Sorry to hear your having a problem.
I have Furman's in a morning sun afteroon shade
situation here it burns out in full sun. My purples
Raspberry,Pastel Purple, Diane and some of my own
selections in 6 Hours of sun in Clay soils and 100
+degree heat (for about 4 Months and they grow fine)
What is your water like?Soil Ph? Mine is 8.5.
There is a S.greggii "Sierra Linda"(Pink)that is the only one surviving in Phoenix were other greggii's fail.Which purple forms have you tried?Some of the S.lycioides hybrids
should do well for you (Pastel Purple)
Sorry we have a few fungal problems here but nothing like
your describing.
Art

hybridsage thanks for your Input.I will give some of these a whirl next spring. To answer your question about soil and water, the soil is about 6.8 ph up here but the tap water is 8.5. My natural red soil is amended quite heavily when I prepare my planting holes, and the drainage is perfect.Maybe I should keep them quite dry next year regardless of the heat.
John

Greggii are somewhat xeric, but at least for me they prefer to have regular water in the growing season. If we have a month or more of summer drought their growth slows and depending on the cultivar flowering can stop. The ones at the edge of my vegetable garden get twice as large as others grown beyond the reach of the hose. Could it be that salts are building up in your soil? You would think that amending your soil would keep such problem in check.

limy9:
Is your Soil red clay ? Your Ph seems to be ok for S.greggii mine is higher on the water side 10.8.
I have raised beds and occasionally mulch to help
keep the ph in check.Do you have a plant lab you to
send samples of your dead plant off to? I can also
go months without watering.The other plants should help with your problems .
Art

Hybridsage ,The locals call my soil red clay but I tend to call it fine red loam as it is not sticky like adobe clay. Since you say you you go months without watering , maybe thats my dilemma, as I do tend to be too kind to my plants.
I have found that Salvia with oval leaves seem to grow like weeds up here but the narrow pointed ones seem to fail.
John

Wardda thanks for the input. You may well be right on the salt build up thing, The plants seemed to be doing well when they were in pots, but regressed after a few weeks in the ground, however they are still alive but very small now compared to how they first were.If its a salt buildup the winter rains should take care of that.
john

A lot of xeric plants need good drainage. Water that is stationary around the roots will cause root rot. Some aeration of the root system is also vital. Observe how fast new roots develop in potted plants periodically during various parts of the growth cycle to get tips.

I grow my Greggii very hard. No watering at all. They are on a sloping limestone rubble bank and some amendment , not much. They started out slow till they got their roots down but they are doing well now. The texas heat does not budge them. They bloomed like the dickens and are healthy but smaller than the lush plants in town where there soil is richer and water come in an unlimited (but rationed) supply.