How do berry mutations work?

Well, I feel silly now, I just found a nice guide that sums it up nicely:

...In essence, what this means is that when certain Berries are planted next to each other, there is a chance for 1 or more of those berries to mutate into a entirely different berry

Known Mutations:

  1. Oran + Peacha = Qualot
  2. Iapapa + Mago = Pomeg
  3. Aspear + Leppa = Hondew
  4. Chesto + Persim = Kelpsy
  5. Kelpsy + Wacan = Apicot
  6. Qualot + Tanga = Ganlon
  7. Aguav + Figy = Grepa
  8. Liechi + Ganlon = Kee
  9. Hondew + Yache = Liechi
  10. Salac + Petaya = Maranga
  11. Pomeg + Kasib = Petaya
  12. Grepa + Roseli = Salac
  13. Sitrus + Lum = Tamato

Sources:

  • Pokemon X and Y Forums
  • Serebii.net
  • Personal Experience (No link for this, sorry)

Lum + sitrus = tamato

I found this one this morning.