Sounds like a fungas infection. Go to the base of one of the trees, and dig down about a foot close to the base so that you can view the tree. Then with a knife, remove some of the outer skin of the tree, not deep. Look. Do you see some brownish discoloration? Is it light red? Or is it grey in appearance? If its grey then yoiu need to add calcium to the wattering of the tree, and that's not fungas, merely a need for more neutrients. If its light red or brownish, then its a fungas infection, and if it is affecting the trees ability to keep leaves on, then you are too late to save the tree. If you see that there are some green leaves on the tree that have not yet turned yellow, then you may have some time left to kill the infection. So, you will need some bleach water, added with other things: Mix up 3 gallons of water, add 1 1/2 cups of bleach (purex works best), add 1/2 cup of white vinigar; add 1/2 cup of brown sugar. Stir well. Then add 1/3 cup of baking power (not Soda), stir well. And let the mixture sit in full sun for 2 -3 hours. Pour 1/2 gallon of the mixture at the base of one of your trees, and continue until all of your trees have received the mixture (you may have to mix up another batch to feed all of your infected trees), you should see the results within 7 to 9 days. -Birdy