Effect of neem cake soil amendment on reduction of damping-off severity and population densities of plant-parasitic nematodes and soilborne plant pathogens

The effects of neem cake, a nutrient-rich organic material derived from neem seed, on plant-parasitic nematodes, Verticillium dahliae, and seedling damping-off diseases caused by Rhizoctonia solani and Pythium aphanidermatum were investigated. In greenhouse trials, 1% neem cake (mass/mass soil) caused a 67%–90% reduction in the number of lesion (Pratylenchus penetrans) and root-knot (Meloidogyne hapla) nematodes in tomato roots grown in three different soils. In the field, 1% neem cake (mass/mass soil) reduced the number of lesion nematodes by 23% in corn roots and 70% in soil around roots. Population densities of free-living nematodes were either enhanced or not affected by neem cake treatment. In laboratory tests, addition of 3% neem cake (mass/mass soil) to soil killed V. dahliae microsclerotia and increased soil pH from 5.2 to 8.7. Killing of microsclerotia appeared to be caused by generation of ammonia during decomposition of neem cake. In growth room assays, addition of 2% neem cake (mass/mass peat-based mix) to R. solani -infested peat-based mix 28 days before planting radishes reduced damping-off severity. In a sandy loam soil artificially infested with R. solani and a muck soil naturally infested with damping-off pathogens, addition of 0.5% neem cake (mass/mass soil) had no immediate effect on damping-off, whereas incubation of the amended soil for 7 days before planting radish or cucumber reduced damping-off severity. This suggested that neem cake was not directly toxic to the damping-off pathogens but that during incubation neem cake may have created a biological climate that was suppressive to disease.