Quite some time ago I made orange trees for my Peninsular war 25mm games. Take one commercially available small tree (about 50mm tall) They all are fairly regular in shape, so look 'cultivated' already. Buy a pack of 'dress making' pins (from the habidashery dept of any big dept. store.) the ones with a 'bobble' on the non sharp end. Cut the shaft down to about 10mm & paint it black (you do not want the very sharp point still on when it is in the tree) Then paint the 'bobble' what ever colour you want the fruit to be. I painted mine orange. Put some glue on the shaft and push it into the tree. My tress were done over 18 years ago and are still going fine.
I did see something similar done with polystyrene balls but thought them grossly overscale for oranges, OK for alien fruit though. Afew years ago I did see (at a model railway shop ) a packet of 'fruit' for trees. It was 2 small packs of tiny coloured beads. One pack orange, the other red. I think the idea is you put glue on the tree and (depending how much fruit you want) you roll the tree in the 'fruit' or sprinkle the fruit over the tree.
Good luck on building some.