Soil-applied Photorhabdus-derived secondary metabolites reduce root infection of Meloidogyne incognita in cotton

Root-knot nematode (RKN) Meloidogyne species cause significant economic damage to crops worldwide, spurring demand for safe, affordable, and sustainable nematicides. Previously, our group identified secondary metabolites (SMs) from an Arizona-native entomopathogenic nematode symbiont (Photorhabdus species) that have successfully controlled RKN both in vitro and in planta. However, the movement and efficacy of Photorhabdus SMs in the soil column remained unknown. In this study, we investigated soil-drenched nematicidal Photorhabdus-derived SMs against RKN to elucidate the movement, bioavailability, and stability over time of SMs for effective RKN management. We tested trans-cinnamic acid (TCA) and (4E)-5-phenylpent-4-enoic acid (PPA), applied to 20-cm long soil columns per SM (1st experiment). Only TCA was applied to the soil columns 10 days or one day before RKN inoculation (2nd experiment). We estimated the bioavailability of soil-drenched SMs by inoculating each layer of the soil column with RKN Meloidogyne incognita in pots, seeding cotton seeds, and then assessing root infection and plant growth. Results showed that SMs applied to the soil columns moved with water to the bottom. RKN root infection was significantly reduced by both SMs across all soil depths. A similar trend was observed in the 2nd experiment. The nematicidal potencies didn’t vary within 10 days. In both experiments, no phytotoxic effects were perceived in cotton plants grown in SM-treated soils during weekly measurements or by six weeks post-treatment. Our results suggest that a single soil SM application will effectively reduce RKN damage without impairing plant growth or inducing phytotoxic effects.