Quantitative plant resistance is supposed to be more durable than qualitative resistance for the control of plant diseases. However, it has been experimentally shown that erosion of quantitative resistance can occur. Cumulation of quantitative resistance loci (QRLs) in the same cultivar is considered to improve the efficiency and durability of quantitative resistance, but the choice of QRLs to be combined is of crucial importance. This study investigated whether the combination of QRLs acting on different stages of pathogen development could improve the efficiency of resistance in the apple scab pathosystem. The efficiencies of three QRLs were evaluated against 10 isolates of Venturia inaequalis and the stages of pathogen development that were affected by the QRLs were defined microscopically. A gain in the efficiency of resistance was observed when QRLs were pyramided compared to when they acted alone. Thanks to the combined effects of the individual QRLs, the pyramiding of the three QRLs hindered fungal development at different stages: before the penetration of the plant cuticule, after the penetration with hypersensitivity reaction, and during the colonization and asexual reproduction. These effects were dependent on the V. inaequalis isolates. These results suggest that the gain in efficiency of resistance by pyramiding may derive from the combination of different and complementary molecular mechanisms underlying QRLs. Thus, the resistance achieved from pyramiding such a combination of QRLs should be durable.
Ascochyta blight, caused by the necrotrophic ascomycete Didymella pinodes, is responsible for severe losses in winter and spring pea crops. Despite different climatic conditions, epidemics on winter and spring crops are due to a single population of D. pinodes, suggesting gene flow either between the two crops or from reservoir sources during the cropping season. This should lead to similar pathogenicity characteristics in isolates sampled from the two crops. However, these hypotheses have never been formally tested. We therefore sampled a total of 520 D. pinodes strains throughout a growing season from winter and spring pea plots (WP and SP, respectively) and from winter and spring trap plants (TWP and TSP). Amplified fragment length polymorphism (AFLP) markers revealed high genetic diversity within subpopulations, whereas pathogenicity tests showed that mean aggressiveness increases over the course of an epidemic. These results support the idea that alloinoculum contributes to the carryover of epidemics between winter and spring crops and that the most aggressive isolates are selected as an epidemic progresses. IMPORTANCEAscochyta blight, caused by Didymella pinodes, is responsible for severe losses in pea crops. While previous studies have shown that ascochyta blight epidemics on winter and spring crops are due to a single population of D. pinodes, suggesting that isolates from the two crops present similar pathogenicity characteristics, that hypothesis have never been tested. Genetic analysis of subpopulations sampled throughout a growing season from winter and spring pea plots revealed high genetic diversity within subpopulations, whereas pathogenicity tests showed that mean aggressiveness increases over the course of an epidemic.T he spatiotemporal dynamics of crop diseases are simultaneously impacted by pathogens, host plants, the environment, and human activities (1, 2). Indeed, whether or not hosts and pathogens interact is determined largely by spatial and temporal components of host and pathogen life history traits (3, 4). These interactions can thus be conceptualized as a continuous sequence of biological cycles, including dormancy, growth, reproduction, dispersal, and pathogenesis (5).Gene flow, resulting from pathogen reproduction and dispersal, can drastically increase the extent to which pathogen epidemics spread across a landscape (6). As such, it is a main factor in the transmission of disease to previously uninfected areas and drives the spatial structure of pathogen populations in fragmented landscapes by influencing the long-term survival and genetic composition of populations (7-9). Individual dispersal events, occurring over periods of days or weeks during both the cropping and intercropping seasons (2), originate from a large number of potential inoculum sources: resting structures in soil (mycelium, oospores, chlamydospores, or sclerotia), infested stubble left on the soil surface, infested seed, and alternative hosts (wild or cultivated plants, including volunteers). The degree ...
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