Every time the character of Fabio smells apples, he is transported back to his country of origin. He also feels nostalgia from his "lost youth" and feels that he is all alone in the world. Hence, the apples are the symbol of nostalgia, exile, sadness, and memories.
"Those trees are beautiful on the hills," I said.
"Autumn's a lovely season. The trees are getting ready to die, and they show their colors, proud-like."
"No such thing in our own country," I said.
That remark seemed unkind, I realized later. It touched him off on a long deserted tangent, but ever there perhaps. How many times did lonely mind take unpleasant detours away from the familiar winding lanes towards home for fear of this, the remembered hurt, the long lost youth, the grim shadows of the years; how many times indeed, only the exile knows.
There are different opinions about the symbolism behind "apples," "apple trees," and "scent of apples" in these short stories by Bienvenido Santos. According to some, they may represent loneliness or sadness. This is possible but I think that the more likely answer is that apples are symbolic of the past.
There is no doubt that in every instance where apples are mentioned, the narrator is reminiscing about the past. He remembers and misses the past, how things used to be. There is a longing for something that has been lost.
The narrator's love of apples and their scent seems to be symbolic of his love of Filipino culture. He makes known in various ways that he is not happy about being away from his culture.
So apple trees, in my opinion, are about the happy past that the narrator is longing for. You should feel free to think about and develop your own opinions about what apples mean in these stories.