I wondered why Alph was allergic to Mango and Avacado in particular so I did a breif research about Mango and Avacado. I discovered that Mango and Avacado both have something in common, both of them contain latex. This concludes in my theory that Alph is allergic to latex (Probably allergic to latex in fruit only?).
These are the references to these theory (I hope these are reliable sources):
http://www.nutrition-and-you.com/mango-fruit.html (allergies section)
(Couldn't find allergy section about mango in the wikipedia so have to find an alternitive)
I am suprised that Alph isn't getting any allergic reaction to the Sunseed Berry (as strawberries also conatin latex):
Feel free to comment or prove me otherwise about this theory