Differential recognition of Phytophthora infestans races in potato R4 breeding lines

Introgression breeding has resulted in several potato lines that are resistant to late blight, a devastating plant disease caused by the oomycete Phytophthora infestans. The traditional differential set consists of potato lines with 11 late blight resistance specificities, referred to as R1 to R11. With the exception of the R4 locus, all the resistance loci in these lines have been genetically mapped or positioned in resistance (R) gene clusters. In this study, we show that potato lines that are defined to carry R4 do not necessarily recognize the same P. infestans strains. Field isolates appeared to be avirulent on either the R4 differential developed by Mastenbroek or the one developed by Black but not on both. Previously, we identified the avirulence gene PiAvr4, which is a member of the RXLR effector family. In planta expression of PiAvr4 revealed that recognition of PiAvr4 is strictly confined to the Mastenbroek R4 differential. Segregation of the trait in two independent F1 progenies showed that late blight resistance in this differential is determined by a single dominant gene, now referred to as R4Ma.