Ingoldian aquatic fungus
The disease is consistent with Diplodia quercina, though it doesn't seem to have been studied from this oak before.
Small portions of the tree are completely brown in branches and leaves. Leaves are otherwise intact and do not wither or drop from the tree easily. The remaining tree appears stable and alive.
Dothideomycetes is the largest and most diverse class of ascomycete fungi. It comprises 11 orders 90 families, 1300 genera and over 19,000 known species. Traditionally, most of its members were included in the loculoascomycetes, which is not part of the currently accepted classification. This indicates that several traditional morphological features in the class are not unique and DNA sequence comparisons are important to define the class.