Learning about plant pests and diseases can be a valuable part of gardening, but we sometimes run into the same problem with plant health research as we do with people health research: the more general the symptoms and the more gruesome the disease, the more convinced we become that — sure enough — that's what we have. Goodbye cruel world!
The Desert Garden disease that is most likely to cause this kind of concern is Cotton Root Rot, also called Texas Root Rot. The scientific name is Phymatotrichopsis omnivora and it's that "omnivora" part that really worries us.
P. omnivora is a soil-borne disease that attacks a wide range of plants, there is no way to diagnose its presence in the soil other than to have a plant die, and there is no cure. The symptoms are very general: plants suddenly wilt and die, or they slowly wilt and die; they wilt and die in hot weather, or they wilt and die in cooler weather; the fungus is present in the low desert, and in the high desert, and in many different soil types.
I would comfortably wager that there is not a single gardener in the Desert Garden who hasn't had a plant inexplicably wilt and die during hot weather, cool weather, or somewhere in-between.
And then we look for answers. And what we frequently find as we search is Cotton Root Rot. And we read that the only visible sign of the disease is a white or tan fungal mat on the soil surface that looks like spilled pancake batter. And then sometime later we see a fungal mat that looks like the one in the picture above. And then we know: we're doomed.
Nope, don't worry: that photo above is of a perfectly harmless fungus that appears on the soil surface in warm weather after a bit of rain; it helps decompose organic material and return nutrients to the soil. I get asked about that fungus three or four times a year, so this year I'm making a pre-emptive strike: that is not Cotton Root Rot. You can see the real fungal mat for P. omnivora here.
Other than having an expert carefully examine the fresh and intact root tissue of your dead plant under magnification, there is no way to tell if P. omivora killed your plant or if it's present in the soil.
If you suddenly lost a plant, there's a very good chance the plant just up and died. Sometimes that happens.