Behavioural differences of Heterodera glycines and Meloidogyne incognita infective juveniles exposed to root extracts in vitro

In vitro behaviour of infective second-stage juveniles (J2) of Heterodera glycines and Meloidogyne incognita was compared in the presence and absence of plant root extracts. In an agar plate attraction-retention assay, with samples applied by agar disc infused with water (control) or aqueous test solutions, H. glycines was 15-fold more responsive to a chemical attractant (CaCl2) than was M. incognita. Control discs retained H. glycines at a rate 2.9-fold greater than M. incognita. Crude extracts (slurries; 40 mg dry root (ml water)(-1)) from roots of six plant species (corn, Zea mays; cucumber, Cucumis sativus; marigold, Tagetes patula; mustard, Sinapis alba; pepper, Capsicum annuum; soybean, Glycine max) differentially affected the two nematodes. Cucumber, marigold, pepper and soybean each attracted H. glycines at rates between 2.2-and 3.6-fold greater than controls. No root preparations were attractive to M. incognita, which were significantly repelled by corn, cucumber, mustard and pepper, relative to controls. Preparation of selected root extract supernatants, which involved vacuum drying, decreased the attractiveness of marigold and soybean to H. glycines by 38 and 82%, respectively, but the effect of pepper was unchanged. Supernatant processing had no effect on M. incognita behaviour. In a liquid-based J2 movement assay, root supernatants from marigold, pepper and soybean at 1 mg dry root ml(-1) each decreased the frequency of head movement in H. glycines and M. incognita relative to controls. However, dose responses were detected only with marigold, with maximum decreases in activity at 16 mg dry root ml(-1) for each species. These decreases were significantly different at 46 and 66%, respectively, for H. glycines and M. incognita. The behaviour of the two nematodes was qualitatively different in assays that required detection of signals across a short distance (agar assay), whereas qualitative responses were similar when juveniles were immersed in treatment solution (liquid assay). In the latter, quantitative responses to marigold differed significantly between H. glycines and M. incognita J2.