Biological Control of Verticillium Wilt of Cotton by Salt Tolerant Rhizobacteria in Extreme Soil Conditions.
Dilfuza Egamberdiyeva, Tashkent State Univ of Agriculture, University Str. 1, Tashkent, 700140, Uzbekistan
Cotton was the major crop produced in Uzbekistan for decades. Its production required an extensive use of chemical fertilizers and pesticides and was accompanied by strong irrigation of fields. The heavy use of fertilizers and pesticides is the cause of increasing soil salination and soil contamination from agricultural chemicals. Because cotton plants are under saline or water unbalance stress, they become more vulnerable to diseases caused by pathogenic fungi. Cotton growth suffers from Verticillium wilt diseases and losses are approx. 40%. It is clear that the situation could be largely improved when the use of chemicals would be replaced by environmentally friendly biologicals. The use of specific microbes which stimulate plant growth and/or are natural enemies of pathogens allows a considerable decrease in the use of agrochemicals which are now being used for plant growth stimulation and control of diseases. Therefore the development of salt-tolerant inoculants in such environments will help improving cropping methods, plant health and productivity. The objectives of our research work were the isolation, selection and characterization of rhizosphere bacteria for the control of cotton Verticillium wilt disease and to increase their growth and nutrient uptake in salinated nutrient poor soils. Soil samples were collected from the field site salinated arid serozem soil, planted to cotton, Surhandarya region, Uzbekistan. The bacterial strains isolated from the rhizosphere of cotton grown and screened for their antagonistic activity against Verticillium dahlia fungal pathogen in vitro. Bacterial strains that were antagonist against Verticillium dahlia were identified as Pseudomonas, Bacillus, and Arthrobacter species. The treatment of cotton seeds with those bacterial strains resulted in statistically significant decreased of cotton infection and improved the plant growth. The 40% reduction in disease symptoms by seed bacterisation is a big achievement and makes the rhizobacteria important in managing cotton wilt disease. The effective bacteria isolates Pseudomonas spp. UzC15, Bacillus subtilis UzC 27 and Arthrobacter globiformis. UzC6 increased the shoot and root dry weight from 13 to 29% significantly as compared to the control. Bacterial inoculation also resulted in higher values of nitrogen, phosphorus and potassium content in plant components from 13 to 45 % as compared to the control. Besides, growth promoting bacteria produced the phytohormon auxin. Some bacteria, which were capable of nitrogenase activity, are produced B-complex vitamins and different proteins. Effective strains were osmo-tolerant. They also have ability to temporarily survive in dry soil conditions. In general, our results support the importance of physiological adaptation of microbes to the whole environment, thus local isolates may be involved.