Design and development of a workflow for microbial spray formulations including decision criteria

Herein, we present a workflow for the development of talc-based microbial inoculants for foliar spray consisting of four steps. These include together with decision-making criteria (1) the selection of additives based on their capability to wet juvenile maize leaves, (2) their adhesion on the plant, (3) their interaction with the biological systems, and (4) the choice of thickener for good dispersion stability. In total, 29 additives including polysaccharides and proteins, polyols, glycosides, oils, waxes, and surfactants (e.g., chitosan, gelatin, glycerol, saponin, castor oil, polyethylene, rhamnolipid) were evaluated. Contact angle and spreading index measurements revealed that the use of 5% Geloil, 1% rhamnolipid, or suitable combinations of Geloil + rhamnolipid and Nurture Yield S 2002 + rhamnolipid enhanced wetting of hydrophobic maize leaves and adherence, similarly to the commercial wetting agents recommended for plant protection 1% Prev B2 and 1% Trifolio S Forte. Interaction of additives with biological systems was based on biocompatibility and phytotoxicity assays, and cell viability monitoring using the endophytic Gram-negative bacterium Paraburkholderia phytofirmans PsJN. Results from biocompatibility assays indicated that in contrast to rhamnolipid and Prev B2 Geloil, Nurture Yield S 2002 and Trifolio S Forte fully supported bacterial growth within a concentration range of 1 to 5%. Dose-dependent phytotoxicity was observed in plants treated with rhamnolipid. Most efficient formulation was composed of PsJN, talc, xanthan, and Geloil. Beyond that, the proposed workflow is expected to generally provide guidance for the development of spray formulations and help other researchers to optimize their choices in this area.