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Question
My mature cotton wood trees leaves have yellow spots on them, almost a hole in them. The leaves are falling off the trees at an alarming rate. Looks like fall!

I don't know what to do ? Help!

Answer
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. 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.

You nay lose some growth but the tree will not die from this disease. The spores are on the leaves and raking the fallen leaves will reduce the amount of spores thereby reducing the effect on the tree the next year. Nothing to worry about in the long run.

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

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Jim Hyland

Expertise

I am an expert in Forestry, Forest Entomology, Forest Pest Control, and Forest Health. Extensive knowledge in Identification of insects and diseases of trees. Expert on Bark beetles and other insects that attack forests. Also a Registrated Forester with extensive knowledge in the management and care of forests.

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34 years as State Pest Management Chief in a Southern state. Extensive knowledge in Forestry.

BS with major in Forest Management and Entomology
Registered Forester
Certified Pesticide Appicator

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