The Involvement of Phytophthora Root Rot and Drought Stress in Holm Oak Decline: from Ecophysiology to Microbiome Influence

Purpose of Review Holm oak is a relevant species, both for its distribution and ecological importance. Among the risks looming over this species, oak decline-influenced by extreme climatic events, and alien-invasive species-is considered the main factor causing the loss of holm oak in Mediterranean open woodlands. The aim of this review is to identify and summarize the effects of drought and pathogen root rot, focusing on tree physiology, and the relationship between the stressors (biotic and abiotic) and the tree response. Recent Findings Symptoms of root rot are often associated with drought. However, it has been shown the presence of a differential response to root rot and severe drought is related with general defence mechanisms triggered by the plant. Soil microbiota has also been shown to be a key factor influencing health status and soil pathogen abundance. The application of next-generation sequencing techniques to forest pathology allows us to study complex relationships between soil, plant and microorganisms. Tolerance of holm oak against Phytophthora cinnamomi root rot is related to specific hydric and photosynthetic mechanisms that differ from those associated with drought. This response involves changes in the metabolism of the photosynthetic organs of the plant which can be linked with changes in functional traits. Studies of the soil microbiome have identified several pathogens, apart from P. cinnamomi, involved in holm oak decline, and the relevance of key fungal species in the management of this syndrome. In this regard, the presence of beneficial microorganisms such as Trichoderma spp. or ectomycorrhizae influences the physiological status of trees affected by root rot.