Efficiency of pyramiding of three quantitative resistance loci to apple scab

Abstract

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.