Question
I have a large cottonless cottonwood tree in my front yard, I believe that it is about 10-12 years old. In the last week a majority of the leaves have turned yellow and are falling off in clumps. There is a dark discharge from the tree. I have read that late in the season it usually is not something too worry about and to have a professional look at it but right now I am concerned that it is too early in the season for me to wait for the tree to go dormant to have it treated. It is not even august. and the tree is close the house. what should I do? The tree provides excellent shade.

Answer
By a dark discharge I am assuming you mean that something is dripping from the tree and the leaves and twigs are turning black. OR there is a discharge from areas on the trunk??  

IF it is the first--I would say it is either aphids or a scale insect. These insects  suck the plant juices form the tree and secrete a substance called honeydew which is high in sugar content and sticky. A sooty mold will grow on the honeydew causing the black. Aphids will also cause the leaves to curl and turn yellow and fall. The sticky under the tree and on the leaves is the clue to these insects. IF this is the case then I would treat the tree with an insecticide called Bayer Advanced Tree and Shrub Insect control. It is applied to the soil around the tree and the roots will carry it to the leaves and twigs and when the insect sucks the plant juices it is killed.

IF there is a discharge form areas on the trunk this sounds like borer damage. This is an insect that will enter the trunk at weak spots and bore under the bark. The tree tries to "wash" out the inset that is what the dark wet spots on the bark would be. These can be controlled with the use of an insecticide called Onyx or Merit. Spray the trunk where these wet areas are located, Check with your local garden type store for these products.

There is also 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. 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).
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  


I know I have given you a lot of different things it could be but what you described fits several problems. I hope this helps.