Mizuna (Brassica rapa L. japonica group) has been cultivated throughout the year in plastic houses of Namegata city, one the main area of production in Japan. Recently, damping-off of mizuna has increasingly become a serious problem in Namegata city, especially in the summer when the market price of mizuna reaches a high. As a first step to determine a control strategy for Fusarium wilt, the main disease, we investigated the isolation ratio of Fusarium oxysporum, and seasonal occurrence of damping-off on mizuna in commercial houses in Namegata city in 2016 to 2017. Inoculation experiments in a plastic house in the Ibaraki Agricultural Centre Horticultural Research Institute (HRI) were used to examine the repeatability of the seasonal occurrence of Fusarium wilt in commercial houses and the seasonal abundance in the soil. The isolation ratio of F. oxysporum from damping-off was 20.0–75.0%. In addition, the isolation ratio of Rhizoctonia solani was high, revealing Rhizoctonia disease on mizuna in commercial houses at Namegata city. Damping-off symptoms and Fusarium wilt on mizuna peaked in the summer both years and were present in all houses examined. Density of F. oxysporum in the soil, however, did not present a clear seasonal trend; the fungus was present all year at approximately 100 cfu/g dry soil at depths of 0–15 and 16–30 cm. These results suggest that in the plastic houses of Namegata city threatened with the Fusarium wilt on mizuna, Fusarium oxysporum stably exists in the soil and causes the disease in the summer every year.