1Laboratory for Pest Management Research, and 2Department of Plant Pathology, ARO, Volcani Center, Bet Dagan 50250, Israel; 3Hevel Maon, Israel; 4Extension Service, Ministry of Agriculture, Israel

Background and obiectives
Soil fumigation is a non-selective control method to eliminate soilborne pathogens and other soil organisms in all soil profiles. Chemical fumigants were found to possess limiting negative attributes, such as health hazards and environmental pollution. With increasing regulatory restrictions on chemical soil fumigation, e.g. methyl bromide, it is necessary to find and develop effective non-chemical strategies for combating soilborne pests and diseases. Combining organic amendments with soil solarization is a different, non-chemical approach to improve the control of soilborne pests [1]. Organic amendments such as plant residues and organic wastes (poultry manure, feather meal) decompose in soil during heating, resulting in the generation of toxic, volatile compounds. Pathogen can be effectively controlled in solarized-amended soil by the combination of thermal killing, accumulation of toxic volatile compounds, and enhanced antagonistic microbial activity. The objectives of this study were to identify toxic compounds generated from decomposing soyabean meal and from plant residues (e.g. Artemisia and Salvia residues) during soil heating and solarization, and to study the role of volatile generation in pathogen control.

Results and conclusions
Significant qualitative and quantitative differences in volatile compounds from heated and non-heated soil amended with plant residues (Artemisia or Salvia) or soya meal were evident in both controlled-environment studies. Concentrations of volatile compounds were higher in heated soil amended with plant residues than from the corresponding non-heated soil. Concentration of volatile compounds evolved was directly related to increased soil heating. High temperatures significantly increased the vapour pressure of compounds present in the liquid or solid soil fractions, resulting in greater release to the soil atmosphere. Under high soil temperature during solarization, the process of material breakdown is faster and the time required for effective pathogen control is shorter. Other mechanisms, such as heat-induced breakdown of more complex compounds and heat-induced release of polar molecules from clay particles, may also be involved. Numbers of propagules of Fusarium oxysporum f.sp. basilici were reduced by more than 95% when the propagules were exposed to volatile compounds from heated, amended soil. In contrast, volatile compounds from non-heated, amended soil had only a weak killing effect on these fungal propagules after 4 weeks of incubation. Microbial activity in heated or solarized soil amended with plant residues increased after the first 2 weeks of incubation or solarization as compared to non-amended soil. Apparently microbial activity is involved in pathogen control during solarization of amended soil.

Viability of fungal propagules was completely eliminated in solarized amended soil to a depth of 50 cm in field experiments. Crown rot of tomatoes and Verticillium wilt of potatoes were effectively controlled by a combination of solarization of soil amended with poultry manure or soya meal. A long-term effect of solarization treatment was evident on Verticillium wilt of potato for three crops after solarization treatment.

1. Gamliel A, Stapleton JJ, 1997. Phytoparasitica 23, 87-92.