Sick Cottonwood Tree

[umlaf]

We are renting our house and we've only been here since November so I don't know the history of the tree, but I have a lovely large cottonwood in my backyard that seems to be ill. In the fall it was the last of the trees in the neighborhood to loose it's leaves and it took it's time dropping them so I think it was happy and healthy then. But right now my summer lawn looks more like a late fall lawn because it is literally COVERED in leaves falling from the cottonwood, and I just raked a large quantity of them up on Friday. Many of the leaves have some odd looking holes in them, but I see no evidence of bugs, other leaves have some brown/black spots, and most of them that fall are yellow or brown. It's been going on for probably three or four weeks now. Does anyone have any idea what is happening and/or what I can do about it. It's really a beautiful tree and it gives a lot of shade to the yard, I would hate for it to die but at the same time it's not my house and I don't want to pour a whole lot of money that I don't have into it.

Thanks,
Beth

[Dchall_San_Antonio]

Has anyone applied anything to the surface in the tree's root zone since you moved in? In particular, compost? Or is there a lawn service that sprays? I'm thinking somehow a herbicide got applied.

[umlaf]

No, we haven't put anything on the tree or on the ground around the tree. And I don't have any reason to suspect that the previous renters did either because when we leased the house the owner mentioned that the previous renters were organic gardeners too. It was suggested in the tree forum that probably it is not getting enough water which could be true because I water the lawn about once a week but I don't ever really focus much on the trees because I figured they mostly take care of themselves. So maybe it's the water....

-Beth