Host Diversification May Split Epidemic Spread into Two Successive Fronts Advancing at Different Speeds.

Host diversification methods such as within-field mixtures (or field mosaics, depending on the spatial scale considered) are promising methods for agroecological plant disease control. We explore disease spread in host mixtures (or field mosaics) composed of two host genotypes (susceptible and resistant). The pathogen population is composed of two genotypes (wild-type and resistance-breaking). We show that for intermediate fractions of resistant hosts, the spatial spread of the disease may be split into two successive fronts. The first front is led by the wild-type pathogen and the disease spreads faster, but at a lower prevalence, than in a resistant pure stand (or landscape). The second front is led by the resistance-breaking type, which spreads slower than in a pure resistant stand (or landscape). The wild-type and the resistance-breaking genotypes coexist behind the invasion fronts, resulting in the same prevalence as in a resistant pure stand. This study shows that host diversification methods may have a twofold effect on pathogen spread compared to a resistant pure stand (or landscape): on the one hand, they accelerate disease spread, and on the other hand they slow down the spread of the resistance-breaking genotype. This work contributes to a better understanding of the multiple effects underlying the performance of host diversification methods in agroecology.

Clubroot resistance QTL are modulated by nitrogen input in Brassica napus.

KEY MESSAGE: Nitrogen levels can modulate the effectiveness of clubroot resistance in an isolate- and host-specific manner. While the same QTL were detected under high and low nitrogen, their effects were altered. Clubroot, caused by Plasmodiophora brassicae, is one of the most damaging diseases of oilseed rape and is known to be affected by nitrogen fertilization. However, the genetic factors involved in clubroot resistance have not been characterized under nitrogen-limiting conditions. This study aimed to assess the variability of clubroot resistance under different nitrogen levels and to characterize the impact of nitrogen supply on genetic resistance factors. Linkage analyses and a genome-wide association study were conducted to detect QTL for clubroot resistance and evaluate their sensitivity to nitrogen. The clubroot response of a set of 92 diverse oilseed rape accessions and 108 lines derived from a cross between 'Darmor-bzh' (resistant) and 'Yudal' (susceptible) was studied in the greenhouse under high- and low-nitrogen conditions, following inoculation with the P. brassicae isolates eH and K92-16. Resistance to each isolate was controlled by a major QTL and a few small-effects QTL. While the same QTL were detected under both high and low nitrogen, their effects were altered. Clubroot resistance to isolate eH, but not K92-16, was greater under a low-N supply versus a high-N supply. New sources of resistance were found among the oilseed rape accessions under both low and high-N conditions. The results are discussed relative to the literature and from a crop improvement perspective.