Abstract
The distribution and spatial pattern of bacterial grain rot of rice, caused by Pseudomonas glumae, were determined by using Morisita's index IB of distribution. An area of5, 000 hills (50 rows×100 hills) 1 and 2 weeks after heading time and an area of 2, 000 hills(20 rows×100 hills) 3 weeks after heading time was assessed for disease incidence in 3 fields, A, B and C. Each assessment was done by using 5 quadrat sizes (rows×hills). The spatial pattern of the hills, whose disease incidences were 2 to 4, was clumped in field A a week after heading time as determined by Morisita's index, but not in fields, B, C. At 2 weeks after heading time, the shape of the IB curve for the 3 fields was typical for a pattern with small clumps. The higher the severity of fields, the earlier was the formation of the aggregations. Hills, with a disease incidence of 4, were frequently surrounded by hills of lower disease incidence. The dissemination of this disease was studied in a paddy field by setting diseased hill planted in 1/5, 000 a pots as a inoculum source at the center of 2.7m×2.88m plots (9 rows×19 hills) at heading time or a week after heading time. For all plots, the further the distance from the inoculum source, the lower was the disease incidence. The disease incidences of 8 plants surrounding the inoculum source were clearly largest for the plot in which a severely diseased hill was set as a inoculum source at heading time. Populations of P. glumae from severely diseased panicles, which were sampled from the paddy field and were incubated in a moist chamber, were clearly higher than those from slightly diseased panicles and symptomless panicles, moreover, the populations of P. glumae from diseased panicles depended on the incubation time in a moist chamber. The results of this experiment show that severely diseased panicles which were occurred earlier, were important in focal formation of bacterial grain rot of rice.
