Biological Control of Fusarium fujikuroi, the Causal Agent of Bakanae Using Some Antagonistic Bacteria in Gilan Province

Introduction: Ricebakanae disease caused by Fusarium fujikuroiis one of the most important diseases of rice in Iran and in the world. Studies show that the disease has spread to a wide range of paddy fields worldwide, with losses in Japan up to 20%, in India up to 15%, and Thailand's northern and central areas have been reported to be 7.3% -14.7%. Symptoms include rice foot blackness and yellowish and then wilting of infected plants. The pathogen is more likely to attack the foot rot which is a sign of the abnormal growth of contaminated plants in the farm. Infected seedlings are slender and taller than healthy plants, and highly infected plants may die before or after transplantation. The tillering is reduced, consequently the leaves die in a short time. The fungus causes the disease threats the human and animal health through the production of phytotoxins. Identification of Fusarium species is currently confusing, on the other hand, several Fusarium species have always been isolated together with rice contaminated with the disease. Therefore, it is not clear which species of this fungus are the main reason of the disease. This fungus is soil-borne and has a long life in heavy soils. It has also a global expansion and is active in most parts of the world. Rice foot rot disease in all major rice producing countries in the world is considered as a seed disease and the transmission of the disease agent from one season to another season is mainly due to contaminated seed, but soil-borne fungus can be as well. Currently, treating the seeds with fungicides is the best method to control this disease, however using chemical pesticides can lead to environmental pollution. In this situation, it is important to achieve a healthy alternate method. For this purpose, biological control is one of the ways in which today a large investment is being made around the world. Materials and Methods: Samples of rice exhibiting the symptoms of bakanae were randomly collected from different parts of Gilan province, Iran. Pieces of organs with rot symptoms were cut and they were surface-disinfected after washing with 0.5% sodium hypochlorite solution. At the next step, they were washed with distilled water and dried on filter papers. Then, they were cultured in Petri dishes containing potato dextrose agar (PDA) culture medium and were placed in an incubator at 28°C for 3-5 days. Afterward, they were placed on a water-agar medium for purification and morphological identification. In this research, from a total of 80 samples collected from rice farms in Gilan province, 18 bacterial strains were isolated and the antagonistic ability of 8 strains of bacteria in the laboratory and greenhouse was investigated. For this purpose, in laboratory, dual culture method, volatile compounds, siderophore and antibiotic production were used. These bacterial strains were inoculated into rice under greenhouse conditions, and then the severity of the disease was determined in the tested treatments. After calculating the severity of the disease, the height of the rice bushes was measured by the ruler. To measure the fresh weight, the rice bush with the roots was removed from the soil and measured by a scale. Each bush was then separately placed for 48 hours in an oven at 80-90°C. After leaving the oven, each of the bushes was re-weighed. This weight was recorded as dry weight. Results and Discussion: A total of 18 isolated bacterial strains, 8 bacterial strains including Bacillus subtilis, Bacillus circulans, Pseudomonas putida, Pseudomonas syringae, Pseudomonas aeruginosa, Pseudomonas fluorescens (N47), Pseudomonas fluorescens (149) and Pseudomonas fluorescens (CHA0) were identified and used for biological control studies in laboratory and greenhouses. P. putida with 39.99 % in the dual culture, B. subtilis with 31.01% in the volatile metabolites, in the method of siderophore production, P. fluorescens (N47) and B. subtilis with 52.10 % and 45.85 % respectively, and in the antibiotic production, P. putida with 59.21% had the greatest effect on inhibiting the mycelial growth of the disease causative agent. Based on the results of the analysis of variance under greenhouse conditions, there was a significant difference between treatments for severity of disease, fresh weight, dry weight and plant height at 1% probability level. Among the studied bacteria, the least severity of the disease belonged to the treatment with P. putida and B. subtilis, respectively. Regarding height, the highest height was related to treatment with P. putida and B. subtilis and in terms of fresh weight and dry weight, the highest weight was related to P. putida. Conclusion: According to the results of biocontrol studies in laboratory and greenhouse conditions, P. putida and B. subtilis strains were the most effective bacteria for controlling rice foot-rot disease.Therefore, isolating and identifying these bacterial strains as much as possible can be promising for the greater effectiveness of biocontrol methods in the management of rice crown-rot disease control.