The mode of development of bacterial grain rot of rice caused by Pseudomonas glumae Kurita and Tabei in the field was studied using a selective medium in relation to factors associated with the disease. Host susceptibility, inoculum density, and climatic factors played a major role in the infection of grain. Studies on the population dynamics of the pathogen in pot and field experiments revealed that P. glumae on flag leaf sheaths was the inoculm source for the primary infection and that diseased panicles played an important role in the formation of foci leading to secondary infection. Pot and field experiments revealed that the pathogen showed a lognormal distribution on individual leaf sheaths, indicating that bacterial populations on the uppermost leaf sheaths in the field may be estimated from lognormal values of population size on each leaf sheath sample more accurately than on bulked leaf sheath samples, and that although the frequency of uppermost leaf sheaths harboring P. glumae drastically decreased with internode growth, the population size on rice plants before at least 30 days prior to heading time affected the disease incidence. As for the forecasting of the disease, the frequency of flag leaf sheaths harboring P. glumae (FFP) was an important indicator of the primary infection because the FFP was closely related to the disease incidence at the early stage just after heading and the presence of severely affected panicles at an early stage enabled to predict disease development after secondary infection.