Question
My cotton less cottonwood tree is losing leaves at an alarming rate. Half the tree is full of dry leaves. The funny thing is the leaves fall in clusters and some of them are new growth and still are green. Most of them are already dead. The twigs are black on the tip of the dead clusters. This has happened every summer but this summer it's really bad. I have treated the tree with "Bayer" in the past thinking aphids because of the small white dots on the black parts.  The trunk is weeping a little at a wound cause by a branch cut off last year.my question is... What is your best guess on what's going on with this tree and can it be treated. Thank you for your time. M

Answer
Sounds like Cottonwood leaf rust. There is a rust that will get on cottonwood leaves causing them to turn and fall early. 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. 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