Combining Single-Gene-Resistant and Pyramided Cultivars of Perennial Crops in Agricultural Landscapes Compromises Pyramiding Benefits in Most Production Situations.

Although resistant cultivars are valuable in safeguarding crops against diseases, they can be rapidly overcome by pathogens. Numerous strategies have been proposed to delay pathogen adaptation (evolutionary control) while still ensuring effective protection (epidemiological control). For perennial crops, multiple resistance genes can be deployed (i) in the same cultivar (pyramiding strategy); in single-gene-resistant cultivars grown (ii) in the same field (mixture strategy) or (iii) in different fields (mosaic strategy); or (iv) in hybrid strategies that combine the three previous options. In addition, the spatial scale at which resistant cultivars are deployed can affect the plant-pathogen interaction: Small fields are thought to reduce pest density and disease transmission. Here, we used the spatially explicit stochastic model to compare the evolutionary and epidemiological control across spatial scales and deployment strategies relying on two major resistance genes. Our results, broadly focused on resistance to downy mildew of grapevine, show that the evolutionary control provided by the pyramiding strategy is at risk when single-gene-resistant cultivars are concurrently planted in the landscape (hybrid strategies), especially at low mutation probability. Moreover, the effectiveness of pyramiding compared with hybrid strategies is influenced by whether the adapted pathogen pays a fitness cost across all hosts or only for unnecessary virulence, particularly when the fitness cost is high rather than intermediate. Finally, field size did not affect model outputs for a wide range of mutation probabilities and associated fitness costs. The socioeconomic policies favoring the adoption of optimal resistant management strategies are discussed.