Optimizing the Effectiveness of Induced Resistance in Tomato for Bacterial Disease Management
As alternatives for managing bacterial speck (Pseudomonas syringae pv. tomato (Pst)) and bacterial spot (Xanthomonas gardneri (Xg)) of tomato (Solanum lycopersicum), the synthetic chemical defense activator, acibenzolar-s-methyl (ASM) combined with the synthetic plant growth regulator (PGR) uniconazole (UNI), the natural chemical defense activator, para-aminobenzoic acid (PABA), and the putative biological defense activator, B. mycoides/weihenstephanensis R17, were examined. No consistent benefits for bacterial spot and speck control or tomato growth were observed in field experiments from 2011-2013 with the combination of ASM and UNI, whether applied to seedlings (<6 weeks old) or post-transplanting. However, greenhouse applied ASM or ASM+UNI reduced late season disease severity 18 to 24% compared to nontreated and CuOH-treated controls for cv. TSH4 in 2012, indicating that ASM applied to tomato seedlings can have long-term benefits under certain conditions. Fitness costs in terms of reduced growth are a concern with ASM, so UNI was added to ASM to determine if that could be ameliorated, but this approach was ineffective. Greenhouse ASM applications to cv. H9909 in 2012 reduced total yield by 20% compared to the nontreated control, indicating a fitness cost, and ASM+UNI treated plants showed a similar loss. Fitness costs may have occurred in 2012 due to stress from dry conditions after transplanting. For PABA, effectiveness was affected by application method, concentration and host genotype. Despite optimizing PABA efficacy under controlled conditions, PABA was ineffective in the field. A bacterial endophyte, R17, was isolated from Solanum arcanum and its ability to putatively induce resistance against Pst under controlled conditions was affected by its concentration, application method, and host genotype. PABA and R17 reduced bacterial speck lesion incidence up to 43 and 51%, respectively, but the lesions that developed were larger in treated than nontreated plants resulting in no reduction in Pst population or total symptomatic leaf area. This suggests that certain defense activators can reduce the ability of Pst to infect but then allow for greater Pst population growth post-infection. While defense activators have potential, they need to be more effective and consistent before they are integrated into bacterial disease management strategies of tomato.