The term ”oak decline” is used by scientists to describe a condition in which the vigour (health) of an oak is reduced by complicated interactions between several damaging agents. The damaging agents can involve both biotic and abiotic factors such as insects, nutrient imbalances, root pathogens or extreme weather conditions. Normally, a healthy tree can withstand most of the biotic damaging agents when they are acting on the tree alone, but if the oak is already stressed by other damaging factors, it is less persistent. Often, oak declines result in a premature death of the trees.

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Although it is believed that several biotic and abiotic factors are involved in the causes of oak declines, the actual process is yet not fully understood. Several studies have been performed on oak stands around the world and factors suggested to be responsible for the oak declines involve summer droughts and winter or spring frosts. Other factors suggested include site conditions, root pathogens of the genus Phytophthora and direct and indirect effects of nutrient imbalances. As mentioned above, oak declines are most often caused by a combination of different damaging agents, wherefore a single factor alone should not be considered the cause of the oak decline.

The symptoms of oak declines are most noticeable in the shape and colour of the crown. Normally, a discoloration of the foliage is the first symptom indicating that something is wrong with the tree. The leaves often appear paler than normal, sometimes completely changing colour from green to more yellowish. Another typical symptom of oak declines is a considerably reduced crown size, and a characteristic appearance with the leaves arranged in tufts at the end of the shoots. This is caused by an abnormally increased twig abscission and dieback of buds and branches in the upper canopy. Other symptoms include slime flux on the trunks, progressive necroses of the bark or cambium and reductions in diameter growth of the trunk. As the oak decline progresses, large parts of the crown, or the whole tree, dies. The diebacks can affect single trees or groups of trees within stands, parts of stands or, rarely, whole oak stands.