Effect of continuous cropping of sesame on rhizospheric microbial communities

China is one of four main sesame producing countries in the world, with approximately 600000 hectares planted annually. When sesame is grown continuously in the same field over several years, yield is reduced due to declining soil health. This phenomenon is called the continuous cropping obstacle. In 2010, the area of continuous cropping of sesame was over 1500 hectares in Jiangxi Province alone. The latest hypothesis for the decline in soil health focuses on shifts in the soil microbial community under continuous cropping. Bacteria, fungi, and actinomycetes are the three main microbial groups in the soil. Their variation in abundance and diversity reflect the level of biological activity in the soil. Soil with abundant bacteria and actinomycetes is more biologically active. When the fungal abundance of soil increases, this usually means a decrease in soil fertility. Bacillus bacteria have vital functions in soil improvement and pest control. Sesame wilt caused by Fusarium oxysporum (FO), and sesame bacterial wilt caused by Ralstonia solanacearum (RS), are serious diseases. We examined changes in abundance of bacteria, actinomycetes, fungi, Bacillus, Fusarium oxysporum, and Ralstonia solanacearum, in sesame rhizospheric soil under four treatments; using dilution plate counting to investigate effects of continuous cropping on the microbes present. The four treatments were: normal rotation with vegetable crop (vegetable-vegetable-vegetable-sesame, VVVS), alternation of sesame with peanut (sesame-peanut-sesame, SPS), 2 year continuous sesame (CS2), and 5 year continuous sesame (CS5). Bacteria, actinomycetes, and Bacillus, decreased in abundance, while fungi, Fusarium oxysporum, and Ralstonia solanacearum increased in abundance, with increasing length of continuous cropping. Bacterial abundance under VVVS, SPS, and CS2 was significantly higher than under CS5, and bacterial abundance under VVVS was significantly higher than under CS2. Actinomycete abundance under VVVS, SPS, and CS2 was significantly higher than under CS5. Fungal abundance under CS5 was significantly higher than under VVVS, SPS or CS2, and fungal abundance under CS2 was significantly higher than under VVVS. Bacillus abundance under VVVS was significantly higher than under CS2 or CS5, and Bacillus abundance was significantly higher under CS2 than under CS5. Fusarium oxysporum abundance under CS5 was significantly higher than under VVVS, SPS or CS2. Ralstonia solanacearum abundance under SPS, CS2