Allelopathy: a possible mechanism of suppression of plant-parasitic nematodes by entomopathogenic nematodes

A possible mechanism of suppression of a plant-parasitic nematode Meloidogy ne incognita by entomopathogenic nematodes is described. Heat-killed entomo pathogenic nematodes Steinernema feltiae and S. riobrave temporarily suppre ssed penetration of the rest-knot nematode M. incognita into tomato roots, but live nematodes had no effect. Infective juvenile M. incognita were repe lled from all entomopathogenic nematode treatments that included their symb iotic bacteria. They were repelled by Gulleria mellonella cadavers infected with S, carpocapsae, S. feltiae, and S. riobrave and from cell-free cultur e filtrates of the symbiotic bacteria Xenorhabdus nematophilus, X. bovienii , and Xenorhabdus sp. "R" from the three nematode species, respectively. Ce ll-free filtrates from all three Xenorhabdus spp. were toxic to M. incognit a infective juveniles causing 95-100% mortality at 15% concentration. Cell- free filtrate of Xenorhabdus sp. "R" also reduced the hatch of M incognita eggs. Application of formulated bacterial cell-free filtrates temporarily s uppressed M, incognita penetration into tomato roots in a greenhouse trial. The short-term effects of cell-free bacterial filtrates, namely toxicity a nd repellency, were almost entirely due to ammonium. These results demonstr ate allelopathic interactions between plant-parasitic nematodes, entomopath ogenic nematodes and their symbiotic bacteria. The likely role of allelopat hy in the suppression of plant-parasitic nematodes by innundative applicati ons of entomopathogenic nematodes is discussed.