Combined inheritance of oligogenes (Sgr1, Sgr4, Sgr5, and Sgr6) and a weakly expressed resistance to infestation with virulent greenbug clones was studied in sorghum. Under these conditions, the resistance was shown to depend on the interaction between minor resistance genes of the host plant and the virulence genes of the pest rather than on the "residual effect" of oligogenes. The minor genes can be independent of or weakly linked to the major resistance gene. They differentially interact with phytophage genotypes and, contrary to Van der Plank's postulates, are not responsible for the long-term (horizontal) resistance. The possibility of rapidly overcoming the effect of minor genes was confirmed by observation of seasonal dynamics of a natural aphid population on a resistant variety.