Suppressive biological factors influence populations of root lesion nematode (Pratylenchus thornei) on wheat in vertosols from the northern grain-growing region of Australia

Soils from the grain-growing region of northern Australia were assessed for suppressiveness to Pratylenchus thornei by comparing nematode multiplication rates on wheat grown in sterilised soil, non-sterilised soil and sterilised soil to which 10% field soil had been added; or by looking for soils in which the nematode failed to multiply under conditions that favoured nematode multiplication. Bioassays with cropped soils indicated that multiplication of P. thornei consistently increased when soil was sterilised by heat or irradiation, with final population densities in sterilised soil typically 22-35 nematodes/g soil compared with only 2-8 nematodes/g in untreated soil. Soils from native vegetation and pasture reacted in much the same way. Results of other bioassays indicated that multiplication of P. thornei was sometimes reduced by transferring field soil to sterilised soil, and that the nematode failed to multiply in soil from some farms, particularly if that soil was collected from the upper 25 cm of the soil profile. These results suggest that biological mechanisms of suppression are operating in many of these soils. However, shoot growth generally improved when soil was sterilised and so it is also possible that increased root biomass or better root health was partly responsible for differences in nematode multiplication rates between sterilised and non-sterilised soil. Alternative ways of assessing suppressiveness and furthering our understanding of the factors that influence population densities of P. thornei are discussed.