Effects of Two-Dried Castor Leaf Formulations on the Population Dynamics and Pathological Activities of Root-Lesion and Root-Knot Nematodes on Water Yam

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The pathological activities of plant-parasitic nematodes affect the marketable value of yam tubers. This study intends to evaluate the nematicidal effects of dried castor leaf powder and dried castor leaf extract on root-lesion and root-knot nematodes under field conditions. Trials were conducted in the Autonomous District of Yamoussoukro in Côte d’Ivoire during the 2015 and 2016 growing seasons. Before yam seed planting, dried castor leaf powder, dried castor leaf extract (suburbs of Abidjan), and carbofuran (Callivoire S.A., Abidjan) were applied at the top of mounds in a randomized complete block design with three replicates. Symptom prevalence, symptom severity index, nematode numbers, and yam yield were determined depending on the treatments. The gall (28.1%) and dry rot (48.9%) prevalences on yam tubers from untreated soils were higher than those from product-incorporated soils (1.11–12.1%) (P < 0.05). The dry rot prevalence (2.78%) was only lower on yam tubers from soil incorporated with dried castor leaf powder compared to that (10.1%) obtained with dried castor leaf extract (P = 0.000). The nematode numbers were lower in the product-incorporated soils and treated yam plants (3–105 individuals) than in the untreated soils and untreated yam plants (37–423 individuals) (P = 0.000). The nematode numbers in the soil, root, and tuber samples were statistically similar regardless of the dried castor leaf formulations. The highest yields (10.1 to 10.48 t/ha) were obtained from yam plants on soils that were incorporated with dried castor leaf powder and carbofuran (P = 0.000). Dried castor leaf powder is an excellent tool to manage water yam nematodes.