Pyramiding of Meloidogyne hapla Resistance Genes in Potato Does Not Result in an Increase of Resistance

High levels of resistance against Meloidogyne hapla have been identified in wild species of tuber-bearing potatoes, but only QTL with partial effects have been identified so far in back crosses with cultivated potato. This study was designed to test if pyramiding of two previously identified resistance genes, RMh-tar and RMh-chcA, will result in improved or even an absolute level of resistance. RMh-tar and RMh-chcA introgressed from the wild tuber-bearing potato species Solanum tarijense and Solanum chacoense were combined in a segregating diploid Solanum tuberosum population. With the aid of AFLP markers, descendants from this segregating population were classified into four groups, carrying no R gene, with only RMh-tar, with only RMh-chcA and a group with the pyramided RMh-tar and RMh-chcA. Upon inoculation with M. hapla isolate Bovensmilde, the group containing only RMh-chcA showed a decline of 88% in average number of developed egg masses compared to the group without RMh-chcA and RMh-tar. The group of genotypes containing only RMh-tar, but not RMh-chcA, showed a decline of 55% in the number of developed egg masses compared to the group without RMh-chcA and RMh-tar. Unfortunately, the latter effect of RMh-tar was not significant. The effect of both loci, RMh-tar and RMh-chcA combined, did not further reduce the number of egg masses compared to the level of RMh-chcA alone.