Perspectives of Anaerobic Soil Disinfestation

Biological soil disinfestation is an environmentally friendly method to disinfest soil. From now on we refer to it as anaerobic soil disinfestation (ASD). With ASD a green manure crop (40 t/ha) is homogeneously incorporated into the topsoil (0-30 cm) after which the field is lightly compacted and irrigated. Subsequently the field is mechanically covered with air tight virtually impermeable film (VIF) to restrict oxygen supply to the topsoil. The soil is left covered for six weeks in the summer. In this period anaerobic conditions develop rapidly in which toxic fermentation products are formed. These conditions are responsible for elimination of fungal and bacterial pathogens as well as parasitic nematodes, insects and weeds. In the Netherlands since 2004 anaerobic soil disinfestation (ASD) was applied on approximately 70 ha mainly for asparagus and strawberry runners production. When the asparagus crop was replanted ASD proved to be profitable for many years by controlling Fusarium oxysporum. Although highly effective against soilborne pathogens and pests ASD is not applied widely in the Netherlands due to the high costs of about € 4000 per ha. Apart from the costs this technique needs special attention to prevent plastic damage. Moreover the mechanism behind ASD is not well defined which makes recommendations for growers complex. A new and promising development in ASD is the application of defined products on a basis of mixtures of carbon hydrates and proteins. The mechanism of ASD will be studied in the next few years with some of these products by measuring gases and fatty acids and microbial shifts in relation to efficacy. Chemical soil disinfestation with fumigants is restricted more and more and a non-chemical approach of soil disinfestation like ASD is urgently needed. INTRODUCTION Intensive crop rotations have increased the problems with soilborne diseases. Disease control with chemicals like fungicides, fumigants or methyl bromide is increasingly limited by environmental restrictions. For that reason there is much interest in non-chemical alternatives for soil disinfestation with natural products without harmful effects on the environment. Anaerobic soil disinfestation can meet the requirements to control all kinds of pests and diseases in soils in an environmental safe way. Essentially anaerobic soil disinfestation is derived from soil flooding. Under water anaerobic conditions prevail in the soil and many pathogens are killed. Soil flooding however is restricted to flat areas with a low water infiltration rate underneath. By replacing the water by an air tight plastic film anaerobic conditions prevail over the whole soil profile under the film. In this way no rapid recolonization is expected as was the case in the top soil layer after soil flooding (Stover, 1955). Anaerobic soil disinfestation (ASD) with incorporation of large amounts of fresh organic matter in soil followed by irrigation and then sealing off the soil with virtually impermeable film (VIF) is a method developed in the Netherlands. The method is based on the principle that under gastight plastic anaerobic conditions are created which convert organic material into other organic compounds. These compounds are supposed to be lethal to soilborne pathogens and pests at the concentrations realized. Moreover the lack Proc. VII IS on Chem. and Non-Chem. Soil and Substrate Disinfestation Eds.: Gamliel et al. Acta Hort. 883, ISHS 2010 278 of oxygen may support the efficacy (Blok et al., 2000). Also an increase in soil temperature and water content of the soil decrease the survival of for instance Fusarium oxysporum (Stover, 1954). All aspects of anaerobic soil disinfestation with green manure are discussed in this paper. THE METHOD OF ANAEROBIC SOIL DISINFESTATION (ASD) The farmer starts growing a green manure in the field to be disinfested or produces the green manure in another field. The green manure is harvested in summer and incorporated with a rotary spading cultivator in conjunction with a pressure roller into the top soil layer of about 30 cm. The amount of manure in this case should be 40 tonnes/ha. The compacted soil is irrigated with 25-50 mm of water. The next day a contractor spreads out the 3.5 m wide strips of VIF. One side of the film is glued to another film strip already fastened on the soil surface and the other side is dug into the soil with discs. Under anaerobic conditions toxic substances evolve from the manure, killing pathogens, pests and weeds. After four to six weeks the plastic may be removed from the land. The method is adjusted for control of replant disease in asparagus. Then 80 tonnes of green manure per ha are rotary tilled to a depth of 80 cm before plastic mulching. Care must be taken that ASD is applied in a correct way. Of course the plastic should not be blown away when there is much wind. Perforation of the plastic by sharp lumps of soil, shells, straws or stems of woody plants should be prevented by smoothing out the surface. Incorporation of the organic manure should be in an early stage before the stems starts to lignify. Plastic damage by animals can be prevented by fencing of the plot. Birds should be scared off (Molendijk et al., 2008). Also a thicker plastic sheet may be used. Crops to be incorporated may be all kind of fresh material grown on the spot or in other fields providing crops are no host plants for parasitic nematodes. On the contrary when plants are used as catch crops for certain nematode species two modes of action are combined. This is for instance the case with Tagetes patula which eliminates Pratylenchus species or with some glucosinolate containing Brassica crops resistant against beet cyst nematodes. In the latter case biofumigation is combined with ASD. Broccoli plants (Blok et al., 2000) or white mustard (Thanning and Gerhardson, 2001) however did not increase pathogen control. When rapidly fermentable material is replaced by slow deteriorating material like ripe composts then unsatisfactory results are obtained. Initially plastic sheets were ensilage films of 0.12 mm thick PE. The oxygen permeability was about 1500 ml/m/day/bar. The use of three layer VIF plastic sheets of 0.035 mm thickness reduced the oxygen permeability to 240 ml/m/day/bar and decreased survival of V. dahliae and F. oxysporum in three experiments (Lamers et al., 2004). THE ANAEROBIC PROCESS When VIF has been laid over the field the oxygen content of the soil decreases rapidly under 1% within a few hours and remains that low for seven weeks (Termorshuizen et al., 2000). The redox potential (Eh) of the soil decreases to low levels indicating high reducing soil conditions (Blok et al., 2000). First the oxygen in the soil is reduced, then with lowering of the redox potential nitrate and Mn are reduced, followed by Fe and sulfate. When the redox potential is less than -200 mV even methane gas can be formed after reduction of carbohydrates or CO2 (Reddy, 1987). Under anaerobic conditions also other gases like ammonia, hydrogen sulfide and nitrous oxide are created. Other fermentation products produced are fatty acids like (iso)butyric acid, maleic acid, (iso)valeric acid, acetic acid, lactic acid, citric acid and propionic acid. Ammonia is known to be lethal to root lesion nematode Pratylenchus penetrans (Min et al., 2007). Nematicidal effects of fatty acids have been demonstrated recently by several research groups (Abdel-Rahman et al., 2008; Mahran et al., 2008; Xiao et al., 2008). The concept of biological soil disinfestation is also applicable to other disinfestation methods like biofumigation, solarization and flooding. Therefore in future