Dose-Dependent Effects of Antioxidants on Root-Knot Nematode Infection in Vegetable Crops and Dosage Standardization for Preventive Measures

Different antioxidant compounds (ACs) were applied to vegetable plants as foliar spray or soil drench before inoculation with root-knot nematodes (RKNs). Different doses of salicylic acid (SA), methyl-salicylate (MetSA), methyl jasmonate (MetJA), and ascorbate (ASC) were tested; doses were chosen according to the size and weights of the plants to be treated. Generally, low doses of ACs increased nematode infection; conversely, when doses were raised, ACs acted as effective resistance inducers and reduced infection, measured as numbers of individuals developed in roots and reproduction rates. The activation of defense often occurred at the expense of plant fitness, although in infected plants, the benefits of treatments on plant growth were caused by relief from the symptoms monitored in untreated plants. Single pre-treatments of SA, MetSA, and ASC, in the proper amounts, almost halved infection variables; repeated applications of SA during nematode pathogenesis annulled the effectiveness of single pre-treatments. MetJA application was generally toxic to plants, and, also when provided in minimal amounts, this compound was always ineffective against nematodes. Other phenols and phenolic acids were tested at the same doses that were effective for SA with no reduction of infection except for duroquinone, which caused almost total suppression of infection, although associated with a decrease in root growth.