Biocontrol Study Of Pseudomonas Sp. 20～#-5 To Tobacco Black Shank

Tobacco black shank caused by Phytophthora parasitica var. nicotianae is a serious plant disease which distributes widely in the world. It has drastic destruction to all cultural tobacco and is very difficult to be controlled. For a long time, people have been trying to control this disease with chemical pesticides, mainly including metalaxyl, yizailing. However, because of disadvantages such as contamination to environment, harm to human body, inducing resistance of the pest, chemical pesticides had been challenged. While biological pesticides have the merits of low toxicity to mammal and little pressure to environment. Therefore, the development of biological pesticides is valuable and potential.Pseudomonas sp. 20^#-5 strain is an antibiotics-producing bacterium. Preliminary study indicated that its metabolites could inhibit the growth of Phytophthora parasitica var. nicotianae. In this paper, we reported the petri dish antagonism test, greenhouse disease-control experiment, field control effect of this strain to tobacco black shank and its colonization in tobacco plant body.1. Antagonism test in petri dish20^#-5 strain and its ferment products were added to the medium respectively to test their antagonism to Phytophthora parasitica var. nicotianae. Results showed that both the strain and active compound of its ferment products (we call it ferment products of 20^#-5 strain for short below) obviously inhibited the growth of P. parasitica. The inhibition zone was 0.5cm after cultivation for 60h. While tested under microscope, the mycelia of the P. parasitica were found to grow abnormally and their walls had been destructed.2. Disease-control experiment in greenhouseArtificial inoculation test in greenhouse demonstrated that ferment products of 20^#-5 strain could control tobacco black shank efficiently. The control efficiency of the treatment with ferment products before disease was 59.32％, while that after disease was only 10.17％. This result indicated that the efficiency of ferment products of 20^#-5 strain to tobacco black shank was "prevention" but not "therapy".3. Field control effect testControl efficiency of each treatment was high at the beginning of disease because of the low attack rate. The efficiency of the treatment with ferment products before disease was 64.39％. It was a little lower than the efficiency of yizailing (72.07％), but higher than metalaxyl. The efficiency of the treatment with ferment products after disease was only 39.88％. Statistical analysis indicated that they were not significant one another. The efficiency of the treatment with ferment products before disease upgraded to 75.68％, higher than that of ferment products after disease (50.00％), metalaxyl (36.58％) and yizailing (29.77％) at intermediate stage of disease. The statistical analysis showed significant difference when compared with yizailing. Notably, the efficiency of two chemical pesticides had decreased.The efficiency of the treatment with ferment products before disease increased to 79.70％at the end of disease. It was significantly high when compared with those of metalaxyl and yizailing, which was 37.84％and 29.16％, respectively; but it wasn't distinct when compared with that of treatment with ferment products after disease.Compared with other treatments, the efficiency of the treatment with ferment products of 20^#-5 strain before and after tobacco black shank upgraded consecutively, which indicated that it had a steady efficiency and long persistence. The efficiency of the treatment before disease was higher than that of after disease proved that the effect of ferment products of 20^#-5 strain was "prevention".4. Colonization testSpontaneous mutant strain 20^#-5* resisting 200μg-ml^-1 streptomycin was obtained by increasing antibiotic concentration in the culture step by step. After being transferred 10 generations on the plate medium, it still maintained the resistance to Stre and the antagonism activity to P. parasitica in petri dish and greenhouse control effects. This mutant strain was used for the colonization analysis.In the colonization test, the tobacco variety and soil were the same with those in the greenhouse disease-control experiment. Results indicated that strain 20^#-5* could be detected in the inner part of tobacco roots, stems and leaves one day after watering roots with the suspension. The amounts had been upgrading for seven days in the inner roots while twelve days in the inner stems and leaves, then decreased respectively. This suggested that 20^#-5 strain had the endogenousness. Besides, 20^#-5* strain could colonize on the surface of tobacco roots, stems and leaves seventeen days after watering roots. It suggested that they could retain for a long time on the surface of tobacco tissues.The amounts of 20^#-5 colonized in tobacco roots, stems and leaves had the tendency of first increasing and then decreasing. The population of 20^#-5* in the roots was more than those in other tissues. The amounts in stems were slightly higher than those in leaves.