Studies were conducted to examine the annual fluctuations of disease intensity in potato wart disease. Each experiment ran over many months and consisted of monthly, weekly and daily inoculations of Arran Victory potato tubers withSynchytrium endobioticumpathotype 2. Treated plants were harvested 2 months after inoculation. The experiments were conducted in the greenhouse and controlled environment growth room. A trimodal waveform (of diminishing amplitude) of percent infections was detected. The three periods of increased infection coincided with the major periods of host susceptibility. A periodicity of about 8 weeks between these infection periods was evident. This paper is part of a continuing study of pathogenesis of S.endobioticum.