I have three flowering crabs, standard form, ("Coralburst") and two of these trees are flourishing, but one planted within 6 feet of the others can barely muster a bud break and now after a month or warm weather in TX, the two healthy trees are fully leafed out and the one is just barely leafing out. Really dramatic since they are juxtaposed.
I have looked carefully at the bark, no evidence of Apple Scab, Fire Blight
Cedar Apple rust, or Powdery Mildew. Since the whole tree is affected, I am thinking it's Cotton Root Rot.
Any cure or thoughts about this problem?
Well, I'm not sure where in Texas you're located, but I can say that crabapples tend not to be the best suited to our neck of the woods! Crabs are definitely suceptible to cotton root rot. If you're sure your tree is not infected with the other common disease problems you mentioned, and it is cotton root rot, then your other trees will most likely succumb as well. Unfortunately, there is no true "cure" for this fungal disease yet.
You will most likely see disease symptoms show up in June and continue through September. This is when the soil temperatures reach 28oC (82oF). The first symptoms are slight yellowing or bronzing of leaves followed by wilting. Plants die suddenly after the first symptoms of wilting. Leaves remain firmly attached to the plant. Affected plants die suddenly, often after excellent growth. Large trees and shrubs may die more slowly.
The best preventative is healthy soil with a balance of nutrients and soil biology. Controlling the alkalinity of your soil can also help. Solutions include adding sulfur and sometimes sodium to the soil at the rate of 4 to 8 ounces per square foot. Treat the soil with horticultural cornmeal at 10-20 lbs./1,000 sq. ft. Organic soil amendments have proven to provide significant control of the fungal disease.
Plant barriers can also help reduce the spread of the disease. This technique consists of planting resistant species around an obviously infected area. These barriers either exclude or limit the spread of the pathogen. When the disease occurs in an ornamental planting, replace diseased plants with resistant species.
One of the best things you can do to prevent the spread of the disease is to plant resistant varieties in your landscape. If you lose a plant to the disease make sure to replace it with a resistant variety.