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Rice plants were inoculated by spraying with Pseudomonas glumae, the causal agent of bacteri grain of rice, at different times before heading time to study population dynamics of the pathogen on ric sheaths in the field and the relationship between the population size and the disease incidence. Th bacterial population on the uppermost leaf sheaths and flag leaf sheaths were detected periodically usin the selective S-PG medium. The results indicate that the rate of leaf sheaths with pathogen populati of more than 103cfu/g fresh weight, which was the detection limit of the selective medium, decreas greatly with the growth of internode. Bacterial populations on individual leaf sheaths varied from lea sheath to leaf sheath showing approximately lognormal distribution by plotting data sets on the cumul tive probability scale. The fact suggests that the bacterial populations on uppermost leaf sheaths in th field may be estimated from lognormal value of population size on individual leaf sheaths more accurat ly than from bulked leaf sheath samples. Correlation coefficient between detection frequency of th 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 fla leaf sheaths is important in primary infection of the disease. The value of frequency is applicable fo estimation of disease incidence of bacterial grain rot of rice in the field

Change in susceptibility of rice spikelets (cvs. Koshihikari, Koganebare and Asominori) to bacterial grain rot, caused by Pseudomonas glumae, with time was examined by spraying bacterial suspension to panicles at different stages of flowering. Percentage of diseased spikelets of these varieties was highest when they were inoculated on flowering day and remained high when they were inoculated within 3 days after flowering. The disease was very low when spikelets were inoculated 2 days before or 4 days after flowering, even though spikelets were incubated under high humidity (RH>95%) condition for 24hr after inoculation. These results indicate that susceptibility of rice spikelets changes with time during a short, critical period before and after flowering and that high humidity at this stage is crucial to the infection of spikelets. Analysis on the relationship between susceptibility of a whole panicle and the flowering rate of spikelets within a panicle showed that panicle susceptibility depends on the rate of flowering per day after heading. This correlation indicates that susceptibility of rice plants to the disease in the field may be estimated from the heading rate of panicle in the field and from the extent of susceptibility of each panicle per day. The relative value for cumulative susceptibility of rice plants (Tt) in the field on the day t after first heading was represented by the value for total susceptibility of all panicles on the day t which was calculated by daily heading rate of panicles on days 0 to t after first heading in the field and by the panicle susceptibility on each day after heading. The Tt value of rice plants increased similarly from heading time, which is defined as the day when more than 40% of panicles have headed, and then decreased 5 to 6 days after heading time. Tt was very low about 12 days after heading time, suggesting that rice plants in the field are susceptible to the disease during a short length of time, i.e., from the heading time to about 11 days after that.

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Species of three viral genera infecting Ornithogalum thyrsoides plants showing mosaic symptoms were identified using RT-PCR and degenerate universal primers for each viral genus. The DNA fragments obtained encoded the coat protein (CP) gene and were sequenced. The plants were found to be infected with one or other of three potyvirus species, one of them was Ornithogalum mosaic virus (OrMV). The other two viruses were previously unrecorded and were named Ornithogalum virus 2 (OV-2) and 3 (OV-3). Direct comparison and phylogenetic analysis with published OrMV isolates revealed that the CP of the three OrMV-like clones were more similar to Pterostylis virus Y (PtVY) than to OrMV. No carlavirus or potexvirus was isolated.

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