Answer: Based on your description it is difficult to make a diagnosis, but it is possible that the tree is infected with bacterial twig blight, or some type of canker, especially if a neighboring tree showed similar symptoms. Willows are potentially susceptible to a large number of problems and for this reason it is always wonderful to see a large old healthy specimen. You might want to ask if your County Extension (747-8320) can help identify the problem with certainty and then possibly suggest what to do (if anything can be done) to save the tree. If the problem is truly unusual they may be able to send it to a plant pathologist for a better diagnosis. Two last thoughts, and both long shots: one is that ants have nested in and destroyed the root system -- I have seen this happen to large rose bushes, although not to trees. If this is the case, the roots would not longer be able to support the tree's needs and it would die progressively from the tips down in proportion to the root damage. The second is that there is something in the soil (or in your water) slowly affecting the plants. Water softener chemicals for example might be a cause, or some sort of mineral being off balance. Your County Extension should be able to help you look into these types of problems as well.
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