1Department of Phytopathology, Wageningen Agricultural University, POB 8025, 6700 EE Wageningen, The Netherlands; 2Applied Research for Arable Farming and Field Production of Vegetables, POB 430, 8200 AK Lelystad, The Netherlands

Background and objectives
Nonchemical methods used to disinfest soils include solarization and flooding. With both methods, an effective control of a number of important soilbome pathogens such as Fusarium oxysporum and Verticillium dahliae can be reached. However, the applicability of solarization is limited to (sub)tropical areas and that of flooding is limited to soils with a suitable hydraulic conductivity. Thus, there is need to develop additional control methods for many agricultural soils in the temperate zone. Here, we report on a new method that is aimed at imposing a general soil anaerobiosis. This strategy was suggested earlier by Menzies [1] but, to our knowledge, has never been tested under field conditions.

Materials and methods
The method involves the incorporation of green plant material (either broccoli or grass, approximately 40 ;t fresh weight/ha), followed by irrigation and application of a plastic tarp with a low permeability for oxygen for 12-15 ;weeks. Plots without plant material and without plastic tarp and plots with incorporation only of green plant material or application of only a plastic tarp served as a control. The effect on survival of pathogens was tested by burying nylon bags with approximately 100 ;ml inoculum at various depths and determining the viability after the treatment. During the experiment, the oxygen concentration of the soil atmosphere, the redox potential and the soil temperature was measured regularly. Field experiments were performed in four consecutive years.

Results and conclusions
Following application of the plastic tarp, the soil rapidly became anaerobic irrespective of the incorporation of plant material. In tarped soil with plant material incorporated the redox potential values dropped gradually to values as low as -200 ;mV at 15 ;cm depth, indicating strongly reducing soil conditions. In tarped soil without plant material the redox potential did not reach these low levels. Soil temperatures did not reach (sub)lethal levels in these experiments.

In tarped soil with plant material survival of F. ;oxysporum f ;sp. asparagi, Rhizoctonia solani and Verticillium dahliae was greatly reduced in all experiments (usually 90-100%). In treatments with organic matter or plastic only, no significant reduction in pathogen survival was found. Pathogen inactivation was similar for broccoli and grass. Pathogen inactivation is suggested to be caused by a direct effect of low oxygen levels in the soil atmosphere, or by products of fermentative bacteria that accumulate temporarily in the anaerobic soil prior to the onset of methane production, or by a combination of both factors.

1. Menzies JD, 1970. In: Toussoun TA, Bega RV, Nelson PE, eds. Root Diseases and Soil-borne Pathogens. Berkeley: University of California Press, pp.16-21.