Mosaics often outperform pyramids: insights from a model comparing strategies for the deployment of plant resistance genes against viruses in agricultural landscapes

Abstract

The breakdown of plant virus resistance genes is a major issue in agriculture. We investigated whether a set of resistance genes would last longer when stacked into a single plant cultivar (pyramiding) or when deployed individually in regional mosaics (mosaic strategy). We modeled the genetic and epidemiological processes shaping the demogenetic dynamics of viruses under a multilocus gene-for-gene system, from the plant to landscape scales. The landscape consisted of many fields, was subject to seasonality, and of a reservoir hosting viruses year-round. Strategy performance depended principally on the fitness costs of adaptive mutations, epidemic intensity before resistance deployment and landscape connectivity. Mosaics were at least as good as pyramiding strategies in most production situations tested. Pyramiding strategies performed better only with slowly changing virus reservoir dynamics. Mosaics are more versatile than pyramiding strategies, and we found that deploying a mosaic of three to five resistance genes generally provided effective disease control, unless the epidemics were driven mostly by within-field infections. We considered the epidemiological and evolutionary mechanisms underlying the greater versatility of mosaics in our case study, with a view to providing breeders and growers with guidance as to the most appropriate deployment strategy.

Mosaics, mixtures, rotations or pyramiding: What is the optimal strategy to deploy major gene resistance?

“…In this context, infectivity towards some of the major genes in the pyramid may already be present in a pathogen population. As shown in recent modelling studies, the initial presence of preadapted pathogens can have a dramatic impact on the durability of the pyramid compared to other strategies (Djidjou‐Demasse et al., 2017; Lof et al., 2017). Moreover, mutations towards multi‐infectivity were considered independent, and our model does not currently include pathogen sexual reproduction.…”

Assessing the durability and efficiency of landscape-based strategies to deploy plant resistance to pathogens

“…However, in addition to long-term measures related to stable equilibria, short-term and transitory periods measures are important because severe epidemics responsible for heavy losses may occur during these stages [125,132]. These periods can be accounted for by averaging the number of healthy hosts over the whole simulation run using an analogy of our Green Leaf Area [35-37, 40, 42, 133] or the number of infected hosts using the area under disease progress curve (AUDPC) [28,32,33,48,50]. Interestingly, these measures offer the possibility to concentrate on different evolutionary phases, for example the short-term period following resistance deployment until resistance breakdown, and the long-term period once resistance is overcome [45].…”

“…mixture and pyramiding: [43]; mosaic and pyramiding: [44,45]). Only a few studies explicitly compare two types of strategies [46][47][48][49][50] and only two studies evaluated more than two strategies [51,52]. As a result, a comprehensive evaluation of different deployment schemes is complicated, and currently only feasible via pairwise comparisons [53,54].…”

Anna‐Liisa Laine

“…The theoretical study by Djidjou‐Demasse et al . () addresses the major problem in agriculture – break‐down of resistance genes following pathogen adaptation. Their comparison of different breeding strategies is likely to guide future empirical work, and I think science is at its best when there is a strong dialogue between theory and data.…”