We started a new garden maintenance contract in a large three/four acre garden a few years ago, during the first autumn we spotted clumps of honey fungus around the base of a large grey poplar, It was growing about 4m away from a rose covered trellis on one side and a shrub bed next to this, a Lonicera nitida hedge runs down the other side, behind the trellis are more flower beds with a large fig tree as a central feature.
The tree was felled and the main root was grubbed out, it was then decided to encircle the tree stump with a 2ft 6” trench about 3m away from where the tree stood.
This was then going to be lined with thick plastic sheeting and the trench refilled, any roots discovered were checked for signs of infection and cut off.
We had no idea how long the tree had been infected as the previous gardeners had not mentioned anything and the owner of the property had not noticed them before, but she admitted that she probably wouldn’t have remembered even if she had as she knew nothing about honey fungus then.
The work was completed and we thought that we had managed to contain it, there were no visible signs of fungus in any of the roots that were cut through.
Three years later.
Clumps of honey fungus started to appear on the outer side of the plastic barrier in areas where the old tree roots were known to be, close to the trellis and inside the side shrub bed where a couple of shrubs were also showing possible signs of infection, poor growth and die back.
There were rhododendrons three metres back into this bed and these led on to a few small trees and more shrubs. More trenches were needed and checks made on all found roots. Luckily the trench across this section did not find any roots going into the rhododendrons, but all roots in the bed were dug up and the two shrubs were also removed when it was found that the infected roots had travelled directly underneath them.
The same procedure was performed on the other areas near the trellis, but we were not as lucky here as the roots had travelled way beyond the trellis and into the fig bed behind and they were showing signs of honey fungus, with black rhizomorphs being sent out near the trellis. Again the plan was to try to move the partition trench further back closer to the fig to see if we could cut off its route. The specific know roots on the outside of the original barrier were found and tracked under the trellises and removed, but there were a few small lateral roots going under the climbing roses which we could not get. The roses would have to be left inside the new surrounding barrier and additional compost would be added throughout the year to try to keep them healthy.
The new trench was dug across two metres behind the trellis and all roots discovered were individually tracked back to the trellis and across to the fig tree taking great care not to damage the fig trees roots.
One of the found roots went directly under an old round granite mill stone, and although the majority of it was dug out, there was one lateral fork that went down into the centre which was unreachable. So another trench was dug all the way round the millstone and it was surrounded by plastic sheeting.
The root in that situation probably would not normally be a problem, there was a remote chance that the fig tree roots could have grown underneath, but the biggest danger was from the black bootlaces, these rhizomorphs can travel up to 30 m in search of new roots to attack.
Rhizomorphs/ black bootlaces .
Most perennial roots do not spread that far, they are not thick and woody and probably do not give the honey fungus mycelium enough energy to send out rhizomorphs to find and attack distant plants, however the plants may become infected and die, and may still be able to infect another root system growing through its own.
Once it infects a large tree and spreads through the root system it can be a nightmare to deal with, especially if the tree is close to other trees and borders. I have had numerous experience of this in Devon valleys.
The reason for this could possibly be traced back to the Dutch Elm disease problem.
There were many elms growing in the surrounding woods and hedgerows and the dead trees were left or felled leaving the stumps for the HF spores to grow on and then spread along the hedgerows and into the neighbouring gardens.
The fungus can spread along a root at 1m a year and it can send out rhizomorphs to find new roots to attack, we have dug up some that have formed a tangled net, which may mean that the bootlace does not have to directly find the new root, it could be the growth of the root that finds these nets.
I have just found a rhizomorph starting to be formed inside an infected root. It is red in colour at this stage with a small white connecting root (mycelium) which is obviously going to provide the necessary food from the decaying root.
In this instance it is growing right in the centre of the root, as to whether this is normal I do not know, but I can assume that the forming of the rhizomorphs is triggered by a critical mass of the mycelium, (small twigs will not produce rhizomorphs) and therefore they probably would not form on the outside.
I think I am right is saying that their energy source is gained through the rotten root and not from the surrounding soil, this fact can make the control of the rhizomorphs slightly easier, as cutting through them with a spade should cut off their food source, they seem to spread just below the surface once they are away from the root.
Removing infected roots obviously does the same. Honey fungus is the only fungi that has developed this ability to form rhizomorphs.
It is strange that it had to develop this new way of finding new food/trees, as it must have been growing in ancient forests for many years ago where nearly all roots are intertwined.
Perhaps it used to be a lot more selective, or only attacked dead or dying trees.
How does the rhizomorph infect a newly found root,
On close examination of the boot lace they look like thin dark tree roots, but when bent they crack along the outside in many places.
Is it this that helps them, as a tree root expands it would push against the boot laces and cause the outer layer to crack, just below this layer is a band of white mycelium which would then be able to spread across onto the new root. It may be able to directly infect the root at this point, or certain roots, but it may also cause the rhizomorph to change direction by producing a lateral rhizomorph which could in theory follow the root back to the plant, - but I am only guessing.
Trees are often found with rhizomorphs growing under the bark near the base.
Spores from the toadstools can also land on exposed damaged roots and infect the plant.
It is the largest living single organism in the world, in Oregon in the USA it has been discovered that a single organism of a variety of honey fungus had spread to cover an area of 880 hectares of land and was about 2300 years old and it is total weight exceeds any other living orqanism.
So it is a creature of unequalled dimensions able to change its shape and structure and live longer than anything else on the planet.