Breeding for Field Resistance to Late Blight of Potato at SCRI

In 1991 a breeding programme was started at SCRI to combine quantitative resistances to late blight (Phytophthora infestans) and the white potato cyst nematode (Globodera pallida) with commercially acceptable potato tubers. The blight resistant parents derived from a programme started by Dr. Black at the former Scottish Plant Breeding Station in 1954. They have complex pedigrees tracing back to Solanum demissum, but involving other species such as S. phureja and S. microdontum as well as S. tuberosum. The breeding programme has involved cycles of crossing, selection between progenies (= full-sib families) and clonal selection within the selected progenies. Progress at the end of the fourth cycle was evaluated by comparing the 77 selected clones with the 36 parents used in 1991 and the 15 parents used in 2003. The assessment confirmed that none of the original 36 parents combined blight and G. pallida resistance whereas one of the 15 parents and seven of the 77 clones did. They had foliage blight scores >= 7.0 (<= 25% necrotic tissue) which were not associated with late maturity, compared with a score of 2.5 (> 80% necrotic tissue) for 'Maris Piper', 25 days after infectors were placed in the spreader rows of the field trial. They also had G. pallida scores of <= 3.08 (square root of cyst number), which was the highest score of the nine original parents with resistance. The main reasons why more clones did not meet these targets were the susceptibility of some parents and clones to new genotypes of P. infestans that have arrived in Britain and have been used in the assessments, and genotype x test condition interactions in testing for G. pallida resistance. Hence realism is required over the levels and durability of field resistance that can be expected in breeding commercially successful potato cultivars.