‘Host of the month’ is a series of Blogs and PDF’s that highlight a tree host and their associated priority pests and diseases that are best seen and recorded in that month. For April we’re looking at oak (Quercus species) and acute oak decline.

Oak trees are deeply connected to the history of Britain; King Charles II reputedly hid in an oak tree following the Battle of Worcester and as a result The Royal Oak is one of the top three pub names in the England, incidentally Charles II’s birthday is commemorated by Oak Apple Day on May 29th. Oak has also been one of the most important timber trees grown in Britain, particularly for use in ship building and timber-frame buildings. Great Britain has two native species; pedunculate oak (Q. robur), and sessile oak (Q. petraea) and a number of non-natives, the most common being Turkey oak (Q. cerris) and red oak (Q. robur). Separating the native species can be tricky and they hybridise readily. There is no one single defining character, instead a range of characters must be considered together. In winter you are limited to bud characters, leaf litter and old acorn cups with attached peduncles (flower or fruit stalks).

Priority disease – acute oak decline (AOD)
AOD is a complex condition where several different bacteria work together as part of a pathobiome, a community of microbes associated with reduced health in the host, to cause symptoms in oaks over 50 years old. Of the species commonly seen in Britain and Ireland pedunculate and sessile oak are the most commonly affected but Turkey and red oaks have also presented with symptoms. Tree death can occur within 6 years of symptoms first appearing but is not a foregone conclusion.

The most visible indicator for AOD is the presence of multiple vertical cracks in the bark and associated bleeds of dark fluid which can appear anywhere along the whole length of the trunk. Individual bleed points are not linked and can occur around the entire girth of the tree or restricted to one or two sides. Canopy dieback is common where stem bleeds are frequent. There is also a strong association between AOD and the D-shaped exit holes of the oak jewel beetle (Agrilus biguttatus) which are often seen in close proximity to the bleeds.

For more information check the Observatree resource pages for acute oak decline, or the Host of the Month for April . You can also test your knowledge with the Host of the month Quiz.

April is an ideal time to seek out oak trees and see if you can identify any of the signs and symptoms of acute oak decline. AOD is a priority disease so please report possible sightings via TreeAlert. Healthy tree data is equally important so please do report those too.