Ganoderma is a very common decay fungus of trees and is something we come across a lot. This post takers a brief look at the disease.
There's four species which we come across most often in this country G.applanatum, G.australe, G.resinaceum and G.pfeifferi. All four are perennial, persisting for years, and produce a bracket-like fungal fruiting body.
Ganoderma resinaceum is common on oak trees whilst Ganoderma pfeifferi is most commonly encountered on beech. G.applanatum and G.australe are also common on beech but can be found on a variety of other trees and in our experience these two are the most commonly encountered of the Ganodermas.
Ganoderma and decay in trees
Ganodermas cause a white rot in the wood of their hosts. They degrade the wood by selective delignification which essentially means the lignin and hemi cellulose (two important components of wood) are broken down and the cellulose is left in tact (or at least in the early stages of decay anyway).
The decay occurs in the base and roots of afflicted trees and can extend fairly high up the stem. The highest we've seen it up a stem is approximately 12m between the fork of two co dominant stems on a lime tree.
The decay leads to cavity formation and can leave trees vulnerable to structural collapse and wind throw. However, very often trees colonised by Ganoderma can remain standing for many years and without any obvious decline in overall vitality. This is normally because the rate or progress of decay has not overtaken the rate at which the tree can add wood to surround the area of degradation. The nature of selective delignification means afflicted trees, if healthy enough, can respond and do something about it. They can add a 'band' of additional wood around the decay as compensatory support which can result in 'bottle-butt' formation, a swelling of the affected part.
Management of trees with decay
There are recognised safety limits for hollow/ decayed trees. Not every tree with a hollow stem or decay is deemed unsafe. Research by Claus Mattheck and others has suggested that the threshold for the amount of wood which must be sound and unaffected by decay is set at between 30-35% of the stem radius. If you can be confident that this threshold has not been exceeded then it may be quite acceptable to leave the tree alone.
If a tree has been identified with Ganoderma and the amount of decayed wood within a stem has been determined as beyond recognised safety threshold limits or is very close to it than management of the crown can be a great way of saving a tree from the chop.
We recently surveyed a tree in Twickenham which had Ganoderma australe at the base. The decay had progressed asymmetrically and on the north side of the stem was worse. With some investigatory probing we estimated the stem at the worst point of decay fell short of recommended safety levels.
We recommended a full crown reduction to entail 1-1.5m of branch shortening all round. This would reduce the sail and wind loading on the tree and allow its safe retention in the short term. In the long term regular monitoring will determine how well the tree responds to the remedial works and how much further the decay progresses.
Ganoderma is a very common fungus on many different types of trees. It causes white rot of the infected wood. In the late stages of decay it can render trees unsafe but management of the crown can allow trees to be retained for many years.