Root lesion nematode numbers in soil and wheat roots were evaluated on the sixth year of a crop rotation and tillage management study in Oregon, USA. Seven treatments were established in 1993 and culminated with all plots planted with winter wheat in 1999. Treatments comprised: (1) two-year rotation of winter wheat and high-residue fallow, using a disc in autumn following harvest and a chisel plough to prepare fallow in spring; (2) two-year rotation of winter wheat and high-residue fallow, using a chemical fallow in autumn following harvest and chisel plough in standing stubble; (3) three-year rotation of winter wheat, spring barley and fallow with tillage as in treatment 1; (4) three-year rotation of winter wheat, spring barley and fallow with chemical fallow as in treatment 2; (5) three-year rotation of rape, winter wheat and fallow with tillage as in treatment 1; (6) two-year rotation of winter wheat and low-residue fallow using a mouldboard plough during spring (current conventional practice with wheat stubble standing through winter following harvest); and (7) continuous no-till spring wheat for five years and winter wheat during 1998-99. Pratylenchus neglectus was the dominant soil lesion nematode and the only species obtained from the roots. P. thornei occurred in soil of some treatments but its ratios were not determined. The highest numbers of lesion nematodes and lowest grain yields occurred in treatments where wheat followed another crop rather than fallow (e.g. annual wheat and the 3-year rotation of rape, winter wheat and fallow). Yield was inversely associated with lesion nematode numbers in roots and soil. There were no relationships among stunt or nonparasitic nematodes and crop history or wheat grain yield.
