The variability in virulence and agressiveness of 20 populations of the potato cyst nematode G. pallida from the Andean region of La Libertad, Peru (2 600-3 500 m.a.s.l.) was determined. Eleven clones selected for their resistance to race P5A as well as some with resistance to P4A, showed good levels of partial resistance to P5A (Pf/Pi = 0.25 to 3.19). Double resistant clones (P4A + P5A) were more effective against race P4A (Huancayo). The clone 278096.10 also confirmed its resistance to this race (Pf/Pi = 0.46). Clones 280090.10 and 280236.6 were the most resistant to all populations (70 and 55%, respectively). The presence of a new virulence group was confirmed and tentatively nominated as race P6A due to its multiplication on all resistant clones. Four groups of aggressiveness were determined. Seventy-five percent of populations were classified as either very aggressive or aggressive and are considered to be the most important. In general, the most virulent populations also were t
