Cardamom Essential Oil Exerts a Curative Effect Against Kiwifruit Bacterial Canker but Fails to Activate Host Defense Mechanisms

Pseudomonas syringae pv. actinidiae (Psa) is the most destructive pathogen of kiwifruit, and the absence of curative measures makes the management of Psa-induced kiwifruit bacterial canker (KBC) particularly challenging. Elettaria cardamomum produces an essential oil (CAR) rich in bioactive compounds with demonstrated potential to act directly against Psa, but its mechanisms of action remain insufficiently explored. Here, we investigated CAR’s mode of action in plants with established mild KBC symptoms, and assessed its potential as a plant elicitor. In the in planta assay, CAR application (0.1% w/v, applied 7 days after inoculation) reduced KBC symptoms, with the strongest effect observed 14 days after treatment (DAT). However, CAR did not significantly affect oxidative stress biomarkers, antioxidant system, pigments and primary metabolism, or the expression of target genes related to systemic acquired resistance or salicylic acid and jasmonic acid pathways. For instance, Psa inoculation significantly upregulated PR1 (≈5.4-fold) and PR5 (≈4.3–5.5-fold), irrespective of CAR application. Complementary in vitro assays revealed a transient, phase-dependent antimicrobial activity of CAR: although the effect disappeared by 32 h in liquid-phase assay and no inhibition was observed under vapor-phase exposure, a strong reduction in Psa viable cells (79.7%) was observed after 8 h exposure in the liquid phase. This study demonstrates that CAR exerts a direct, albeit transient, antibacterial effect against Psa, conferring curative activity when applied to plants with mild KBC symptoms. Consequently, repeated applications may be required to maintain disease suppression, as CAR does not appear to induce a sustained preventive defense response in the host.