Direct Delivery of Inoculum to Shoot Tissue Interferes with Genotypic Resistance to Ralstonia solanacearum in Tomato Seedlings

Abstract

Employing known susceptible and resistant genotypes and pure bacterial inoculum (0.1 OD; 108 CFU/ml−1), five different inoculation methods were tried to assess the response of tomato genotypes to Ralstonia solanacearum. This included seed‐soaking inoculation, seed‐sowing followed by inoculum drenching, or at 2‐week stage through petiole‐excision inoculation, soaking of planting medium with inoculum either directly or after imparting seedling root‐injury. Seed‐based inoculations or mere inoculum drenching at 2 weeks did not induce much disease in seedlings. Petiole inoculation induced 90–100% mortality in susceptible checks but also 50–60% mortality in normally resistant genotypes within 7–10 days. Root‐injury inoculation at 2‐week seedling stage appeared the best for early and clearer distinction between resistant and susceptible lines. The observations suggest a role played by the root system in governing genotypic resistance to the pathogen. Direct shoot inoculation is to be adopted only for selecting highly resistant lines or to thin down segregating populations during resistance breeding.

Influence of Seedling Age on the Susceptibility of Tomato Plants to <i>Ralstonia solanacearum</i> during Protray Screening and at Transplanting

“…Data were analyzed for single factor ANOVA employing the data analysis package of Microsoft Excel-2007 as earlier [7]. The response of different age groups was assessed based on the extent of mortality on the scale: ≤10%, high resistance (HR); 10% -40%, moderate resistance (MR); >40% to 70%, moderate susceptibility (MS) and >70%, high susceptibility (HS) [7].…”

“…Seedlings of 2, 3, 4, and 5 weeks from the date of sowing in protrays were employed for root-injury inoculation as per Thomas et al [7]. Briefly, the roots were injured by inserting a No.…”

“…The experiment was repeated twice. Another experiment was set up under identical conditions where the seedlings were given simple cocopeat soaking inoculation without imparting injury to the roots [7].…”

“…Arka Vikas released from this Institute was employed as the standard susceptible genotype [7]. The seedlings were raised in 98 cavity portrays in pasteurized cocopeat, a common coconut husk-based organic growing medium for raising vegetable seedlings (M/s Varsha Enterprises, Bangalore, India) as described else-where [7].…”

“…Evaluating different seedling-stage inoculation methods for screening tomato (Solanum lycopersicum) genotypes against R. solanacearum, we have observed that direct delivery of inoculum to the shoot tissue interfered with the resistance characteristic of proven resistant genotypes [7]. Root-injury and inoculation on two-week-old seedlings appeared the best approach to facilitate the discrimination between known resistant and susceptible genotypes with the knocking down of most of the susceptible plants before field planting.…”