K. WYDRA, ZEKARIAS LEYKUN
University of Hannover, Institute of Plant Diseases and Plant Protection, Germany
Bacterial wilt caused by Ralstonia solanacearum is difficult to control due to the high variability of the pathogen and its capability to survive easily in adverse environments. Therefore, use of resistant genotypes is the key option for integrated control. However, latent infection may cause breakdown of resistance, pathogen dissemination or substantial yield reduction even with low wilt incidence. Therefore, the level of latent infection was quantified in fourteen `resistant' tomato genotypes in relation to wilt incidence. R. solanacearum was detected in the collar of all symptomless genotypes and, hence, initial root infection was not limiting for bacterial colonisation. Highest differences in bacterial numbers were observed in the mid-stem parts of `resistant' genotypes suggesting existence of effective resistance mechanisms on mid-stem level in some genotypes. Immunohistochemical analysis revealed differential reactions in xylem cell wall structure and composition in resistant and susceptible genotypes. Bacterial numbers and wilt incidence were positively correlated, with a higher correlation coefficient in the mid stem parts than in lower or upper plant parts. Therefore, quantification of bacteria in the mid"=stem is suggested as a complementary criterion in addition to wilt symptom evaluation to identify genotypes, which suppress the latent pathogen multiplication.
The ability of R. solanacearum to enter the viable but non-culturable (VBNC) state in planta was examined in different genotypes after infection. A significant percentage of R. solanacearum cells entered the VBNC state in xylem vessel, increasing in number with time after infection in symptomatic plants. The influence of the plant on the phase change of the bacterium could be an additional characteristic for the selection of resistant genotypes.
Keywords: Bacterial wilt, host plant resistance, Ralstonia solanacearum, tomato