Combining Ability for Foliar Resistance to Late Blight [<Emphasis Type="Italic">Phytophthora infestans</Emphasis> (Mont.) de Bary] of Potato Cultivars with Different Levels of Resistance

Combining ability for resistance of potato to late blight (LB) was analyzed by testing segregating seedling progenies obtained from a full diallel crossing design between 3 potato parent cultivars with different levels of LB resistance and a North Carolina Mating Design II with the same resistant parents as females crossed to 2 LB susceptible male parents, against a complex race of P. infestans. Artificial inoculation was conducted under greenhouse conditions in Dalat, Vietnam at 1500 m above sea level. Percentage of foliage infection was recorded visually at 5 days intervals until 25th day after inoculation when the susceptible check was totally destroyed. General combining ability effect was significant and predominant, indicating that in the inheritance of resistance to LB, additive gene actions are more important. Specific combining ability was significant only for crosses between resistant parents, but insignificant for those between resistant and susceptible parents. In almost all cases, no significant reciprocal effect was found. Cultivar C88 was the best general combiner for resistance to LB. There were evidences that 1) the major genes R1, R3a and Rpi-blb1 alone are not enough to condition a good level of resistance to the isolate used; 2) the resistance of C88 was plausibly mostly due to presence of the gene R2, which is probably the main genetic factor of the durable resistance of this cultivar and, 3) the resistance to LB in potato seems background dependent, the background being the set of nuclear factors (major or minor genes) whose expression collectively affects the resistance level.