Snow fountain cherry trees

Early this year, I was at one of the warehouse-type discount stores, and saw some snow fountain cherry trees in 20-gallon containers for a very good price. I figured "Why not?", because I wanted to put some trees in a park strip anyway.
Well, they did quite well for the first 3 or 4 months. But last month, one declined and died. Now, the other two are following the same pattern. Here's what's going on:
The actual tree trunks (to which they've attached the cherry grafts) are alive and thriving, while the actual cherry grafts are slowly drying up, going brown, and dying. The actual trunks are putting out stems and leaves quite happily - but they aren't, of course, cherry stems! I've made sure that the watering is appropriate, but because the trunks are doing so well, I'm a bit baffled.
Does this sort of thing mean that I got bad trees from the start, or are there environmental/care-related factors that would cause the graft to die while not the trunk?
steve
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The graft has simply failed, which is a common enough occurrence on grafted trees IF insufficient time has been allowed for the graft to take or less than proper care was exercised during the grafting process. This is much more likely to happen on trees which are cheaply mass produced for discounters or box stores than by quality growers. 20 gallon trees are pretty large, so perhaps the graft was attempted when both the root stock and scion were too mature. BTW, that IS a cherry growing from below the graft union, just not a very desirable one.
pam - gardengal

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the other problem might be incompatible grafting rootstock. I had several "dwarf" fruit trees from Miller nurseries do fine for 2 years after planting and then die. typically they graft onto sand cherry which is incompatible. Ingrid

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