Influence of Soil Microorganisms On the Reproduction of Helicotylenchus Dihystera and Its Pathogenicity To Millet

Soils were collected from 3 agro-ecosystems in Senegal: a millet cultivated field, a one-year-old fallow field and a 17-year-old fallow field. Microbial biomass was higher in the old fallow soil than in the two other soils; bacteria and fungi were more abundant in the old fallow soil, particularly fungi belonging to the genus Mucor. The soils were autoclaved and inoculated with their own soil filtrates, either sterilised or unsterilised. Millet seedlings were planted in the different soils and Helicotylenchus dihystera was added. Irrespective of the microflora treatment, the reproductive index of H. dihystera was similar in the cultivated and the one-year-old fallow soils (Pf/Pi = 20 for an inoculum of about 800 H. dihystera per dm 3 of soil) and the nematodes had no measurable effect on plant dry weight. The native microorganisms had no effect on the reproductive index of H. dihystera. Reproduction in these sandy loam soils was greater than in the old fallow soil which was a loam (Pf/Pi = 6 for an inoculum of about 800 H. dihystera per dm 3 of soil). Soil texture may explain the difference. In the loam soil of the old fallow, the reproductive index of the plant-parasitic nematodes was doubled (Pf/Pi = 8 versus 4) in the presence of the native soil microorganisms. In this treatment, the dry weight of millet was 15% less than that in the nematode-free control. The pathogenicity of H. dihystera increased in soil with a high microbial status and high population of saprophytic fungi.