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Characteristics of Pseudomonas spp. causing grain discoloration and sheath rot of rice, and associated pseudomonad epiphytes.


Ninety-five strs of fluorescent (presumed to be Pseudomonas spp.) and non-fluorescent pathogens of rice causing grain and sheath rot, dirty panicle and manchado de grano, and 21 strs of non-pathogenic fluorescent pseudomonads isolated from rice grains and sheaths from 22 countries were compared with 26 reference strs (P. avenae, P. fuscovaginae, P. glumae, P. marginalis and P. syringae) with the use of morphology, serology and 77 physiological traits. A Ward's min. variance cluster analysis grouped the strs into 7 clusters corresponding to 6 bacterial species: P. fuscovaginae (2 clusters), P. avenae, P. fluorescens, P. glumae, P. putida and P. syringae. Every str. from Chile that caused sheath and grain rot was P. syringae. The rest of the pathogenic fluorescent strs were consistent with P. fuscovaginae but were grouped into 2 different clusters, suggesting that further study of this pathogen is warranted. The non-fluorescent pathogens were either P. avenae or P. glumae, and no pathogenic Erwinia spp. were encountered. Approx. 25% of the strs of P. syringae and P. fuscovaginae agglutinated with their respective heterologous antisera and up to 75% of P. avenae and P. glumae agglutinated with their respective antisera. There was <10% agglutination between fluorescent and non-fluorescent pathogens and their respective antisera. The non-pathogenic strs did not react with any antisera. It is concluded that 4 bacterial species causing grain or sheath rot of rice were encountered in this study. They are P. fuscovaginae, P. syringae pv. syringae, P. avenae and P. glumae, with P. fuscovaginae and P. avenae the most common. These rice pathogens, formerly believed to be of limited distribution, are shown to be distributed worldwide.