Due to rapid spread of fireblight, the Rural Development Administration is supplying bactericides to farmers. However, research on inhibitory effects of main active ingredients in these bactericides on fireblight is lacking. Chlorophyll fluorescence analysis is a non-destructive method for analyzing the photosynthetic efficiency of plants, enabling time-series data analysis of pathogen progression and allowing for large-scale studies. Therefore, this study analyzed inhibitory effects of main active ingredients in bactericides on fireblight using chlorophyll fluorescence response analysis. Flowering pear trees (three-year-old ‘Shingo’ variety) were sprayed with control agents and fire blight pathogens on flowers. Chlorophyll fluorescence responses were then measured at seven-day intervals. Twenty-eight days after bactericide treatment, the fluorescence of the O-J transition stage in the untreated group was twice as high as in the average bactericide-treated group presumably due to inhibition of electron transport in the PSII donor side caused by pathogen infection, leading to leaf necrosis. Additionally, the electron transport efficiency (ET2o, RE1o) decreased, reducing the driving force of photosynthesis (DF total ABS) to 20% of the average bactericidetreated group, indicating chlorophyll damage and reduced photosynthetic capacity. In conclusion, chlorophyll fluorescence technology can be used to quantitatively evaluate the efficacy of fire blight control agents during the flowering period of pear trees.