I can sympathize with the posters above who have found grafted apples to be a bit problematic.
They are.
I've got trees on 7's, 111's, 118's, and G30.
None of them are as speedy as my age requires.
But they are coming along and are bearing......a little more fruit each year depending upon fox squirrels, raccoons, and possums.
(Deer predation is a little easier to manage with fencing.)
Re: crabs.
My best luck...by far...is in finding a cluster of crabs along a country road, or railroad bed that seemed to be the progeny of the largest crab nearby. I think what such 'clusters' tell me is that the parent is throwing off seed that is coming true to the parent. So, if I like what I see in the parent.....I gather seed. So, far ....after 10yrs of doing this....I'm growing ever happier with the results. Critter predation is less. Bug & disease issues are hugely less.
So that's my story with gathering wild crabs.
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For pears....I have now, for the last 3 years, had fantabulous success with some seed-grown pears.
I gathered these seed in 2008 from two farmyard trees that have been consistent, persistent, prolific producers of fruit. (I also have those same trees on grafted rootstocks...and they are producing like Mom & Dad.)
But it is the seedlings from one of those two trees that now is my habitat 'home run'.
They began putting out significant fruit (about 1" to 1-1/4" diameter) in 2016....and are growing exponentially every year since.
And best yet --- they hold long.....real long. Into January.
And they are the absolute #1 'deer magnet' for my property.
I have a gabillion trail cam shots of deer rearing up on their hind-legs to snip one of those small fruits.
In 2017 & 2018 the fruit hung into late January of those years. (Mom & Dad pear trees are all dropped by mid-November)
This year, 2018/19, my trailcam showed a raccoon found the trees in December and he (she?) wiped out the fruit in a week's worth of work.
But the upshot of the story is that now I am concentrating on growing more seedlings from the parent's seeds.