Response of vegetative growth and grain yield of wheat Triticum aestivum cv. Condor to the control of cereal cyst nematode Heterodera avenae by nematicides applied with the seed, in the drill row was assessed in twenty field trials. These trials were conducted in 1978 on three soil types near Coonalpyn, South Australia. Aldicarb was used at all sites and fosthietan and terbufos at four sites. Significant grain yield increases to aldicarb were obtained at 12 sites while yields were increased by the three nematicides at three sites. Numbers of eggs of H. avenae were determined in soil taken in January 1978, and these counts showed that all sites were infested over the range 0.03-8.5 eggs/g soil. Plant assays of the soils assessed the reduction in the length of seminal root axes (range 0-45%) and the severity of the root knotting caused by H. avenae. The egg densities in the soil, reduction in the length of the seminal root axes and disease ratings in the plant assay were highly correlated with each other (r = 0.75; P< 0.001 to 0.91, P< 0.001). These variables were not significantly correlated with grain yield increase due to aldicarb on the two major wheat soils studied, although a correlation, explaining 32-42% of the increase, existed when all sites were considered. A mathematical model based on cropping history and an estimated annual hatch of eggs of H. avenae failed to show a relation between these variables and the yield increase from nematicide. H. avenae caused severe disease and yield loss on calcareous loams and red duplex soils but had only minimal effects at the sites on siliceous sands.

Significant yield responses of up to 0.9 t/ha were obtained with the nematicides aldicarb and dibromochloropropane in seven of eleven field trials with the wheat variety, Condor. Both nematicides reduced the numbers of white cysts of Heterodera avenae on the roots of wheat. With aldicarb the increase in wheat yields varied directly as the decrease in white cysts: dibromochloropropane gave similar increases in yield as aldicarb with a greater reduction in cyst numbers. There was no yield increase with either nematicide when cereal cyst nematode was not present. An analysis of covariance indicated that over all the sites 64% of the increase in yield due to aldicarb could be explained in terms of cysts of cereal cyst nematode.