Thank you for your question and for sending photos.

To help me to understand the situation, I have a few questions for you:
1. What is the sun exposure (east, west, south, north)?
2. Is the tree planted in turf?
3. Is it watered by a sprinkler system, drip system or by hand?
4. Can you send a picture of the whole tree? This helps to see the overall health of the tree. 
5. What were the symptoms exhibited that made you diagnose Apple Scab? What fungicide did you use?

From the pictures you sent, the leaves look chlorotic. Yellow leaves with green ribs. Chlorosis is a disorder that can be caused by a number of factors: overwatering, over-fertilizing, compacted 
soils, root damage, root stress or girdling roots can contribute as well. Girdling roots are roots that actually circle the tree underground and often cut off the availability of water and nutrients to the tree. Root damage symptoms often occur on one side of the tree rather than the whole tree. Chlorosis can be caused from the trees inability to pull iron from the soil. We usually have plenty of iron in our soils but the tree may not be able to use the iron from the soil. 

Apple scab symptoms on leaves typically shows up as dark necrotic areas on the leaf. Apple scab is a fungus and you may be able to see the mycelium on the underside of the leaf. Spring Snow variety is susceptible to Apple Scab and Fire Blight, but is resistant to Cedar-apple rust and Powdery mildew. 

Here is a Fact Sheet on Flowering Crabapple Trees with a section on diseases and other disorders: 

https://extension.colostate.edu/topic-areas/yard-garden/flowering-crabapple-trees-7-424/

You can do a bit of exploring in the soil around the tree to check for girdling roots or other root issues. Even though these trees were planted at the same time, there may be something that is causing one tree to show signs of stress. 

I will wait for the answers to my questions and possible another picture. Thank you. 

Thanks so much for the response...

1. Exposure of the front of the house is South east or so, but the house is a ranch, it is sunbaked and gets sun the vast majority of the day. The grass out there takes extra water to stay green. I water overnight and try to use as little as possible without killing the grass. The only shade is provided on the side of the tree that is struggling in the late afternoon by a large pine tree.

2. It is planted in turf with a small area of mulch around it.

3. The water is from a sprinkler system that is mostly designed for the grass.

4. The fungicide I used is fertilome systemic fungicide II, active ingredient is Propiconazole. It attaches to a hose.

5. I thought the leaves resembled those of apple scab (maybe actually cedar-apple rust despite the resistance to it and I am misremembering now) as described online the first year this was happening. Long and short I thought fungus for some reason after looking at the leaves.  To me they don't resemble that this year though, and I was pretty diligent with the fungicide in the Spring the last two years.   I did have an old apple tree die of what I assumed was Fireblight around the time these were planted as well.

Only other relevant info I can think of is we do have pretty heavy clay soils here, it takes a lot of work for multiple years to keep new plantings alive. These two trees were perfectly healthy for 3 or 4 years, then this one started struggling a few years ago.  

Thanks again for any thoughts you have.

Thank you for answering the questions and sending more photos.

The whole tree has pretty sparse foliage but you are right, one side definitely has lost most or all the leaves. I cannot see from the photos where the root flare is. You should see the root flare at the base of the trunk just above soil level. The mulch may be too deep. If you do not see any root flare at the base of the tree when the mulch is removed then the tree may be planted too deep. A tree planted too deep is unable to get adequate moisture, nutrients and oxygen to the roots. You could carefully dig around the tree to check the root system. When a tree is more affected on one side, one of the possibilities is inadequate roots, girdling roots or root damage on that one side. The roots should be going out from the base of the tree and will often extend beyond the canopy or drip line. 

At the same time check the moisture level of the soil. Is it too dry? Or is it too moist? If dry, you may have to supplement the watering . If too wet, then you may have to cut back on the water. Trees do not like to sit in constantly wet soil. 

I cannot say for certain what is going on with your tree, I can only give possible causes. All of the symptoms, chlorosis of the leaves, premature loss of foliage particularly on one side, lack of complete foliage in the whole tree are indicative of a stressed tree. The stress can be from root damage or issues, being planted too deep, too little or too much water, over fertilization or possibly other factors or a combination of these factors.