Molecular interaction between obligate filamentous pathogen and its host plant has not been well studied under field environments. Unstable temperature and humidity outsides potentially influence disease resistance of the host plant and virulence of pathogen. There may be unknown mechanisms of plant-pathogen interaction that have not been clarified under the laboratory conditions. To reveal plant-obligate filamentous pathogen interaction at molecular level under the field environments, we studied seasonal gene expression dynamics of both the powdery mildew Blumeria graminisand its host plant Aegilops umbellulataZhuk., a diploid wild wheat relative with UU genome, using RNA-sequencing for infected leaves of the field trails of three years from 2016 to 2018. Infection level of the powdery mildew changed gene expression of the host and the pathogen, although it was difficult to find common dynamics of their gene expression among the years. In 2018, the infection level radically increased at the end of March together with upregulation of genes involved in response to biotic stimulus. Subsequently, gene expression of photosynthetic genes deceased. On the other hand, in 2016, the infection level moderately increased and reached the maximum level at the seed maturing stage of the host plants, and then photosynthesis genes were downregulated. The genes involved in response to biotic stimulus did not show radical upregulation. It was found based on these observations that the radical increase of the infection level in 2018 made the activity of photosynthetic genes depress almost a month earlier than in 2016. In addition, only in 2016, cleistothecia, which are closed fruiting body with ascospores and enable to survive after dying host plants, were observed on the surface of the infected leaves. By investigating frequency of two mating types of B. graminis, we revealed that both types coexisted with equal frequency at the seed maturing stage of the host plant. By contrast, in 2017 and 2018, there were no cleistothecia and one of the types dominated at the end of the seasons, suggesting that the powdery mildewisolates could not leave offspring to next season. In conclusion, dynamics of infection level alters gene expression patterns of both B. graminis and Ae. umbellulata may make an impact on demographic changes in the B. graminisisolates. Radical increase of infection level would cause serious damage not only to the host plant but also to the obligate pathogen itself from the point of view of leaving offspring to next season.