Selecting cork oak and holm oak trees for stable tolerance to combined drought and Phytophthora cinnamomi stress

Abstract Using trees tolerant to drought and Phytophthora cinnamomi ( Pc ) infection is the most promising approach to restore Quercus suber and Q. ilex forests under decline. Breeding programs require heritability estimates for tolerance to combined stress. Moreover, the genetic variation underlying genotype-by-environment ( G × E ) interactions in oak tolerance to combined stress has not been explored. We evaluated life expectancy of approximately 10,400 seedlings from 87 Q. suber and 90 Q. ilex half-sib families from Spain under Pc , Pc + drought, and drought + Pc scenarios. Intraspecific variation in tolerance across scenarios, additive genetic variance, and G × E interactions were analyzed. To ensure optimal selection strategies, family genotypic stability was quantified. Under well-watered conditions, Q. suber showed higher tolerance than Q. ilex to Pc , but under the Pc + drought scenario, mortality was similar between species. Geographic variation was detected in cork oak, with populations from eastern Spain showing greater susceptibility. Both species exhibited significant additive genetic variation under the Pc and Pc + drought scenarios, and narrow-sense heritability estimates ( h i 2 ) ranged from 0.10 to 0.16, indicating potential for selective breeding. We report a relevant G × E interaction, primarily due to family rank changes across scenarios. Genotypic stability analyses enabled selection of families with increased and stable tolerance, and the Pc + drought scenario was deemed optimal for selection. A backward selection strategy targeting the 20% most tolerant parents would allow genetic gains of 18% in both oak species. This is the first time oaks have been screened for enhanced and stable tolerance to combined stress.