Biochar as an integrated management tool against Phytophthora cinnamomi, a key driver of Quercus suber decline.

Portugal has over one-third of the Quercus suber L. world area, a natural treasure threatened by a progressive decline, which has been witnessed during the last decades by increasing tree mortality, where Phytophthora cinnamomi Rands is considered the main cause. Biochar has proven to be a product of great interest for application in agriculture due to its benefits in terms of soil chemical and physical properties. There is also evidence of phytosanitary status improvement, including P. cinnamomi, namely inducing resistance in plants. Three biochars of different origins were tested under lab and greenhouse conditions: one prepared from Acacia spp., another from a mixture of woody plant species, and a third from macroalgae Oedogonium intermedium Wittr. The aim was to evaluate the biochars effect on root rot disease caused by P. cinnamomi infection testing their ability to inhibit P. cinnamomi mycelium and reproductive structures and on young Q. suber plants. Generalized linear mixed models (GLMMs) identified dose-dependent biochar responses, with Acacia at 1% and wood_mix at 5% (w/w) emerging as optimal concentrations. These treatments markedly suppressed P. cinnamomi reproductive structures (p < 0.001), promoted cork oak plant growth (root recovery 79-80% above pathogen-challenged controls), and maintained favorable soil chemical parameters, representing a compelling integration of pathogen suppression with plant health benefit. The possibility that biochar may act as a duplibiotic in relation to soil microbiota is discussed. Our observations allow us to conclude that biochar could be an option to include in an integrated strategy plan to control the root rot disease in cork oak caused by P. cinnamomi.