Effects of para-aminobenzoic acid on bacterial speck symptom development and Pseudomonas syringae pv. tomato populations in tomato leaves

Para-aminobenzoic (PABA) is reported to induce resistance against a range of plant pathogens in different crops in a salicylic acid-dependent manner. However, factors affecting its efficacy are not well understood. Foliar PABA applications on tomato seedlings reduced lesion incidence caused by Pseudomonas syringae pv. tomato (Pst) in a dose-dependent manner in distal leaves up to 18 mM under controlled environment conditions, but only three out of six commercial processing tomato cultivars tested showed a response to PABA. Leaves in direct contact with 9 and 18 mM PABA of both PABA-responsive and PABA-nonresponsive cultivars showed phytotoxicity. In a PABA-responsive cultivar, one, two and three PABA applications were equally effective at reducing lesion incidence in distal leaves, but the duration of control only persisted for approximately 7 days. Although PABA application reduced lesion incidence in distal leaves, the Pst population in leaves was unaffected. Lesions on PABA-treated plants were larger than nontreated plants, and thus the proportion of leaf surface area with lesions was unaffected by PABA treatment. In in vitro assays, 18 and 72 mM PABA produced zones of inhibition against Pst 15 and 50% larger than the ethanol control, demonstrating direct antimicrobial effects of PABA. PABA application did not affect symptom development in a mixed infection of Pst or Xanthomonas spp. in one field experiment with a PABA-responsive cultivar. Further research is needed to understand why PABA was unsuccessful in the field before it is to be used as a practical disease management tool for foliar bacterial diseases of tomato.