Similarities and Differences Among Factors Affecting Complex Declines of Quercus spp., Olea europea, and Actinidia chinensis

The decline of perennial plant species, including oak, olive, and kiwifruit, is a phenomenon currently observed in many areas of the world. In this review, such species are chosen precisely because, despite the differences in their botany, native distribution, and current utilization, they are all affected by significant global or local declines. An analysis of the main common causes involved could be useful for a better understanding of the phenomenon. Quercus species are impacted by “Chronic Oak Decline” (COD), “Sudden Oak Decline” (SOD), and “Acute Oak Decline” (AOD). In Italy, olive groves are severely damaged by “Olive Quick Decline Syndrome”, whereas kiwifruit orchards are struck by “Kiwifruit Vine Decline Syndrome” (KVDS). Among the abiotic inciting stressors, drought, warmer temperatures, and waterlogging, all within a climate change scenario, are involved in all declines described herein as well as in plant dysbiosis. The involvement of some aggressive phytopathogens is another common feature of all these declines. Oomycetes contribute to COD, SOD, and KVDS; Xylella fastidiosa subsp. pauca and Botryosphaeriaceae affect olive, and some enterobacteria are involved in AOD, all representing decisive contributing factors. These declines are quite complex, and a comprehensive approach is required to dissect all the facets involved. A better understanding of altered host–microbial community relationships can lead to a more tailored approach to understanding and managing declines. Maintaining tree resilience in a warmer Earth remains a primary goal to achieve for preserving both natural ecosystems and profitable crops.