Symptoms of oak decline

 

Initial and subsequent symptoms

Typically, the first symptom observed on a declining tree is a deterioration of the foliage. Leaves may be smaller than normal, and are often pale green or yellowish. In some cases the foliage may be sparse or thin over the entire crown.

Over the next year or so, death of fine twigs may be followed by the death of small branches; at this stage the foliage can be very thin. In some cases the process continues through to death of large branches, and eventually the whole tree may die.

Very often serious decline or terminal decline is signalled by bleeding patches on the trunk of affected trees:

However, some trees can make a partial recovery through the production of epicormic shoots on the trunk and surviving branches:

Chronic decline

Oak decline takes different forms. In some trees the decline is chronic, and dieback may occur over many years, even decades, and some individuals may stabilise and make a partial recovery.

Typically trees affected in this way are described as ‘stag-headed’ with skeleton branches protruding from the crown:

Very often the smaller area of the remaining crown has leaves of normal colour and size showing there has been recovery, although further episodes of dieback can strike resulting in chlorotic leaves and further twig and branch dieback which accumulate over time.

Acute decline

Records of oak decline in the early 1990s described a disorder that in a proportion of affected trees progressed rapidly and could be very acute: trees moved from the early stages foliar symptoms to extreme deterioration and death in just 4 to 5 years. In other trees, their condition stabilised following some symptom development and remained little changed when reassessed over the next 3 to 5 years.

A striking feature of the acute decline recorded at this time was the presence of the buprestid beetle Agrilus pannonicus:

Early stages of attack by Agrilus were often marked by the presence of a dark exudate (sometimes called tarry spots or bleeding) on the bark surface. When the bark was removed it contained extensive galleries produced by tunnelling Agrilus larvae and this contributed to the death of the tree because of the amount of phloem disruption that already weakened trees then suffered: