Meetings of The Royal Botanical Society of The Netherlands

A new non-chemical method for the control of soilbome pathogens is being developed. This method involves the incorporation of green plant material into moist soil, directly followed by the application of a plastic tarp with a low permeability for oxygen for 12-15 weeks in summer and autumn. Results of field experiments in 4 consecutive years show that in this manner a strong reduction in the inoculum density of Fusarium oxysporum f. sp. asparagi and Verticillium dahliae (usually >90%) is achieved. No significant reduction was observed in treatments with organic material or plastic only. Following application of the plastic, the treated soil became anoxic rapidly and redox potential dropped gradually to values as low as —200 mV at a depth of 15 cm. Products of fermentative bacteria (e.g. organic acids, aldehydes, alcohols) that accumulate temporarily in the anoxic soil layer prior to the onset of methanogenesis are suggested to be responsible for the inactivation. These products can cause inactivation of fungal resting structures either by acting as a toxicant or by inducing germination followed by lysis. A simulation model was developed to describe organic matter decomposition and transport processes in treated soil. With this model it can be shown that key factors controlling the concentrations of fermentation products in treated soil are (1) the amount and quality of the organic matter; (2) the amount of available inorganic electron acceptors, such as oxygen, nitrate and reducible Fe oxides; (3) the initial biomass of methanogenic bacteria in the soil; and (4) soil temperature. Further research aims at obtaining more insight in the mechanism and dynamics of inactivation during the treatment since this will allow an efficient optimization of the control method.