What has caused these black thread-like structures under the bark of this old felled sycamore, Acer pseudoplatanus, log? The black sheet-like growth attached to the threads is also part of it and the whole structure extends throughout much of the log which is around 1 metre long. Look out for the answer next week....
The black structures are caused by a fungus of the genus Armillaria, better known as Honey Fungus or Bootlace Fungus. There are up to 10 species of Honey Fungus in the genus Armillaria that are found in Britain, the most common (and likely what we have here) being Armillaria mellea and Armillaria gallica.
Honey Fungus is parasitic - it takes it's nutrients from a living tree whilst having a detrimental effect on the host organism, however it can also live as a saprobe, meaning it can survive on dead and decaying plant matter. It causes what is known as 'white rot' in trees. The fungus produces enzymes that break down lignin (the woody substance that gives trees their strength). This leaves behind cellulose which is a lighter colour than lignin, hence the name 'white rot'.
Symptoms of Honey Fungus include the black 'bootlaces' that you see in the picture. These are called rhizomorphs and can be thought of as similar to a plant's roots. The rhizomorphs are the fungus' main method of spread. They move through soil at a rate of around 1m per year, infecting new plants along the way. Other symptoms include a white sheet-like formation underneath the bark of the tree's stem base and roots, called the mycelium. This has two main functions, firstly it is through the mycelium (comprising millions of smaller structures called hyphae), that enzymes are secreted and the resulting nutrients from the decayed wood are absorbed. Secondly, it plays a part in vegetative reproduction and can give rise to fruiting bodies. The mycelium usually has a strong smell of mushrooms. Finally, you may see the fungal fruiting bodies at the base of infected trees and on infected dead wood ihn the autumn. They are honey coloured (hence the name), usually up to around 10cm in diameter and grow in clumps of several fruit bodies together.
Armillaria mellea and Armillaria gallica are common the the UK, with the other species varying in scarcity. Susceptible tree species include sycamore, birch, beech, rowan, wild service, guelder rose and wayfaring tree. However, there are some trees that have a natural resistance including the native yew and common whitebeam and others such as black walnut and box elder.