- 14 Jan 2010
Lottie, in view of your last post this definitely sounds like honey fungus. A characteristic symptom is exactly as you describe: sheets of white fungus mycelium between bark and wood, smelling of mushrooms.
If you dig down to look at the main roots, at least some of them will be affected in the same way. The key feature is that honey fungus grows in and kills roots by forcing its way between bark and wood. After killing the root, it then feeds on the decomposing wood. When the advancing infection reaches the base of the tree it starts to encircle it and at this point you start to see symptoms above ground, just as you describe. It is too late by this stage to save the tree.
Quote from Strouts, R.G. and Winter, T.G. (2000) Diagnosis of ill health in trees. 2nd ed. The Stationery Office (P.39):
"English or persian walnut, Juglans regia, is particularly susceptible to killing by honey fungus (Armillaria), whereas observations and experimentation indicate that J. hindsii and J. nigra are probably immune."
Honey fungus spreads through the soil by means of rhizomorphs, black threads which can then infect new roots on contact. However, the rhizomorphs can only live when they are attached to the dead roots they are feeding on, they do not live independently in the soil. To deal with an infection you have to do two things. Firstly, excavate all the affected roots and burn them. Secondly, try and trace where the infection came from, which is almost certainly another old stump in the near vicinity. If you can be sure you have got rid of all the roots and stumps that might be a source of re-infection, you will be safe to replant. However, it would still be safest to replant outside the root zone of the old tree, because many trees do not thrive if replanted directly where one was previously growing.
Juglans nigra would be a safer bet than J. regia in view of its strong resistance to infection. It is stated to have edible nuts, but I'll leave others to comment on how good they are, in my experience it is grown primarily for its timber.