Pathogenicity Process of Pseudomonas fuscovaginae, the Causal Agent of Sheath Brown Rot of Rice

Sheath brown rot of rice caused by Pseudomonas fu.ʹ;- covaginae has been described 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 rice at booting stage was studied in a growth chamber at midrange and low temperatures. Analysis performed at 13 C, 18 C and 23 C in two rice cultivars showed that pathogenicity of P. fuscovaginae was explained by the general model of the independent action. The inoculum dose necessary to obtain 50% of diseased sheaths decreased with increase of temperature. Analysis of m planta bacterial population dynamics and mean response time pointed out that low temperatures affected pathogen multiplication in host before and after symptoms development. In consideration of our results, it was concluded that low temperatures acted negatively on the pathogenicity process of P. fuscovaginae. Therefore, occtirrence 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.