Question
I have several curly willows here in northern California near San Francisco, a medium size one has died from the tips of the branches on down, with dark brown circular areas along the branches.  I have found the same dark brown spots on the largest tree. One side of the trees are watered with the garden sprinklers the other 1/2 is in a dry field.  Thank you for any help to save these trees.

Answer
Sounds like you may be descrbing a fungal canker disease.  There may be no real cure for the problem.  If you can spray the entire tree, you can try to use a fungicide applied as a foliar spray.  I would use Daconil fungicide according to label instructions for ornamental trees.  You can concentrate you spray to the branches and try to wet the bark really good.  The fungicide is a protectant and will not kill what is already there.  There are no good systemic fungicides for use on trees.  I wish you Good Luck.