Evaluation of Rhamnolipids and Chitosan-Based Products for Managing Shoot Trauma Blight in Apple Trees

Most damage associated with fire blight disease (Erwinia amylovora) is due to blossom blight, so less research has been dedicated to shoot infections. Current control of shoot trauma blight, associated with wind or hail damage, relies on copper and antibiotics, yet sustainable alternatives are needed. This study evaluated the efficacy of chitosan, rhamnolipids, and caprylic acid against shoot trauma blight. We demonstrated the antimicrobial activity of all three compounds in vitro and optimized a host inoculation method mimicking high-infection pressure conditions. Caprylic acid, with high antimicrobial capacity in vitro, exhibited high phytotoxicity at effective bactericidal concentrations and provided no disease control in plants at nonphytotoxic concentrations. The efficacy of commercial chitosan-based products was dependent on product formulation. Only one of the three tested commercial products provided consistent control efficacy and/or symptom reduction levels comparable to those of antibiotics, copper, and/or acibenzolar-S-methyl in two greenhouse trials under high infection pressure. Rhamnolipids were effective at reducing symptom severity in two independent greenhouse assays and were the only treatment with significant effects in a field trial. Realtime PCR results showed that treatments with chitosan activated salicylic acid (SA)-dependent gene expression, whereas rhamnolipids induced genes linked to both SA- and jasmonic acid-dependent responses. Moreover, rhamnolipids enhanced kasugamycin and streptomycin activity in vitro, indicating their potential as coadjuvants for enhancing antibiotic activity at lower concentrations. Overall, our findings provide preliminary data for the implementation of these treatments in shoot trauma blight management, potentially useful under moderate or low infection pressure conditions.