Use of glucuronidase (gus A) gene as a marker to monitor the fate of Ralstonia solanacearum (Smith 1896) Yabuuchi et.al. 1996 comb, nov. in tomato (Lycopersicon esculentum Mill) and in the soil

The colonization pattern by the pathogen in six tomato varieties, reportedly by varying degrees of resistance to bacterial wilt, revealed similar infection pattern. The bacterium was observed colonizing the merging roots of susceptible as well as resistant varieties as early as 12 h from inoculation indicating that resistance to Ralstonia solanacearum was not expressed early in the colonization process. However, examination of transverse sections of the infected parts of the primary and secondary roots as well as of the stem one week after inoculation indicated a direct correlation between susceptibility or resistance with degree of colonization. Infection in the susceptible varieties was more severe, reaching the mid cortex, totally plugging some of the cortical cells in the primary root, including the entire vascular bundle of the secondary roots and stem. The resistant varieties had significantly lesser infection. Intracellular infection was observed up to the mid cortex but intercellular infection was limited to a few cortical cells in the primary root. In the stem, infection reached far into the inner cortex but the vascular bundles remained uninfected in both the primary root and stem sections. GUS activities of the different varieties measured by flourometry 48 hours after inoculation correlated well with the results from histochemical studies where generally higher GUS activities were obtained from the more susceptible varieties. Detection of the pathogen in the soil was enhanced with the use of both double antibiotic and GUS markers. Survival of the strain was monitored in three soil types at two moisture regimes and at two temperature conditions. Moisture level affected the survival of the pathogen. Consistently low recovery was obtained from all soils maintained at permanent wilting point. The type of soil likewise had a significant effect on the survival of the pathogen. Clay loam and sandy clay were found to be conducive while sandy loam was suppressive to the growth of the pathogen. In the clay loam and sandy clay, the population of the pathogen was maintained, falling only to its original number even after 13 weeks. In the sandy loam, the population drastically dropped after the first week and was undetected in soil after 8 weeks. Temperature difference of 10 deg C caused a significant effect on the survival of the strain. Growth of the strain was more favorable at 25 deg C than at 35 deg C