Abstract: The effect of extremely low frequency (ELF) amplitude modulated (AM) square waves in the frequency range 0.5 to 20 Hz on the S. rolfsii activity was studied. The effect of time of exposure to ELF AM waves on the linear growth and biomass gain of the fungus in vivo and in vitro was also included. The results indicated significant reduction in the linear growth and biomass gain of S. rolfsii after being exposed to square AM waves for 24 h in vivo or in vitro. The number of sclerotia, percentage of germination and germ tuble length was dramatically affected. Sensitization of the fungus for 24 h counteracted the potency of the fungus exposed to frequencies above 0.5 Hz 5.0 and 15 Hz were even stimulatory to seedling emergence of sugarbeet to values around the control non-infected plants.