A genetic analysis of quantitative resistance to late blight in potato: towards marker-assisted selection

Late blight caused by the oomycete Phytophthora infestans is the most impor tant fungal disease in potato cultivation worldwide. Resistance to late bli ght is controlled by a few major genes (R genes) which can be easily overco me by new races of P. infestans and/or by an unknown number of genes expres sing a quantitative type of resistance which may be more durable. Quantitat ive resistance of foliage to late blight was evaluated in five F-1 hybrid f amilies originating from crosses among seven different diploid potato clone s. Tuber resistance was evaluated in four of the families. Two of the famil ies were scored for both foliage maturity and vigour. The five families wer e genotyped with DNA-based markers and tested for linkage with the traits a nalysed. QTL (quantitative trait locus) analysis identified at least twelve segments on ten chromosomes of potato having genes that affect reproducibl y foliage resistance. Two of those segments also have major R genes for res istance to late blight. The segments are tagged by 21 markers that can be a nalyzed based on PCR (polymerase chain reaction) with specific oligonucleot ide primers. One QTL was detected for tuber resistance and one for foliage vigour. Two QTLs were mapped for foliage maturity. Major QTL effects on fol iage and tuber resistance to late blight and on foliage maturity and vigour were all linked with marker GP179 on linkage group V of potato. Plants hav ing alleles at this QTL, which increased foliage resistance, exhibited decr eased tuber resistance, later maturity and more vigour.