Custard apples are an edge species native to North America and mostly found in the eastern U.S. both North and South. George Washington ate them for dessert. In the Pacific NW the problem is slugs. The hungry devils girdle saplings whose bark hasn’t toughened up.
Slugs don’t like copper, at least fresh copper without a patina. In the past I’ve hammered used copper pipe flat and made rings to protect young hostas. I had problems with slugs going under the rings on uneven ground.
Solution… BB’s. I think a wide pool of copper coated BB’s might keep slugs at bay. There is shade cloth on our baby trees so the metal shouldn’t super heat and roast the trunks. I also have a big magnet I use on our fire ash to extract the occasional nail that might get repurposed when the mulch gets redressed.