A. Judging from your description, most likely your tree has a disease called ash decline. It is important to know the scientific or Latin name of this tree, Fraxinus velutina, because it is called by many, names in the nursery trades including velvet ash, smooth ash and desert ash among others.
|Ash tree showing the first stage of yellowing due to ash decline|
|Close-up of leaves yellowing and scorching due to ash decline.|
In the mid-1980s in the Las Vegas Valley in North Las Vegas on Modesto ash and we tried everything we could to cure, rectify and remedy this problem with no success whatsoever. The trees ultimately died. We involved the state plant pathologist who sent tissue samples to Florida thinking it was a disease called ash yellows and it came back negative.
Personally, if these trees are getting adequate water, not too often and not too little, assume it is ash decline and remove them as soon as possible. We don't know much about the disease, we don't know how it is spread so we should get rid of them and not plant them again until we can figure out what this problem is and how to stop it.
Look for suitable replacement trees. If these replanted in a rock/desert landscape then replace it with a desert landscape tree that will give you the same benefits. To our knowledge this disease is not present in the soil and is restricted to certain types of ash trees so replacement trees should be fine if they are not ash.