The Identification, Action Mechanism And Application Efficiency Of Biocontrol Streptomyces Against Tobacco Target Spot Disease

Tobacco target spot disease(Rhizoctonia solani Kiihn), a newly-reported disease in recent years, has widely blasted and spread in northeast China, and made a large loss in both tobacco leaf quality and yield. Currently among methods in prevention and cure for tobacco target spot, chemical control has still played a dominated role with less information reported in the biocontrol level. In this paper, we screened and obtained a biocontrol Streptomyces S128, which was naturally endowed with high antagonism against tobacco target spot disease. Therefore, in terms of this isolate, we also made further studies in such areas as its taxonomy and identification, activity in germs inhibiting, interaction mechanism, optimization of fermentation conditions in solid medium culture, and application effect in prevention and cure for tobacco target spot disease in two different circumstances including greenhouse and field. The main achievements are as follows:1. The biocontrol strain named S128 with characteristics which inhibited against tobacco target spot disease well was screened and obtained from the soil of tobacco field in Dandong, Liaoning province, this isolate S128 was also identified at the same time. The Phylogenetic tree showed that the strain S128 and Streptomyces griseus subsp (AB045867) were classified into the same evolutionary branch in the phylogenic zone. Couple with its physiological and biochemical characteristics. The strain S128 was identified as Streptomyces griseus.2. The inhibiting Germs style of the biocontrol Streptomyces S128 had been confirmed and the function mechanism of that had been also preliminarily studied. In the experiment of testing the inhibiting activity of the biocontrol Streptomyces S128 against Rhizoctonia solani Kiihn, results showed that it had exhibited quite evident inhibiting effect on the growth of tobacco target spot disease, with the width of the inhibiting zone covering about 9 mm and inhibiting rate reaching 88.70%. From the results of the further experiment about the biocontrol Streptomyces S128 influenced the morphology of Rhizoctonia solani Kiihn, the results indicated that the biocontrol Streptomyces S128 could lead to the malformation of the mycelia of the pathological fungus, to the extend that fungal mycelia malformation include mycelia shortening, thickening, distorting, and increasing separating floors in the mycelia. Furthermore in testing the inhibiting spectra of the biocontrol Streptomyces S128, this strain displayed inhibiting effect not only on Rhizoctonia solani Kiihn, but also another over ten kinds of pathological fungus in different degree, exposing broad spectrum inhibiting activity. The results also showed the capacity of the biocontrol Streptomyces S128 to produce cell well degrading enzyme, the final results exposed that it may generate and discharge chitinase, cellulose, glucanase, protease, whose function style was perhaps speculated in the way of degrading the cell wall enzyme.3. The optimal fermentation conditions of the strain S128 in solid culture medium and the main impact factors in the fermentation were conducted. Finally a conclusion was drawn that the optimal cultivation fermentation condition was in such a series of impact factors including the optimal culture medium, millet; water volume,40 mL; temperature 28℃; bottle volume,20 mL; inoculation amount,3 mLand carbon source, glucose.4. Comparing with the results about indoor and outdoor inhibiting effect of the seven commercialized agents of biocontrol, it had been performed that the test of indoor inhibiting effect and the prevention effect in greenhouse and field for the supplied agents. Among the seven supplied agents of biocontrol in the inhibition test, although they displayed the obvious inhibiting effect, Bacillus subtilis showed the best inhibiting effect at the inhibiting rate of 94.26%, surpassing the other six medicines. The biocontrol Streptomyces S128 was second to the Bacillus subtilis, with the rate of 90.88%. In the results of the effect of indoor toxicity, Bacillus subtilis had exhibited the strongest toxicity at the EC50 value of 2.6422 cfu/mL, followed by the S128 with the EC50 value of 2.7008 cfu/mL. In the results of potting cultivation in greenhouse, the prevention effect of the biocontrol Streptomyces S128 which was second to the Bacillus subtilis surpassed the other six agents of biocontrol, in the meantime together with the supplied seven agents of biocontrol, they commonly showed the better prevention effect over the chemical medicines. The results of prevention effect in the field, Bacillus subtilis showed the best inhibiting effect at the inhibiting rate of 88.83%, the biocontrol Streptomyces S128 was second to the Bacillus subtilis, with the rate of 86.87%. The biocontrol efficiency of other six agents of biocontrol exceeded above 59.18%. The control effect of chemical control of was 54.37%.All the results showed that the biocontrol Streptomyces S128 has good development prospects to control tobacco target spot disease.