Association between late blight resistance and foliage maturity type in potato : Physiological and genetic studies

Keywords: Earliness, Foliage maturity type, Late blight, Phytophthora infestans , Potato, QTL analysis, Resistance, Solanum tuberosumPotato ( Solanum tuberosum ) is grown throughout the world and is among the most important crops for global food supply. Late blight, caused by Phytophthora infestans , is the most important and destructive disease wherever potato is grown. Potato varieties with resistance to late blight can be used to contest the disease. The application of race-specific resistance has turned out not to be durable, because P. infestans is able to overcome this type of resistance. Race-non-specific resistance is expected to be more durable but, unfortunately, this type of resistance against P. infestans is consistently associated with late foliage maturity. The purpose of the research described in this thesis was to unravel the nature of the association between race-non-specific resistance to late blight and foliage maturity type in potato. The main goal was to determine whether the association between the two traits is genetic or physiological. As part of the physiological studies, single-node cuttings of potato were tested for their potential to serve as a model to predict foliage maturity type at any time during the cropping season. Single-node cuttings did not reflect the stage of tuber development or tuber induction of the plants from which they were taken, and they do not appear to reflect the physiological state of the whole plant adequately. Consequently, single-node cuttings cannot be used to predict foliage maturity type.A possible physiological association between late blight resistance and foliage maturity type was studied by relating the effects of plant age, leaf age, and leaf position on resistance. Leaf position had the largest and most significant effect: apical leaves were far more resistant to late blight than basal leaves. Plant age and leaf age had only minor effects. Thus, the resistance of a specific leaf remained about the same during its entire lifetime.Genetic studies were performed with seven different diploid potato progenies. Recombinant genotypes with early foliage maturity and resistance against P. infestans were not identified. Therefore, it cannot be concluded that the association between late blight resistance and foliage maturity type is due to closely linked genes. QTL analyses revealed loci for resistance to late blight on chromosomes 3, 5, and 10, and for foliage maturity type on chromosome 5. The locus for foliage maturity type could not be distinguished from the most important QTL for resistance: the allele of molecular marker GP21 that is associated with late blight resistance is also associated with late foliage maturity.