## 5 Conclusions  
This study confirms the widely extended phtoplasma _C. P. pini_ infection in all pine populations analyzed and also shows strong evidence that phtoplasma symptoms play a major role in the decline of Aleppo pines populations. Our findings show that not only drought-induced declining processes operate in our forests. However, the visible phtoplasma symptoms and the decline processes associated in the trees only appeared under more stressful conditions, reflecting the synergistic effect of the lack of water and biotic diseases. Since both phtoplasma disease expression and drought act as stressors by affecting the plant's capacity to provide a plastic response, we may expect their combined occurrence would have additive negative effects and could lead more rapidly to exhaustion. This would result in a more accelerated decline process. However, the mechanisms that lie behind _C. P. pini_ symptoms in forests are still poorly known and more studies are needed to better understand how _C. pini_ acts pathologically and the level of microorganism concentration to reach a threshold for the symptomatic expression.  
In terms of management applications, we suggest promoting straightforward actions for forest management of drought-pathogen interactions to prevent as much as possible both water stress in trees during drought episodes and pathogen control measures to avoid phtoplasma from spreading to healthy individuals and stands. In this sense, pine forests resulting from past reforestations in dry to semiarid climates conditions beyond optimal ranges could be more vulnerable to \"hotter droughs\" mediated by biological damaging agents.  
**Supplementary Materials:** The following are available online at [[http://www.mdpi.com/1999-4907/10/8/608/s1](http://www.mdpi.com/1999-4907/10/8/608/s1)]([http://www.mdpi.com/1999-4907/10/8/608/s1](http://www.mdpi.com/1999-4907/10/8/608/s1)), Figure S1: (A) General view of affected pine forests in Cerros Concejiles (Madrid, Spain), with affected pines in the first term. (B) Tree crown detail with defoliated branches. (C) Initial stage of defoliation with dry leaves. (D) Morphological alterations of twig growth and leaves development. (E) Recently dead tree which retain brown leaves. (F) A percentage of dead trees were affected by bark beetles (_Tomicus destruens_ and _Orthotonicus erosus_) with high amount of sub-cortical galleries surrounding all the trunk and holes where young beetles have gone.  
**Author Contributions:** Conceptualization, L.M., D.G., E.G. and A.V.; methodology L.M., D.G., E.G. and A.V.; software, L.M. and D.G.; validation, L.M., D.G. and A.V.; formal analysis, L.M., DG and A.V.; investigation, L.M., D.G. and A.V.; resources, D.G., E.G. and A.V.; data curation, L.M., D.G., E.G. and A.V.; writing--original draft preparation, L.M. and D.G.; writing--review and editing, L.M., D.G. and A.V.; supervision, A.V.; project administration, A.V.; funding acquisition, E.G. and A.V.  
**Funding:** This research was funded by Seccion de Defensa Fitosanitaria (Conservacion del Medio Natural, Consejeria de Medio Ambiente, Comunidad de Madrid) and the Survive-2 project (CGL2015-69773-C2-2-P MINECO/FEDER) from the Spanish Government. The CEAM Foundation is supported by Generalitat Valenciana.  
**Acknowledgments:** We thank Pablo Cobos Lab from the Polytechnic University of Madrid for its assistance in the molecular identification of samples.  
**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.