Dying Sycamore Tree - Herbicide Poisoning?

Sad to report that the most significant sycamore tree in the woodland looks to be dead. It has a tree protection order (TPO) because it is so significant and lovely.

This is how the tree looks today

- about 80% of the tree has lost it leaves and there's scant sign of any secondary buds. There are no other affected trees in the area although another sycamore stump close by showed similar signs of leaf loss and leaf discolouration. 
The tree officer, Ellen, at first thought of lightning, but that was ruled out when the other tree stump had the same signs. Then sooty bark disease was suggested which could arise later following a fungal infection. Ellen strongly advised me to contact the Forestry Research to investigate and I have done that. Samples have been sent to their office and the pathologist said he would get back to me. I waited.....quite some time before eventually they got back to me. From the conversation I gather there's no indication of any fungus but the office seem very focussed on use/abuse of herbicide. I had to look up what herbicide was. I now know that Round Up is a herbicide and I do use that very sparingly on weeds in the street. Well diluted and in a spray. Not enough to kill a mature tree that's for sure.

A few weeks later I am told another pathologist will be in touch and indeed Steve comes along and manages to take samples from higher up the tree. Once again he asks about herbicide and if perhaps the tennis club have used any. I have no idea but the club was open and I ask if I can look. I take a look around the vicinity adjacent to the ailing sycamore.

I find this.

a cross cut tree stump and it's a sycamore. The only reason to cross cut a stump is to aid the action of a stump killer such as a Roundup. 

I then learn about the possibility for trees of the same species to share a root through some sort of fungal? mycorrhizal? activity which promotes the formation of a shared root. It all sounds a bit hard to believe.

There is no sign of any other reason for the death of the tree. Sycamores are robust trees and rarely succumb to fungal infection, this tree has died suddenly and the decay looks like herbicide damage. And yet not another plant in the vicinity of the tree has been affected. So it can't be from overspray. Hard as it is to believe it does look as if the application of a herbicide to the tennis club tree stump may have been translocated into the adjacent sycamore stump and the dying mature sycamore. If this is the case, we need to write this up for others to be aware of the potential for mature healthy trees to be affected by application of herbicide to tree stumps. 

If anyone else has any idea what might have caused the loss of this sycamore I am interested to hear. If anyone else has experience of herbicide use in tree stumps affecting nearby same species trees, again, please get in touch.

It does look as if this huge tree is going to have to be felled. Thats (a) expensive and (b) going to produce a huge stock of wood, more than we will need given our current stock piles. 

We will give it every chance to recover, see if it produces new buds, but it is not looking good. The upside for me is that I actually don't particularly like sycamore and I will be able to plant something else. Then there's the increased light. That's good for my garden plants. But somehow the fact that this tree has been "poisoned" albeit not deliberately, is very sad. It is/was a magnificent sycamore.

- lets hope some of my neighbours are not moved to say I deliberately poisoned this tree myself. Why would I think that might be possible I wonder?


PS
http://www.extension.iastate.edu/forestry/tree_biology/roots.html

Useful info on this link.