Phytoparasitic nematodes cause heavy annual yield losses and thus can be considered among the most dangerous pests for a number of vegetable crops. Management of these parasites has been traditionally based on synthetic pesticides, but because of their high environmental impact we need to find more sustainable products to control nematodes. Biocidal compounds, naturally occurring as products of plant secondary metabolism, may represent a large source of biocompatible nematicides. This review briefly reports the main botanical groups and plant secondary metabolites with a nematicidal potential, as well as the most effective techniques for their exploitation. Nematotoxic volatile products, such as isothiocyanates and nitriles released from the degradation of glucosinolates present in tissues of Brassicaceae plants or cyanide deriving from the cyanogenic glucosides of some Sorghum species can be exploited for nematode control through the incorporation of fresh or dry plant material into the soil. Commercial liquid formulations based on seed oils from neem (Azadirachta indica Juss.) or sesame (Sesamum indicum L.) or water extracts from quillay (Quillaja saponaria Molina) bark or tagetes (Tagetes erecta L.) are already available for soil treatments over the crop cycle and/or seedling root dipping before transplant . Many plant species from Asteraceae family, such as Calendula spp., Chrysanthemum spp. and Artemisia spp., have been also demonstrated to be a potential source of nematicidal products for the control of root-knot and cyst phytonematodes economically relevant to vegetable crops. Green manures or crop rotations with indigenous (Medicago spp., Trifolium spp., Vicia spp., Lupinus albus) and tropical (Mucuna spp., Crotalaria spp.) Leguminosae species, as well as soil amendments with their formulated plant biomass, have been reported to achieve a satisfactory nematode suppression, due to the high content of bioactive metabolites (phenols, alkaloids, alcohols and mainly saponins) of these plants. Essential oils from a wide range of aromatic and medicinal plants and their volatile bioactive components (terpenes, aldheydes, ketones and more) have been largely acknowledged for a high nematicidal activity, both in vitro studies and as soil treatments in water solution or by fumigation. Full exploitation of the high nematicidal potential of these products is strictly related to the development of technical formulations suitable to delay the release and degradation of active components into the soil. Phenolic compounds, such as terpenoids, flavonoids, tannins and coumarins, can also play a relevant role in the formulation of innovative nematicidal products. Commercial formulations of chestnut tannins demonstrated to be highly suppressive to root-knot nematode infestation. Nematicidal effect were reported also for soils amendments with tannin-rich plant materials. Coumarins are among the main agents of the strong nematicidal activity reported for plants from genus Ruta, such as R. graveolens and R. chalepensis. Susceptible of exploitation could be also the nematicidal properties reported for the pyrrolizidine and steroidal alkaloids from many Asteraceae, Leguminosae and Solanaceae species. Review of literature data confirmed the effectiveness of plant products for a sustainable phytonematode management, also due to the low environmental persistance and toxicity to humans, animal and plants of their active compounds. Multicomponent activity of most plant products also avoids the raising of nematode resistance, as frequently reported for repeated treatments with synthetic nematicides.
Keywords: Phytoparasitic nematodes, control, sustainability, biocidal plants