Ask Mr. Smarty Plants

From a Mr. Smarty Plants previous answer:

Fantex is a cultivar of Fraxinus velutina (Arizona ash). Rather, Fantex is a "sport", or mutant form, of ash that was selected and developed by Fanick's Nursery in San Antonio for its different leaf type—its leaves are smooth; whereas the regular F. velutina leaves are somewhat fuzzy on the underside. There is, however, great variety in the texture of the leaves of this species. Additionally, Fantex apparently has a thicker leaf and is sterile (producing no seeds). It is always grafted onto Fraxinus velutina (which is native to Nevada) root stock according to the Pima Arizona Cooperative Extension. Although the name sounds like it should be a "Texas ash," Fraxinus texensis (Texas ash) is a different species, and native to Texas, but not to Nevada. This more or less puts "Fantex" out of our frame of reference, as hybrids are not considered native plants, since you cannot determine exactly what effect either species that was combined in the hybridization has had. However, we will see if we can find out what may have been causing your ash tree problems. As for the Morus Alba, Fruitless Mulberry, that died the year before, that tree is native to China and also out of our range of expertise, so we have no idea if the same thing caused problems in both trees.

Some characteristics that are common to members of the Fraxinus genus are that they are fast-growing and short-lived. They need a lot of water and fertilizer. One horticulturist has actually labeled them "trash trees" because of their short lives, demanding requirements and dropping of seed pods and leaves.

Ezine articles Arizona Ash Trees - "Arizona ash trees, like many other plants, are susceptible to various pests and diseases. These include cankering, mildews and various fungal infections, leaf scorch, rust diseases, and pests such as mites, webworms, carpenter worms, and borers. Ash trees are particularly vulnerable to Verticillium wilt, which is a soil-borne fungus. In some parts of the country (primarily the midwest), the emerald ash borer has killed many tens of thousands of ash trees. Luckily, Arizona ash tree varieties have not yet been affected by the destructive emerald ash borer. Trees that endure poor environmental conditions are more vulnerable to these problems, so it is important to keep the tree's defenses up by watering and fertilizing adequately.

Purdue University Verticillium Wilt of Shade Trees

USDA Animal and Plant Health Inspection Service Emerald Ash Borer

University of Illinois Extension Ash Tree Problems

Again, since we only have information on the native Fraxinus velutina (velvet ash) and not on the results of grafting which produces the cultivar Fantex, we can't say for sure what might be going on. Since the Arizona Ash is native to Nye County, NV in southwestern Nevada, the climate would seem appropriate, but we noticed several referrals to ash trees needing lots of water. If we were trying to link the decline of both trees, we might point a finger at Verticillium Wilt, which is a soil-borne fungus. We would suggest that you contact the University of Nevada Cooperative Extension Office of Southern Nye Co. to see if other trees in your area are experiencing the same  symptoms, and what treatments are recommended.