Pond Boss Forums AQUATIC VEGETATION Balance of plants in the environment Cottonwood Trees

Sounds like some sort of fungus or leaf rust.

http://www.ipm.uiuc.edu/diseases/series600/rpd605/

http://www.ipm.uiuc.edu/landturf/diseases/anthracnose/

Many good resources on the net. I would take a sample to local cooperative extention for analysis. Good luck.