Bean common blight is one of the most destructive diseases of bean world wide. During recent years the disease caused a serious yield loss in Iran. Since infected seed is a major source of primary inoculum and is the exclusive tool of pathogen to disseminate to far distant area, in the first part of this study, several detection methods including Indirect ELISA, Direct PCR, Bio-PCR and Ic-PCR were used to detect X. axonopodis pv. phaseoli in bean seeds. Infected bean seeds were collected from Arak bean farms. Seed samples were incubated in seed washing buffer. Indirect ELISA was done using IgG purified from X. axonopodis pv. phaseoli polyclonal rabbit antiserum. In Direct-PCR seed extracts were boiled and 10?l applied for PCR amplification. In Ic-PCR 100?l of seed extracts were loaded in wells which were coated with 10-1 dilution of purified immunoglobulin. After incubation at 37°C for 3 hours, 30?l of sterile distilled water was added to each well and wells were put in 100°C for 10 minutes. Then 18?l of each sample was subjected to PCR. In Bio-PCR aliquots of the extracts were plated onto semi-selective modified nutrient broth yeast extract agar. After 48 hours incubation, 1ml of sterile distilled water was added to each media plate and the resulted suspension was boiled and 10?l applied for PCR amplification. In all PCR-based methods a species specific pairs of primer X4c\X4e was employed. The sensitivity of Indirect ELISA was low and at least 105 cfu was needed as detection threshold. Direct PCR was unable to differentiate between viable and dead cells and Ic-PCR was an expensive method. The Bio-PCR technique was more sensitive than the other techniques described previously. On the other hand in Bio-PCR only the viable cells could be detected. Therefore Bio-PCR is recommended for assay of bean seeds for X. axonopodis pv. phaseoli. In another part of this study role of factors influence epiphytic population of X. axonopodis pv. phaseoli in epidemiology of bean common blight was investigated conducting four experiments in greenhouse and field. Greenhouse experiments carried out to clarify the effect of relative humidity rate, genotype, type of infection, sampling time and their interactions on epiphytic population size and number of bacterial spots per plant. A factorial experiment using a randomized complete block design with seven replications and two different relative humidity rates (53% , 73%) was used in greenhouse. The result showed that there were significant differences among the genotypes, type of infections and sampling times for epiphytic population size and number of bacterial spots per plants. Generally the epiphytic population size and number of bacterial spots per plant were significantly higher on Khomein cultivar and spray infection. The relative humidity rates didn’t significantly affect the epiphytic population size and number of bacterial spots per plant. The interaction of type