Effects of biopesticides on the progression of Olive Quick Decline Symptoms caused by <i>Xylella fastidiosa</i> subsp. <i>pauca</i>

Xylella fastidiosa subsp. pauca (Xfp) is the causal agent of olive quick decline syndrome (OQDS), a devastating disease threatening olive groves in Apulia, Italy. Efforts are underway to identify effective strategies to limit its spread. In this study, two biopesticide formulations, one derived from an onion extract rich in organosulfur compounds and the other based on the bacterium Paraburkholderia phytofirmans strain PsJN, were tested under field conditions. Treatments were applied eight times per year over a four-year period to assess their potential to reduce Xylella populations and disease progression. The results indicated that neither formulation lowered Xylella populations. Nonetheless, after four years of experimentation, the formulated PsJN treatment reduced both newly developed and pre-existing Xfp symptoms when applications started at the early stages of infection. In contrast, no effect was observed when treatments were applied to trees already exhibiting widespread infection and severe symptoms. Physiological analyses of stomatal conductance revealed that both treatments alleviated the drought stress associated with Xfp infection, with the formulated PsJN treatment showing statistically significant improvements. Consistently, treated olive trees exhibited higher stomatal conductance and lower canopy temperatures compared with untreated controls. Based on these encouraging results, further studies are needed to explore the use of these products under different conditions and application timings, as well as in combination with other treatments, to support the development of an integrated management strategy for controlling infections.