Ask Mr. Smarty Plants

We are so sorry to hear about your trees. Carya illinoinensis (pecan) is not native to Nevada; in fact, the nearest state where it is native is Texas. Here are the Growing Conditions for this tree from our Native Plant Database:

Water Use: High
Light Requirement: Sun
Soil Moisture: Moist
CaCO3 Tolerance: Low
Cold Tolerant: yes
Soil Description: Rich, moist, well-drained soils. Sandy, Sandy Loam, Medium Loam, Clay Loam, Clay, Caliche type
Conditions Comments: The sweet, edible nut, makes pecan the best hickory for fruit production. The tree does not bear liberal quantities of fruit in the northern part of its range, but makes an interesting ornamental there. Susceptible to galls, twig girdlers, aphids, borers, weevils, pecan scab, tent caterpillars, and webworms. Slow-growing. Difficult to transplant because of a large taproot.

Some of the things that occur to us from these Growing Conditions are that it is a difficult tree to transplant because of the taproot. Also, it is susceptible to many pests and diseases. There is always the possibility that there were already some problems with the trees that had not become apparent when they were purchased.  It sounds like you did everything you could to protect the trees from the conditions in your area. We are not plant pathologists, and could not begin to diagnose the problem from a distance. We suggest you contact the University of Nevada Extension Office for Clark County. They may, in turn, recommend you have a trained, licensed arborist look at the trees, as do we. If there was pre-existent damage or disease, you might be able to get your money back from the retailer where you purchased the trees.