Suppression of soilborne pathogens in mixed cropping systems

Since the green revolution, agricultural production has increased tremendously due to synthetic fertilizers, chemical crop protectants and high yielding plant varieties. However, soilborne pathogens remain yield-limiting factors in agricultural production. Hardly any sustainable solutions are available to control soilborne diseases and effective chemicals are limited and often have unwanted environmental side effects. Therefore, more sustainable ways of cultivating crops and agricultural production are needed. The general goal of this thesis research was to investigate the effect of mixed cropping on soilborne diseases in organically managed soils and to unravel the mechanism(s) involved. First, the disease suppressiveness of organically managed soils against the soilborne fungal pathogen Gaeumannomyces graminis var. tritici and the role of antagonistic Pseudomonas bacteria were investigated. Take-all disease severity was lower in organically compared to conventionally managed soils. Naturally occurring populations of 2,4-diacetylphloroglucinol producing pseudomonads were lower in organically managed soils. The initial establishment of an introduced fluorescent pseudomonad was hampered in these soils and its biocontrol efficacy was lower in the organically than in the conventionally managed soils, indicating that these pseudomonads do not play an important role in disease suppression in organically managed soils. Rather, microbial activity was likely important for disease suppression in the organically managed sandy soils. Disease levels are often lower in mixed crops compared to single crops. In this thesis we present an overview of soilborne pathogens in mixed crops and discuss the disease suppressive mechanisms that are involved. Furthermore, we studied the effects of mixed cropping (Brussels sprouts-barley and triticale-white clover) in organically managed soils on suppressiveness against three soilborne pathogens. Mixed cropping did not result in enhanced disease suppressiveness compared to the single-cropped soils. Also soil microbial community structure and activity in mixed cropped soils were indistinguishable from single-cropped soils. Finally we investigated the effects of mixed cropping triticale-clover on the development of Take-all disease under field and greenhouse conditions and the mechanisms involved. Under field conditions, Take-all disease severity increased for both single- and mix-cropped triticale over three consecutive years. Disease increase tended to be lower in the mixed- than in the single-cropped triticale. In the fourth year this trend was however reversed, probably due to bad clover establishment. In the greenhouse, Take-all disease was lower in the mix- than in the single-cropped triticale during five consecutive cycles. In both the field and greenhouse experiments clover biomass was negatively correlated with Take-all disease severity. Potential disease suppressive mechanisms stimulated by the clover biomass could be a change in triticale root architecture, host dilution, enhanced microbial activity, stimulation of antagonists in the rhizosphere and possibly increased ammonium concentrations in the rhizosphere. We conclude that effects of mixed cropping systems on soilborne pathogens depend more on the crops or cultivars growing at that moment than on the mixed crops grown previously. Disease suppression in mixed crops is therefore a plant-driven process and effects are mainly observed in the rhizosphere and not in the bulk soil. Root architecture might have a large influence on the interactions that can take place in mixed cropping and the selection of the crops or cultivars to be included in the mixture should be made with great care to have significant effects on soilborne plant pathogens