Accuracy of genomic selection and long-term genetic gain for resistance to Verticillium wilt in a genetically diverse strawberry population

Verticillium wilt, a soil-borne disease caused by the fungal pathogen
Verticillium dahliae, threatens strawberry (Fragaria × ananassa)
production worldwide.  The development of resistant cultivars has
been a challenge since the disease was first reported on strawberry in the
early 1900s.  The empirical evidence suggests that genetic gains
have not been negligible and that the genetics of resistance to this
pathogen is quantitative.  While resistant cultivars have been
developed, a comparatively small percentage are highly resistant, whereas
a large percentage are moderately to highly susceptible.  To
develop insights into the genetics of resistance, we studied a population
of elite and exotic germplasm accessions (n = 984), including 245
cultivars developed in public breeding programs since 1854.  We
observed a full range of phenotypes, from highly susceptible to highly
resistant.  Broad-sense heritabilities ranged from 0.70 to 0.76,
whereas narrow-sense genomic heritabilities ranged from 0.33 to
0.45.  Less than 3% of accessions were classified as highly
resistant.  Several of the most resistant accessions were
heirloom cultivars or exotic germplasm accessions developed before the
introduction of pre-plant soil fumigation in 1960.  We show that
genetic gains for resistance to Verticillium wilt have been negative over
the last century and that genomic selection has significant potential to
increase genetic gains.  Genomic prediction accuracies ranged
from 0.41 to 0.49 and generally increased as training population diversity
increased.  Our results suggest that heirloom cultivars and other
exotic germplasm accessions carry favorable alleles for resistance to
Verticillium wilt that are not present in modern cultivars.