Holm oak decline threatens dehesa sustainability. Though the pseudofungus oomycete Phytophthora cinnamomisería is thought to be the main cause of holm oak decline, climate conditions have been shown to influence it, as well. Even so, this puzzle has yet to be solved.
source https://www.lifetechnology.com/blogs/life-technology-news-blog/dehesa-health-starts-from-the-ground-up