Diplodia corticola – Coraebus florentinus: a new emerging association in declining cork oak forests in Sardinia (Italy)

Abstract: Oak decline is considered a complex disease caused by the synergistic interaction of
multiple abiotic and biotic factors. In the last few decades, a constant expansion of decline
events has been observed in Sardinia (Italy), mainly in cork oak woodlands. The canker-causing agent Diplodia corticola has been recognized as one of the main pathogens involved. In addition, the black-banded oak borer Coraebus florentinus has emerged as a serious threat to oak forests in this region. Recently, the involvement of C. florentinus in the spread of fungal propagules of some important oak canker-causing agents, particularly of D. corticola, has been demonstrated. However, it is not clear whether C. florentinus plays a primary role as a vector of the pathogens from diseased to healthy plants, or whether the larvae simply contribute to spread the pathogen along their feeding galleries. Therefore, a study was carried out to characterize the fungal species associated with necrotic wood tissues surrounding the C. florentinus gallery systems of declining cork oak trees. Field surveys were carried out in
fourteen cork oak stands in the central part of Sardinia, where the co-occurrence of tree decline symptoms and attacks of C. florentinus were previously observed. Isolations carried out from gallery systems of symptomatic branches yielded a total of 180 fungal isolates. Morphological features and DNA sequence data revealed 26 species belonging to 17 distinct genera. Approximately 36 % of isolates were identified as D. corticola, which represents the most widespread species as it was found at thirteen out of fourteen sampling sites. Diplodia corticola was isolated along the entire gallery systems from both sawdust and woody tissues on the margin of the galleries. Although our findings confirm the involvement of C. florentinus in the diffusion of D. corticola, the specific role played by the insect in the spread of the pathogen still remains unclear.