Acute and Chronic Oak Decline in urban and forest ecosystems in Southern Italy

Since recently, a decline of holm oak (Quercus ilex L.) is observed in Salento (South Italy), this caused concern regarding the protection of native biodiversity and local forests. Symptomatic trees show browning of leaves, thinning of canopy, and loss of branches. The aim of this study was to understand the biotic factors involved in the observed decline. During the monitoring activity, the use of phenotypic descriptors highlighted the presence of two types of decline with distinct symptoms among oaks in rural and urban areas, which could be classified as Acute Oak Decline (AOD) and Chronic Oak Decline (COD), respectively. Following this, diagnostic analyses were carried out to assess the presence of various holm oak pathogens on twigs, trunk, and soil. These analyses revealed the presence of several Botryosphaeriaceae oak pathogens, present ubiquitously between the two environmental contexts, with Diplodia seriata being overall the most prevalent. Contrarily, bacteria associated with AOD were only present in the rural areas and included: Brenneria goodwinii, Gibbsiella quercinecans, and Rahnella victoriana. As far as known to the authors, this is the first time these pathogens have been found in the Mediterranean coastal regions. Lastly, soil analyses performed to detect the presence of Phytophthora cinnamomi were negative. Our results highlight the presence of several pathogens, which indicates that oak decline is a multifactorial phenomenon for which the predisposing, initial, and contributing factors are not yet known particularly for the recently described AOD and COD. Finally, the identification of AOD-related bacteria in Salento is a cause of concern for the need of urgent management actions aimed to contain their spread and to preserve the forest heritage of the region.