The Effect of Plant Host Density on Disease Incidence-A Meta-Analysis.

Plant spacing plays a crucial role in determining the degree of interaction between them. It changes plant physiological processes, plant architecture, microclimate in the canopy and the dispersal and growth of pests and pathogens. This study aims to quantify the relationship between planting density and disease incidence across studies. A meta-analysis was conducted to analyse the relationship between disease incidence and planting density, synthesising 44 publications consisting of 191 experiments, with in total 531 data disease incidence observations, based on > 500 000 sampling units. Four statistical models reflecting different hypotheses on how planting density affects disease incidence were developed and tested in a Bayesian framework. For viruses, disease incidence roughly decreased by a half when doubling planting density, while for fungi no relationship with planting density was observed, regardless of the feeding style or dispersal mode of the fungus, or whether the disease was monocyclic or polycyclic. The negative relationship for virus incidence and planting density and the absence of this relationship in fungi is presumably related to the dispersal mode of the pathogen: active dispersal of viruses by vectors while fungi are generally dispersed passively by water or wind. Implications for ecology and agronomy are discussed.