Abstract: The aim of this study was to investigate the main reasons of apple replant disease（ARD）around Northwest Loess Plateau area. The characteristics of fungi in the soil of different replanted apple orchards were studied by combining the construction of fungi clone library and terminal restriction fragment length polymorphism（T-RFLP）. The correlation between the seedling growth of Malus hupehensis Rehd. and the dominant fungal species in the apple replanted soil was also studied. The results showed that Ascomycetes were the main fungi in the replanted apple orchards in Northwest Loess Plateau area at phyla level；Fusarium，Mortierella，Chaetomium and Cryptococcus were the dominant microflora at genus level. Among the 20 orchards，5 belong to Gansu Province，which had a relatively high Shannon diversity index，Pielou index，Margalef index and low Simpson index. The soils from Shaanxi Fuping in Pinglu and Saline Lake area in Shanxi showed more serious ARD and higher abundance of Fusarium. The severity of ARD positively correlated with Fusarium（r = 0.585，P < 0.01），but negatively correlated with Mortierella（r =–0.473，P < 0.05）. Overall，Fusarium in the apple replanted soil was the key factor for ARD around Northwest Loess Plateau area.

Key words: apple, replant disease, fungal clone library, T-RFLP；Fusarium, fungal community structure