Fire blight ( Erwinia amylovora ) is a destructive disease of pome fruits worldwide which has recently been emerged in quince orchards of Natanz and Kashan in Isfahan province. The incidence of the disease in some locations and its absence in other areas of Isfahan province led to believe that environmental conditions might influence on bacterial population dynamic which increase the prevalence of fire blight. In this research, therefore, the ecological behavior of E. amylovora and its population development to cause disease was studied in different humidity and thermal conditions in vivo and in situ . First, four detection methods including culturing the leaves washing suspension on Nutrient Agar Sucrose (NAS), indirect ELISA, direct PCR and Bio-PCR were evaluated to achieve a sensitive, precise and rapid method for detection and estimation of E. amylovora population on Quince leaves. None of the culturing the leaf washing and indirect ELISA methods were sensitive enough for detection of bacterial low population and the amplification results from the direct PCR reactions was inconsistent.However, Bio-PCR technique with capability to detect low population of E. amylovora was evaluated as a sensitive and relevant method for detection of viable bacteria in epidemiological studies. Bio-PCR methodwas also more sensitive, accurate and simple technique for detection of Viable but nonculturable (VBNC) state of E. amylovora comparing with apple inoculation, Pelargonium leaf infiltration and direct PCR methods. To determine environmental impact on population dynamic of E. amylovora that cause fire blight, the effect of two relative humidity rates (51% and 73%) in combination with three thermal treatments (6?, 20?, 28?) were examined during three months with 10 sampling case. The results revealed that the relative humidity rate has no considerable effect on population dynamic of E. amylovora and the bacterial population reduction trend from 0 to 57 days was similar in both conditions. On the other hand, the active population amount of E. amylovora was increased in 6? comparing to 20? and 28? treatments. In cold condition (6?), the population of E. amylovora decreased gradually, however the presence of bacterial community was documented continuously up to 64 and 84 days using culturing enriched leaves suspension and Bio- PCR, respectively. Employing these methods allowed figuring out the population fluctuation of epiphytic E. amylovora on leaves and flowers of symptomless quince trees in Rahmat-Abad (Natanz) and Isfahan University of Technology orchards. From the results of Rahmat-Abad, it can be concluded that the population of epiphytic E. amylovora increases from March until June in both leaf and flower tissues steadily which leads to establish fire blight disease on the trees sampled. Contrary, in Isfahan University of Technology orchard, the epiphytic population of the bacteria detected on May was too small to develop the disease. Comparing the environmental condition and disease incidence in these orchards let to conclude that the raining in March and April facilitates the releasing and spreading of bacterial cells from the canker of diseased trees to leaves and flowers. Then, in suitable thermal condition (above 18?), the bacterial population develops rapidly and induce disease occurrence. In June with daily temperature above 30?, the epiphytic population was reduced significantly and the disease progress ceased. The results revealed that the expansion of E. amylovora population under appropriate environmental conditions is essential for fire blight development. To such a degree, it would be possible to find fire blight on healthy trees in future by development of epiphytic population of E. amylovora . Therefore, the data obtained in this study might be helpful to find a way to manage the prevalence of fire blight in quince orchards of Isfahan province.