Genetic variation in susceptibility of <i>Phytophthora cinnamomi</i>-infected holm oak in the absence or presence of severe drought

Abstract The evergreen oaks Quercus ilex and Quercus suber are exposed to widespread Phytophthora infestation in natural forests. To restore diseased forests, deploying trees less susceptible to combined stress is the most promising approach. We aimed to determine whether drought affects the susceptibility of Q. ilex and Q. suber seedlings to Phytophthora cinnamomi (Pc) differently. Additionally, to provide scientific support for a genetic improvement program to reduce the susceptibility of holm oak to decline, genetic variation and heritability in susceptibility to Pc in Q. ilex in the absence or presence of drought were estimated. About 7000 seedlings of 66 Q. ilex and 9 Q. suber trees from the Extremadura region (Spain) were inoculated with Pc at age 1. The following year, half the experimental blocks were regularly watered and half were exposed to severe drought, and inoculated again with Pc. In the absence of drought, Q. ilex was more susceptible than Q. suber to Pc (mortality 97% and 59%, respectively), but in the presence of drought after Pc infection the species were equally susceptible (~97% plant mortality). It could therefore be expected that under the climate change scenarios predicted for the south of the Iberian Peninsula, offspring of Q. suber will be as compromised as offspring of Q. ilex to soil infestation by Pc. Significant additive genetic variation and heritability in the susceptibility of Q. ilex to combined Pc infection and drought were observed (hi2 = 0.46 for time to death of plants), indicating that breeding for tolerance to combined stress is possible. Family variance component estimates of time to death in Q. ilex were highest in the presence of drought, and genetic control of susceptibility in Q. ilex decreased over time as plant stress increased. This is the first study to define a production population against combined stress in oak.