Control of citrus blue mold by the antagonist yeast Pichia guilliermondii Z1: Compatibility with commercial fruit waxes and putative mechanisms of action

Pichia guilliermondii strain Z1, which was previously proven to be effective against blue mold of citrus fruit, has been further tested in controlled conditions to determine whether the yeast, as an alternative for synthetic fungicides, would be compatible with other postharvest practices used commercially. In particular, commercial fruit waxes can reduce their survival and effectiveness. The commercial fruit waxes tested, in combination at 20% with strain Z1, included microcrystalline, ester gum, candelilla, beeswax, montan, paraffin, rice bran, rosin maleic, carnauba, shellac, and one mixture shellac plus carnauba. Beeswax, paraffin, rosin maleic, carnauba, and shellac increased significantly strain Z1 survival in Petri dish assays. Candelilla, beeswax, rice bran, rosin maleic, carnauba, shellac, and shellac–carnauba mixture did not significantly reduce the strain Z1 yield on orange fruit surfaces compared to other waxes. With the exception of rosin maleic wax, none of the commercial fruit waxes or mixture increased significantly the ability of the formulated product of strain Z1 to control the postharvest pathogen Penicillium italicum on wounded orange fruit. When the formulated product of strain Z1 was used in combination with beeswax, strain Z1 retained the same efficacy. In contrast, microcrystalline, ester gum, candelilla, montan, paraffin, rice bran, carnauba, shellac, and shellac-carnauba mixture significantly reduced the effectiveness of the formulated product of strain Z1. No antibiosis was detected for strain Z1 against P. italicum. Strain Z1 inhibited the spore germination for the low juice concentration (up to 5%) when compared to the control. However, the addition of fresh juice after antagonist removal allowed the restoration of the germination of P. italicum spores; suggesting the possibility of competition for nutrients in the biocontrol activity of strain Z1. The study demonstrates the potential commercial application of strain Z1 with beeswax and rosin maleic wax for postharvest control of citrus blue mold. As biocontrol relies on competition for nutrients, an enriched formulation with nutrients is needed for reliable antifungal activity of this yeast strain.