Disease suppression in intercropping systems: a mechanistic perspective

The mechanisation, specialisation and intensification of agriculture have resulted in highly efficient and productive agricultural systems. However, this mode of agriculture also comes at a cost; it contributes to numerous forms of environmental degradation, biodiversity loss, and land and water pollution. Monocultures - (large-scale) fields where a single crop type is grown - are a key element of such agricultural systems, and relatively vulnerable to pests and diseases. Intercropping, the simultaneous cultivation of multiple crop species within the same field, offers a promising strategy to enhance agroecosystem diversity and suppress plant diseases. Although numerous studies demonstrate disease-suppressive effects of intercropping, the relative importance of underlying mechanisms and the influence of management practices, crop traits, and pathogen characteristics remain poorly understood, especially in strip cropping systems compatible with modern machinery.

This thesis investigated the drivers and mechanisms of disease suppression in intercropping using a two-fold approach. First, a meta-analysis across diverse crop–pathogen systems assessed general patterns of disease suppression and factors influencing them. Second, detailed field experiments and modelling focused on potato late blight, caused by Phytophthora infestans, within a strip cropping context.

The meta-analysis demonstrated that intercropping consistently reduced plant disease compared with monocropping by lowering both initial disease incidence and subsequent infection rates. These effects were robust across crop combinations and planting patterns. Species that were taller than the focal crop significantly suppressed vectored plant pathogens in the focal crop more than companions of similar or shorter height, suggesting a potentially important role of the barrier effect. However, high variability among systems and experimental designs limited the ability to disentangle specific causal mechanisms.

To examine mechanisms in detail, three years of field experiments in the Netherlands evaluated potato strip cropping with companion crops differing in stature. Strip cropping with grass or maize significantly reduced late blight severity relative to monoculture, whereas faba bean had no significant effect. Measurements of microclimate, spore dispersal, and host resistance revealed that disease suppression arose from multiple interacting mechanisms. Grass reduced relative humidity within potato strips, while maize primarily acted as a barrier to spore dispersal but could also increase humidity later in the season, creating time-dependent trade-offs.

A simulation model integrating host dilution, microclimate modification, and barrier effects showed that no single mechanism fully explained disease suppression. Instead, combined effects best predicted observed disease dynamics, with mechanisms varying in strength and occasionally counteracting one another while remaining overall suppressive.

In the final chapter, I discuss the usefulness of meta-analysis for studying disease suppression, discuss challenges in designing and implementing strip-cropping systems in conventional agriculture, and consider their potential to reduce chemical inputs. Lastly, I stress the importance of looking beyond optimisation and (short-term) techno-fixes, advocating instead for a radical transformation of the food system and a change in power structures to actually address the negative side effects of modern agriculture.