Plant-plant interactions in wheat mixtures modulate mean and variance of susceptibility to Septoria tritici blotch.

Varietal mixtures are a promising agro-ecological approach to stabilizing yields by reducing diseases. The effects of mixtures stem from modifications of epidemiological processes and underestimated plant-plant interactions, which could explain some of the paradoxical observations made in the field. However, the role of plant-plant interactions in modifying bread wheat and durum wheat susceptibility to Septoria tritici blotch remains to be elucidated. Our study aimed to determine the effect of such plant-plant interactions, by producing full matrices of binary mixtures in the absence of epidemics, on Septoria symptoms-specifically necroses (lesions) and pycnidia (spore-containing structures). We employed statistical modeling to compare the mean and variance of focal plants' phenotype in all mixtures versus pure conditions and in each mixture versus pure condition. Our findings demonstrate significant effects of plant-plant interactions on wheat susceptibility to Septoria. Notably, these interactions had specific rather than general effects, with some but not all genotypic combinations significantly influencing focal susceptibility to Septoria. Furthermore, mixtures resulted in reduced necrosis with lower variance, but increased pycnidia formation. These results reinforce the need to consider specific plant-plant interactions for their contribution to trait means and variances in varietal mixtures.