots suggests that active root growth is required to initiate infection.