Paclobutrazol and acibenzolar-S-methyl induced tomato seedling growth response and resistance to bacterial speck (Pseudomonas syringae pv. tomato) (Pseudomonas syringae)

Paclobutrazol, a plant growth regulator and/or acibenzolar-S-methyl, a plant defense activator were postemergence sprayed on two-week-old tomato plug seedlings to evaluate growth response and bacterial speck disease severity. Paclobutrazol induced significant changes involving reduced shoot height, leaf area, fresh shoot and root weights and increased stem diameter and leaf greenness. Acibenzolar-S-methyl had no marked effect on seedling growth except senescence of the lower leaves at higher concentrations. When these two chemicals were combined, the seedlings showed similar growth effects as seen with paclobutrazol but the senescence caused by acibenzolar-S-methyl was reduced. Two repeat applications of acibenzolar-S-methyl as compared to a single application had no significant effects on seedling growth. Bacterial speck foliar symptoms and speck lesion growth were reduced by acibenzolar-S-methyl and the combination of paclobutrazol and acibenzolar-S-methyl showed improved disease control. Higher doses of acibenzolar-S-methyl at either one application or two split applications did not improve disease control over one application. Acibenzolar-S-methyl at 30 mg L−1 with paclobutrazol 5 mg L−1 was the better combination in terms of seedling development and bacterial speck control. The response of different cultivars to paclobutrazol and acibenzolar-S-methyl were similar for growth effects and bacterial speck disease control.