Fallopia japonica（Japanese knotweed, Polygonaceae）, native to Japan, has been a serious invasive perennial weed in North America and large parts of Europe, especially in the UK, where there is an urgent need for a classical biological control strategy. Surveys have confirmed the presence of specialized plant pathogens in the native range of Japan. The results of the surveys showed that three fungal diseases of two rusts and a leaf-spot disease were predominantly common and widespread in the field. These rusts were identified as Puccinia polygoni-amphibii var. tovariae and Aecidium polygoni-cuspidati. Furthermore, a leaf-spot fungus morphologically identified as Mycosphaerella polygoni-cuspidati is also distributed widely in Japan. In the field, P. polygoni-amphibii var. tovariae and M. polygonicuspidati were confirmed to have severe infectivity to F. japonica. Thus, it is suggested that these two pathogens have high potential to become classical biological control agents for F. japonica.