Fallopia japonica, commonly known as Japanese knotweed or "itadori", is an increasingly invasive and troublesome weed in Europe and North America. Classical biological control is being assessed as a potential management strategy in the UK. During surveys in its native Japan, a distinctive rust fungus which commonly appeared in spring was recorded. Based on spore morphology, this was identified as Aecidium polygoni-cuspidati. Field observation throughout 2003 at Kusu (Oita Pref., Japan) showed that the rust occurred mainly from April to June but it was not associated with severe host damage or defoliation. No secondary infection in the form of uredinia was observed. Attempts to infect F. japonica with aeciospores in the laboratory failed. Literature searches confirmed this to be the heteroecious rust, Puccinia phragmitis, which alternates between F. japonica and Phragmites communis. Thus, this species has no potential as a classical biological control agent of Japanese knotweed.