PATHOGENICITY PROCESS OF PSEUDOMONAS-FUSCOVAGINAE, THE CAUSAL AGENT OF SHEATH BROWN-ROT OF RICE

Sheath brown rot of rice caused by Pseudomonas fuscovaginae has been d escribed in areas where low temperatures occur during the rice booting and heading stages. To analyse the relationship between pathogenicity of P. fuscovaginae and low temperatures, pathogenicity process in ric e at booting stage was studied in a growth chamber at midrange and low temperatures. Analysis performed at 13 degrees C, 18 degrees C and 23 degrees C in two rice cultivars showed that pathogenicity of P. fusco vaginae was explained by the general model of the independent action. The inoculum dose necessary to obtain 50% of diseased sheaths decrease d with increase of temperature. Analysis of in planta bacterial popula tion dynamics and mean response time pointed out that low temperatures affected pathogen multiplication in host before and after symptoms de velopment. In consideration of our results, it was concluded that low temperatures acted negatively on the pathogenicity process of P. fusco vaginae. Therefore, occurrence of P. fuscovaginae in areas where rice cultivation is restricted by low temperatures can not be explained by a direct effect of temperatures on pathogenicity.