Question
I have 2 Cottonless Cottonwood trees that I planted in my yard 3 years ago in Denver Colorado.  At the time of planting they were approximatley 4-5 inches so I imagine they are close to 5 years old.  Today July 15th I noticed that the leaves on one of the trees have all turned yellow and are falling off while the 2nd tree still has all of its leaves which are a dark green.  The trees are approximatley 15-20 feet apart.  Is this tree dying or infected as the other tree looks outstanding.  Any help would be greatly appreciated.

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. Her 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).
To make sure I would call the local office of the CO State Forestry agency and ask one of their Foresters to come out and take a look. Of very well maybe something else. Also the city of Denver may have an Urban Forester on staff that could help. Their numbers should be in the local phone book.

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