Mixed Cropping and Suppression of Soilborne Diseases

Abstract

Soilborne pathogens are difficult to manage, especially since the use of methyl bromide has been phased out in most countries. Resistance against many soilborne pathogens is hardly available and fungicides are effective only to a limited extent. In organic agriculture, many problems related to soilborne pathogens are avoided by applying wide rotations, but still some polyphagous soilborne pathogens can be highly problematic, especially since most chemical crop protectants are not allowed. In addition, wide rotations are often economically unprofitable. Therefore, alternative practices to manage soilborne pathogens are needed. In this review, the occurrence of soilborne pathogens in three types of cropping systems are evaluated: (i) continuous cultivation of single crops in monoculture, (ii) crop rotation, and (iii) mixed cropping, i.e., cultivation of multiple crops in the same field at the same time. Both continuous cropping and crop rotation have been investigated extensively. Therefore, in this chapter we focus on mixed-cropping systems in relation to soilborne pathogens, their potential to suppress soilborne diseases, and the mechanisms underlying disease suppression. In general, mixed cropping is practiced to optimize nutrient uptake, control soil erosion, suppress the epidemic spread of airborne pathogens, and improve crop yields per unit of area. While mixed cropping has received attention for its effects on airborne pests and pathogens, the effects on soilborne pathogens are poorly known. In 30 out of 36 publications, mixed cropping showed a significant reduction in soilborne disease and in six, no or a positive effect on disease incidence or severity was found. Diseases caused by splash-dispersed pathogens were less severe in mixed-cropping systems in ten out of 15 studies. The magnitude of disease reduction in mixed compared to single crops varied, from a 63% reduction to a 100% increase in disease. Host dilution appeared to be the most important mechanism of disease suppression for both soilborne and splash-dispersed pathogens (12 and five cases, respectively). Although the use of mixed cropping for soilborne disease suppression is still in its infancy, the wide range of biological effects and interactions observed holds promise for further optimization and management of soilborne diseases, for example, by selecting plant species and cultivars that provide an optimal combination of root architectures.