Heat-inactivated Lysobacter capsici AZ78 cells effectively control Peronospora belbahrii and Plasmopara viticola through direct and indirect mechanisms

The bacterial strain Lysobacter capsici AZ78 is an effective biocontrol agent against several plant-pathogenic oomycetes. This study demonstrates that heat-inactivated L. capsici AZ78 cells retain strong protective activity against Plasmopara viticola and Peronospora belbahrii, comparable to that of viable L. capsici AZ78 cells, as well as a copper-based fungicide and a plant resistance inducer. Microscopic analyses showed that both heat-inactivated and viable L. capsici AZ78 cells were toxic to P. viticola sporangia. Moreover, microscopic analyses also revealed their ability to induce callose deposition and reactive oxygen species accumulation in basil and grapevine leaves, indicating the stimulation of plant resistance mechanisms. Chemical analyses identified dihydromaltophilin and maltophilin, two heat-stable polycyclic tetramate macrolactams, in the membranes of L. capsici AZ78 cells. Isolated dihydromaltophilin and maltophilin, individually or in combination, were highly effective against P. viticola and triggered callose deposition in grapevine leaf discs. These findings demonstrate that L. capsici AZ78 and its heat-stable polycyclic tetramate macrolactams act through a dual mechanism, direct antimicrobial activity and induction of plant defence responses. The use of heat-inactivated L. capsici AZ78 cells and their secondary metabolites offers a promising, environmentally friendly alternative to copper-based products for the sustainable management of downy mildews.