| Phytophthora parasitica var. nicotianae, a soilborne pathogen, causes tobacco black shank. A lot of methods have been carried out to prevent or cure this disease, among which, biocontrol was proved to be very promising. Biological control is one of important ways to overcome soil-borne diseases, because of its effective and environment-friendly. Screening antagonists against pathogens is the basic work in biocontrolling plant diseases.Twelve potential antagonistic strains against Ph. parasitica var. nicotianae were screened out of more than 289 strains isolated from rhizosphere soil of healthy tobacco plants grown in the field strongly attacked by tobacco black shank disease. The strain C-5 was most effective among 12 antagonistic strains against Ph. parasitica var. nicotianae, with an inhibition rate averaged 71.1% and an inhibition diameter averaged 2.8 cm.Strain C-5 also showed antagonistic effect on other plant pathogens, with the inhibition rates being more than 53.3%. The strain of C-5 could inhibite the growth of Fusarium osyporunm f. sp melonis, P. capsici and Verticillium dahliae Kleb. by 73.3%, 66.7% and 66.8%, respectively. Furthermore, the inhibition rate of the strain C-5 against F. oxysporum f. sp. Cubense, F. osyporunm f. sp. cucumerinum and Rhizoctonia solani kulhn ranged from 53.3% to 55.0%. But there was no inhibitory effect of the strain C-5 on F. osyporunm f. sp niveum.In order to determine the taxonomic placement, the identification of strain C-5 was performed with traditional bacterial systemic identification method. Although the physiological and biochemical character of strain C-5 is similar to that of Bacillus and that of Paenibacillus polymyxa, it was very difficult to ascertain its systematic position. Therefore, the molecular identification of the strain C-5 was performed. Through the comparative 16S rDNA sequences analysis, the 16S rDNA sequence of strain C-5 had high homology with Pa. polymyxa (100%). And the constructed phylogenetic tree proved that strain C-5 was indeed subjected to Pa. polymyxa. Strain C-5 strain could produce proteinase and cellulase, which may be the mechanism of the strain against the Ph. parasitica var. nicotianae. The hydrolysis spot diameters of proteinase and cellulase were 23 mm and 3.1 mm. However, other factors such as antibiotic substances could also contribute to antagonistic effect.The optimum cultivation temperature, pH and aeration for the growth of strain C-5 were 30℃,7.0 and 20%, respectively. Moreover, the optimum source of carbon and nitrogen for the growth of strain C-5 were sucrose and peptone, followed by glucose and ammonium sulphate, respectively.Pot experiments showed that application of bio-organic fertilizer containing strain C-5 (BIO) could decrease the incidence of tobacco black shank by 80% compared with that of control (CK2) in which neither C-5 strain nor organic fertilizer were used in continuous cropping soil (diseased soil). Application C-5 strain (ANT) and BIO could change the microbial population of rhizosphere soil. After application of ANT and BIO, the population of bacteria was increased by 2.4 and 3.5 times, actinomyces by 4.4 and 11.9 times, but fungi decreased compared with that of control.The fresh weight, dried weight and height of tobacco in BIO were increased by about 1.8 times,1.7 times and 1.4 times, respectively, compared with CK2. The fresh weight and plant height of tobacco in BIO were increased by about 1.4 times and 1.3 times compared with that of CK1 with healthy soil. The fresh weight, dried weight and plant height of tobacco in BIO were increased by about 1.4 times,1.4 times and 1.2 times than those of OF. This indicates that the bio-organic fertilizer has growth-promoting effects on tobacco, which can also improve tobacco production. |
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