I am not sure what autopsy one can do without a microscope (100-200X). The cruciform hyphae are diagnostic as no other plant pathogen is known to have them. But spotting their brownish mycelia by eye or with a magnifying glass (at least 10X) on a dirty root is not so easy.
Delayed planting makes no difference. While any infectious mycelium will die back the sclerotia can readily live dormant near indefinitely in the soil awaiting a plant root to make contact and have been found down to 12 feet depth. If the organism is present in your soil it is present in your soil. You will likely lose the rest either this summer or the next two if the rootstocks are all the same. But I have to admit it is a weird finger of fate kind of disease. Sometimes trees predicted to be next get skipped for no obvious reason.
By now the leaves should be completely dried on the tree and intact. In fact you should not be able to readily dislodge any part of them if it is Texas Root Rot (TRR) (aka Cotton RR and Ozonium).
The fact you note poor drainage in that specific planting site rather than the others may indicate poor drainage indicative of allowing some other root rot. When you decide to dig down, does the soil in the area appear waterlogged, black with a foul odor?
Presumably your soils are alkaline as you are correct TRR is only found in the arid alkaline soils of the southwest. This fungus thrives in the heavy, calcareous, alkaline soils causing poor soil drainage of the southwest and south central United States and is present throughout the southwestern United States and northern Mexico. Soils low in organic matter are associated with the disease as well. It is most common in the low desert areas where winters are mild, but also occurs at higher elevations, up to at least 5000 ft.
If your property was once alfalfa or cotton fields and flood irrigated would increase the odds of TRR being present.
If you find any of the characteristic brown hyphae I would take them into a Master Gardener or Cooperative Exchange Plant Diagnostic clinic. I’d call ahead and confirm testing availability. Unless their protocol says otherwise, collect several samples of rotting and discolored roots on which the outer or cortical tissue still remains attached. The samples should be pencil size or slightly larger and at least 6 inches long. Leave soil attached and keep the roots cool in a plastic bag (refrigeration is fine). Do not add water or wet paper towels. Submit the sample. [protocol per Jeff Schalau] The sooner after digging up you transport it the better.
This fungus can infect (likely) all dicots (over 2,800 species and growing last count). There is no known treatment for homeowners [some EPA approved commercial antifungals applied to annual fields] or preventative. Some rootstocks or species appear resistant. These are area natives and plants exhibiting drought tolerance … though eventually TRR may take them as well, usually in the hot summer months once soil temps exceed 80F a foot down and the air temperature in the plant canopy is above 104°F.
Citation and Mazzard (cherry) rootstocks appears very susceptible. I no longer plant them. All my losses have been on this until this year when I lost a apricot on Lovell likely due to it [It like another cherry-plum on Citation never broke dormancy while my last cherry on Mazzard swelled up its buds beautifully, popped its first leaf and promptly died. Months later it is still perfectly preserved leading me to believe TRR killed all three. All 3rd year in ground. First losses other than summer.]
“treatments:” [really best practices]
1. Infrequent deep waterings, only when needed
2. Maintain soil pH at 6.5+/-0.3
3. Heavy organic mulch layer to lower soil temperature and increase organic concentration
4. Avoid susceptible rootstocks, plant "resistant" rootstocks
5. Dust or wet bareroots with beneficial soil inoculum before planting trees
http://www.cals.ncsu.edu/course/pp728/Phymatotrichopsis/index.html [2007 ref shows distribution map]
