So if I wanted to plant blue star junipers near Prairie Fire crabapple this wouldn't work. What are the best scab free crabs?
I garden under old Italian Plums that sucker everwhere and about 3 times a year I'm busy with loppers. Is some safe chemical easier than that?

There's no cultural reason to avoid planting Juniperus squamata 'Blue Star' near crabapple trees in this region. Probably you have heard about cedar-apple rust which does not pertain to this situation. There is a reason to think a flowering crabapple tree that drops its leaves in summer and suckers from the rootstock is having a bad time. Maybe it just needs to be watered more, although if the rootstock sprouts are growing well that might not apply, I suppose.

I didn't understand the 'big words' in the article about crabapples and juniper not getting along, and suspected it wasn't the problem since it wasn't a common warning. But the juniper were in awful condition. The dead areas were getting bigger every year, so I dug them out. I thought suckers were very bad things and a sign of trouble. Am I wrong? I cut them off but they come right back.

Another thing about this tree. I have never seen fruit on it. This year I have seen blue jays fly down from high in its branches with little green orbs in their beaks. They drop it on the ground, then pick up leaves and other debris and drop them on top of what I assume are green crabapples. Is that odd? (Maybe it is normal and I have just not observed it before.)

I don't think we have enough information to properly diagnose your tree. Yes, it does sound sick!

Are the suckers from the roots? That means the roots have suffered some damage. Suckering from the trunk, especially below the graft? Yeah, the trunk might have been hit by a weed eater or something else and now you have a suckering mess. Why are suckers bad? Well, mostly because it's a pain in the rear to keep cutting them back. You don't want the suckers to grow big and start depleting the rest of the tree of it's regular nutrients. As for your magic sucker potion: if it sounds to good to be true...

Dropping leaves in the summer is odd! What color were the leaves? Yellow? Probably to much water. Brown? Might need more.

The birds or other wildlife striping your tree of fruit before the crapapples have a chance to ripen, is perfectly normal.

Fruits shouldn't be green on that cultivar. It's a purple-leaved crabapple. If true to name your plant will have purple leaves and stems, and purple fruits. If you got something else instead (and it's a crabapple) it might be a scab- or rust-susceptible one that is becoming infested and shedding leaves in summer. As with orchard apple trees there is a large number of kinds of flowering crabapple trees, with only certain ones being successful in each region.

Just went out and took a closer look at it. First, most of the leaves are green and not healthy looking - but they are on the tree. They don't look spotted or have holes, etc., just dull gray-green. However, here and there among the green ones, I saw healthy-looking shiny purple leaves with purple stems. Many of the branches have what looks like dried moss on them. And, a few little hard purple fruits! As bad as this sounds, just having leaves on the tree is an improvement over previous years. Second, the suckers look healthiest of all - big purple leaves - much bigger than the leaves on the tree. They grow out of what appear to be tree roots that are barely under the surface.

This tree is in area I have just begun to tackle. Removing old landscape cloth, putting down compost, watching what reemerges (like some beautiful staghorn sumac that was somehow surviving underneath ratty looking juniper.) It would be satisfying to feel I nursed this tree back to health but don't know if it would be an exercise in futility. Don't know if it ever was healthy. There are a variety of trees on this street but this is the only crabapple. Maybe that should be a clue that this isn't the place for them!

Most likely the crabapple has scab, a common problem locally. That may explain the "dull gray-green."

See info here about scab
http://ipmnet.org/plant-disease/disease.cfm?RecordID=364
When you read the list of potential sprays, it's important to know that only those coded with a H in a box are for home use.
Also notice the sites has a link to a list of tolerant cultivars.

The multiple suckers from roots seem to be "normal" for crabs.

Moss on the branches means the tree is growing slowly, also that you have good air quality.

The shriveled fruits maybe leftovers from last year. (The robins usually strip my tree by end of January.)

Here is a link that might be useful: scab; code H in box for home use

Dull gray-green could just be the normal late summer color of a purple-leaved crabapple. If it came with the place and wasn't labeled or otherwise identified for you in some way by the party that planted it then it isn't necessarily the same cultivar - there are many kinds of purple-leaved crabapple trees.

The redder leaves would be new growth, the red tinting diminishing as the leaves age. If the leaves and shoots coming from near the surface of the ground are the same color etc. as those on the rest of the tree these may not be from a rootstock but rather lower on the same tree - perhaps it was actually raised from a cutting rather than grafted, or is even a seedling.

Mulching should help, it usually produces quite an improvement.

Interesting reading on the link above.
My zumi calocarpa should not have scab but it does.....

About that zumi calocarpa

I had to look that up because I'm not familiar with it.

That said, "resistance" against any particular plant disease is seldom, if ever, 100 percent.

As with roses (same plant family) crabapple trees are assigned different levels of resistance to diseases in trials; how resistant each kind is determined to be varies with the regional location. With certain common rose cultivars it is possible to see varying levels of infestation on different sites in the same town. I've even seen a large native stand of wild roses that were all free of rust save for one little section. I have not been back to check to see if it was a more virulent strain that went on to infest the rest of the population or if instead there was a small section of a susceptible individual among a much bigger area of resistant ones.

Since leaf fungi mutate rapidly cultivars free of infestation or lightly infested for years on a site may become more seriously infested later, after a more virulent strain becomes present.