Question
I have several weeping willows in my yard (south-central Tennessee). The largest is about 20' feet tall and four years old. The leaves have developed small, dark spots, and then the leaves yellow and drop. I believe the problem may be fungal.

I've sprayed the leaves with a liquid copper fungicide, fertilized the trees, and raked up the fallen leaves. The problem persists and the leaves continue to yellow and fall. Had the same problem last year, although not quite as severe. We had a cool, wet spring and lower than normal summer temps with above-average rainfall.

The trees are growing beautifully and I don't want to loose them. Help...

Answer
It does sound like leaf spots but this late in the year I would do nothing except rake the leaves when they fall and destroy them by burying or burning. This will cut down the amount of spores that can infect the tree next spring. Then next spring spray the new foliage with a fungicide for leaf spots. What you are doing is the recommendation--the spraying after the spots are there will not eliminate the spots but should prevent new spots.
Spraying three times at two-week intervals with a copper fungicide (Hi-Yield Copper Fungicide, Ferti-lome Black Spot & Powdery Mildew Control) or mancozeb (Bonide Mancozeb Flowable, Southern Ag Dithane M-45, or Dragon Mancozeb Disease Control) starting when the leaves begin to unfurl in spring will provide control. Read and follow all directions on the label.

These spot fungi will not kill the tree but can cause some growth loss due to the loss of foliage but overall are not a real health hazard to the tree.