Mod by Shara RedCat that causes default apples to regrow on trees.
This uses node timers, instead of laggy ABMs, so should be very light to run.
Apples only regrow when they were placed by mapgen, not when they were placed by players, and if you remove the tree leaves, the apples no longer regrow.
NOTE: This functionality will be included in the 0.5 release of Minetest Game. This mod can be used to add apple regrowth early, and can then be disabled when you update to 0.5.
Code for this mod is released under MIT (https://opensource.org/licenses/MIT).