| Fusarium wilt disease (FWD), a vascular wilt of cucumber plants caused by Fusarium oxysporum f. sp. Cucumerinum J. H. Owen, is one of the most devastating diseases. Organic amendments are routinely used to improve soil structure, soil biochemical and microbial properties and plant nutrition. Some chemical and physical way, can control soil borne pathogen to some extent, but biocontrol is a best promising way, The objectives in present study were to make and evaluate a bio-organic fertilizer containing Paenibacillus polymyxa and Bacillus anthracis for cucumber production in a continuously cropping soil where Fusarium wilt disease is a problem and to quantify Fusarium population after and before bio-organic fertilizer application, the results obtained were as follows.1. Two antagonistic strains (No.005 and No.021) were isolated from the rhizosphere soil of health cucumber plants in the field of severe diseased Fusarium wilt of cucumber plant. The inhibition rate of them to Fusarium oxysporum is more than 70%, and the inhibition rate of the Bacteria-free filtrate of antagonistic bacteria is more than 40%. No.005 and No.021 were Bacillus anthracis and Paenibacillus polymyxa by the ways of confirming of culture, physico-biochemical characteristics, microscopical evaluation and molecular identification. The co-culture test showed that the two strains had no antagonistic action if they were cultivated separately in the medium plate. Therefore, they should be applied in mixed culture of two strains.2. Pot experiments were carried out to investigate effect of application of amino acid fertilizer (AAF) on growth of cucumber and soil microorganism under continuous mono-cropping. The soil which had been planted with continuously mono-cropping cucumber for 4 seasonal crops and had a severe continuous obstacle, was used in the experiment. Results show that application of AAF increased chlorophyll content, leaf area, photosynthesis rate, and dry weight of the cucumber plants. Moreover, after the application of AAF with high-quality carbon and nitrogen, the number of actinomycetes was increased by 1.57-2.18 folds, fungi by 2.22-8.43 folds, while the number of Fusarium oxysporum, a witling disease causing fungi, was significantly decreased. The ratio of fungi/bacteria was significantly increased while the ratio of F.oxy. to fungi was significantly decreased. The controlling rate of Fusarium wilt was more than 80%. The results suggested that AAF application could significantly promote the growth of cucumber plants, stimulate microbial activities in the soils, and alleviate the adverse effect of continuous mono-cropping of cucumber.3. Soil sterilization can eliminate the adverse effect of soil borne pathogen on crop growth, Pot experiments were carried out to investigate effect of sterilization and application of AAF on growth of cucumber plants and soil microflora in a continuously mono-cropping soil. The results indicated that soil sterilization could improve growth of cucumber plants to greater extent, and SPAD value, plant height, leaf area and fresh weight of cucumber plants grown in sterilized soil, for example, were much higher than those in non-sterilized soil. The differences of cucumber growth between the sterilized soil and non-sterilized soil decreased with increase of AAF application, showing that the application of AAF could solve some problems coming from continuous mono-cropping system. SOD, POD and CAT activities in the leaves of cucumber plants grown in sterilized soil were higher than those in CK, and soluble protein contents of all treatments in sterilized soil were much higher than those in non-sterilized soil while MDAs were significantly lower. The number of bacteria and fungi in sterilized soil was increased by 1.89-3.03 folds and 6.96-2.43 folds, respectively, compared with those in non-sterilized soil and the ratio of fungi to bacteria was significantly increased. In addition, Actinomycetes were slightly enhanced. The analyses of PCR-DGGE profiles showed that the number of bands on the lanes of DGGE profiles in sterilized soil was more than those in non-sterilized soil, with low similarities of DGGE profiles, and the diversities of soil microorganism in sterilized soil were increased compared to those in non-sterilized soil. The incidence of Fusarium wilt of cucumber plants grown in non-sterilized soil was up to 31.2%-68.8%, but no incidence was found in sterilized soil.4. Four field experiments were conducted to evaluate the effect of organic fertilizer application either with or without antagonistic bacteria on the impact of Fusarium wilt disease in cucumber. The incidence of Fusarium wilt disease was 5.3-13.5% for cucumber plants treated with bio-organic fertilizer while it was 30.3-90.4% in controls applied only with organic fertilizer. The inhibition rates of Fusarium wilt disease were 73.6%,82.5%, 96.3% and 92.6% in the four independent field experiments and accordingly, cucumber yields treated with bio-organic fertilizer were increased by 98.8%,25.6%,73.3% and 198.6% respectively compared with CK. The application of Bio-organic fertilizer had a great potential for the control of Fusarium oxysporum wilt disease in cucumber plants.5. Denaturing gradient gel electrophoresis (DGGE) and Biolog analysis showed a significant change in soil bacterial community after bioorganic fertilizer application. The numbers of colony forming units (CFU) of Fusarium oxysporum in bio-organic fertilizer applied soils were significantly decreased compared with controls. Scanning electron micrographs of cucumber basal stems showed a presence of mycelia-like mini strands accompanied by an amorphous substance within the xylem vessels. This amorphous substance and mini strands were richer in calcium and phosphorus but had a lower carbon and oxygen than the living mycelia.
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