Population Dynamics of Pseudomonas glumae, the Causal Agent of Bacterial Grain Rot of Rice, on Leaf Sheaths of Rice Plants in Relation to Disease Development in the Field.

Abstract

Rice plants were inoculated by spraying with Pseudomonas glumae, the causal agent of bacterial grain of rice, at different times before heading time to study population dynamics of the pathogen on rice sheaths in the field and the relationship between the population size and the disease incidence. The bacterial population on the uppermost leaf sheaths and flag leaf sheaths were detected periodically using the selective S-PG medium. The results indicate that the rate of leaf sheaths with pathogen population of more than 103cfu/g fresh weight, which was the detection limit of the selective medium, decreased greatly with the growth of internode. Bacterial populations on individual leaf sheaths varied from leaf sheath to leaf sheath showing approximately lognormal distribution by plotting data sets on the cumulative probability scale. The fact suggests that the bacterial populations on uppermost leaf sheaths in the field may be estimated from lognormal value of population size on individual leaf sheaths more accurately than from bulked leaf sheath samples. Correlation coefficient between detection frequency of the pathogen on flag leaf sheaths and disease incidence on panicles a week after heading time was high (r=0.78, p=0.01), but not significant 2 and 3 weeks after heading time, suggesting that the pathogen on flag leaf sheaths is important in primary infection of the disease. The value of frequency is applicable for estimation of disease incidence of bacterial grain rot of rice in the field.