Influence of preharvest polyoxin-D applications with or without spray oils on brown rot development and tree health of peach

Polyoxin-D is a microbial fermentation product registered to control multiple crop diseases. However, there is limited knowledge of the effect of polyoxin-D against peach brown rot or potential efficacy improvements through combinations with essential or mineral oils. In this study, we evaluated the efficacy of polyoxin-D with or without thyme oil or mineral oil to control brown rot on peach caused by Monilinia frucicola in a multiyear study. Treatments were applied preharvest and included OSO 5% (polyoxin-D), OSO 5% + ThymeGuard 0.25% (polyoxin-D + thyme oil), and OSO 5% + PureSpray Green 0.5% (polyoxin-D + mineral oil). Assessment of preharvest brown rot revealed that the OSO 5% treatment had less disease incidence (9.4% on average) compared to the untreated control (20.0% on average), but no significant differences were found between OSO 5% + PureSpray Green (11.8% on average) and OSO 5% + ThymeGuard (11.4% on average). All three OSO 5% treatments reduced postharvest brown rot compared to the untreated control, but there were no significant differences in disease incidence between experimental treatments. Phytotoxicity was observed in the form of premature leaf drop. The greatest leaf drop was recorded for OSO 5% + PureSpray Green with 36.5% blind nodes on average. In summary, polyoxin D (OSO 5%), when applied by handgun to runoff using 1870 L/ha water volume, significantly reduced preharvest and postharvest brown rot disease but caused some premature leaf drop. The addition of thyme oil or mineral oil did not improve its efficacy.