2.10.1
EVIDENCE FOR THE BIOLOGICAL NATURE OF SOIL SUPPRESSIVENESS AGAINST HETERODERA SCHACHTII

A WESTPHAL AND JO BECKER

Department of Nematology, University of California, Riverside, California 92521, USA

Background and objectives
A field at the Agricultural Field Station at the University of California, Riverside, supported only a low population of Heterodera schachtii on continuously replanted host crops of the nematode. Initially, a high population causing plant damage had been observed after infestation of the field with the sugar beet cyst nematode in 1975. The aim of this study was to determine the nature of the suppressiveness.

Materials and methods
Samples of the nematode-suppressive Hanford fine sandy loam soil were collected at a depth of 0-10 ;cm for the use in greenhouse experiments. In the first experiment the soil was treated with one of the following: metam sodium, methyl bromide, methyl iodide, formaldehyde or pasteurized with aerated steam (87.5C for 30 ;min) or not treated. In a second experiment, suppressive soil was heat-treated in a water bath at 45, 50, 55, 60 or 65C. The heat-treated soils and untreated control soil were mixed 1:9 with fumigated field soil. Both experiments were planted with Swiss chard, Beta vulgaris, and inoculated with second-stage juveniles of H.nbsp;schachtii. After 2 ;months the experiments were harvested and nematode populations determined.

Results and conclusions
At harvest of the first experiment the nematode populations were significantly higher than in the untreated control after each of the biocidal treatments and the pasteurization. In the heat-treatment experiment the nematode multiplication in the 45 or 50C treatment did not differ from the untreated control. However, treatments of 55C or higher permitted a greatly increased nematode multiplication. There was no significant additional increase in nematode multiplication after higher temperature treatments. Both trials provided evidence for the biological nature of the suppressiveness. The heat treatment at 55C eliminated the biological suppressiveness.