The effect of the synthetic plant-growth substance, 2, 4-dichlorophenoxyacetic acid, on the host–parasite relationships of some plant-parasitic nematodes in monoxenic callus culture

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Abstract
Some callus tissues induced by 2,4–D and derived from plants resistant to plant-parasitic nematodes lost their resistance to the nematodes. Red clover callus supported a large population of A. ritzemabosi, whereas red clover seedlings did not. Six races of D. dipsaci were cultured on lucerne and red clover callus tissues and reproduced rapidly, although races usually multiplied more on callus from susceptible than from resistant plants. A. ritzemabosi, normally only a foliage parasite, reproduced equally well in stem and root callus. H. rostochiensis did not reproduce in callus culture.Nematodes multiplied most in callus that grew fastest; both reproduction of A. ritzemabosi and the growth of callus were greatest with 0.125 mg/1 of 2,4–D. Reproduction was inhibited by 5.0 mg/100 ml of 2,4–D. 2,4–D influenced nematode reproduction indirectly by making callus, which provides a better environment for nematode feeding and reproduction.Nematode extracts added to an agar medium caused callus formation on red clover seedlings and nematodes feeding on these tissues reproduced faster than on normal seedlings. Hence, the substances secreted by a nematode into a plant may act on the tissues in a manner similar to 2,4–D. The host–parasite relationship is probably partially controlled by the host's plant-growth substances and the effect on them of the nematode's secretions.We thank Mr C. T. Drakes for assistance, and Mr K. Smith of Imperial Chemical Industries for advice with the initial callus culture.