Zinc nanoparticles combat boscalid-resistance in Alternaria alternata

The potential of ZnO nanoparticles (NPs) to control Alternaria alternata isolates resistant to the succinate dehydrogenase inhibitor (SDHI) boscalid was evaluated both in vitro and in vivo. ZnONPs could effectively inhibit mycelial growth and suppress disease symptoms in both boscalid sensitive (BOSC-S) and resistant (BOSC-R) isolates. A high synergistic effect against BOSC-S and BOSC-R isolates was observed when ZnO-NPs was combined with boscalid both in vitro and when applied in artificially inoculated tomato fruit. The positive correlation between nanoparticles and their ionic counterpart ZnSO4 and the neutralization of the ZnO-NPs fungitoxic action in the presence of EDTA suggested that zinc ion release is the most probable fungitoxic mechanism of ZnO-NPs. The disruption of cellular ion homeostasis mechanisms by zinc NPs could account for the enhanced effectiveness of ZnO-NPs against A. alternata  compared to ZnSO4. ATP-dependent ion efflux and ROS production could contribute to the fungitoxic action of ZnO-NPs as indicated by bioassays with ATP- and antioxidant-inhibitors. Boscalid acting as a “capping” agent for ZnO-NPs, significantly reducing NPs mean size, probably accounted for the synergy observed against BOSC-S and BOSC-R isolates. Concluding, ZnO-NPs are effective against A. alternata both alone or in mixtures with boscalid, and can be used as an effective, eco-compatible anti-resistance tool for reducing the environmental footprint of synthetic fungicides.