How old are the bushes?

Both too little and too much water can show as yellowing leaves.

Assuming that water is the problem as well as the fact that they are willows, I would lean toward too little water. The weather has been been getting hot lately and it has been quite dry in my area the last couple of weeks.

Sounds like anthracnose to me. fungal disease that is causing all sorts of yellowing leaves right now. Willows are susceptible as are many other trees. It is not fatal, it would be good to clean up the leaves off the ground, especially at the end of the season. You could try an anti-fungal on it, but once it sets in in a season and takes hold, you kinda have to ride it out. I am open to correction. Good luck...Helen.

Thank you Ginkgo and Helen..I appreciate the feedback about my willows.I watered them just a bit today and cut back the yellow leaves. I think that if it's a fungal problem (which I've had with my irises), I'll just go with it for now and hope next year will be better.Strangely enough,they were gorgeous until about 3 weeks ago.Thanks again!

You might be able to determine whether it is lack of water or disease by which leaves are turning yellow and how.
---- if the individual leaves are turning all yellow at one time, like they would in fall, and the oldest leaves on the stems (whereever that may be) are turning yellow while newer ones on the same stem are normal - this indicates lack of water, or too much heat, causing a deficit of water.

---- if the individual leaves yellow with black or brown spots (round or angular), or the yellowing is inconsistant throught the individual leaf - this indicates disease or some other malady. Diseased leaves will tend to be in one part of the plant, for instance the lower part, as opposed to older leaves throughout the plant.