Question
I have a cottonless cottonwood tree that I planted in an area that tends to stay wet hoping, since they like lots of water, that it would do well there.  It's only a very young tree, maybe ten feet tall or so, and for the past couple of summers I've struggled to keep it from losing its leaves.  They start to turn yellow in mid-June and fall off.  Is the issue from it getting too much water or too little?

Answer
Sounds like cottonwood leaf rust. Yellow or orange pustules, containing spores, form on the under-surface of the leaves in midsummer. All rust diseases need two different hosts species to complete the life cycle. The orange pustules (uredia spores) are the summer reproductive state of the fungus. They are followed by dark brown pustules (telia spores) which develop in fall and winter. Here in the South, the alternate host (larch) is not present in the forest, and the fungal life cycle is reduced to the urediaurediospore cycle only. There I would think there are conifer species growing in the area that would act as the alternative host. Some families are immune to rust infection and disease-free trees or groups of trees often occur in the midst of other heavily infected trees. That maybe why one tree is infected and the other is not. Treatment in a yard situation is to rake the fallen leaves and destroy them cutting down the amount of spores which may reduce the amount of yellowing next year. The extent of the yellowing will depend on the weather when the spores from the conifers are germinating (damp cool weather are ideal for the spores).
If the majority of the leaves are effected for several years the rust can cause some growth loss. These is not a fungicide that can be used to prevent the disease.--the removing of the leaves and destroying them will reduce the spores for next year.

Here is a web link to more information on Cottonwood Rust:
http://www.for.gov.bc.ca/hfp/publications/00198/Cottonwood_leaf_rust.htm