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

This is a little out of the water but I am having problems with all my Cottonwood trees dying. I have Oak, Maple, Pecan etc around the pond and they are all doing well. The Cottonwood leaves turn black and fall off. The trees put out new leaves, and they turn black and fall off. Any suggestions? By the way, trees are 4 yrs old and have done great til now.