RELATIONSHIP BETWEEN INEFFECTIVE R-GENES AND EXPANSION RATE OF LESIONS ON POTATO LEAVES, CAUSED BY PHYTOPHTHORA-INFESTANS

Progenies from eight tetraploid potato crosses were assessed for the a ssociation between ineffective R-genes and lesion expansion rate (LER) in mm per day on detached leaves that had been inoculated with Phytop hthora infestans. In two of the progenies, a significant association b etween ineffective R-genes and LER was measured, which could be an ind ication of residual effects. No association was found in the other pro genies. These results and other corroborative evidence do not support an approach to plant breeding based on the stacking of R-genes to accu mulate residual resistance effects. Using the same detached-leaf test, differences in aggressiveness between two isolates of P. infestans co uld be measured in all eight progenies. LER values were consistent wit h previously published ranges and were distributed continuously and un imodally in all progenies, suggesting that differences in relative agg ressiveness of isolates were probably not affected by R-genes.