Interesting story. I had a bit of a problem following the timeline though. Maybe you could put some type of division between the different periods. This could be as simple as a long dash. From the way I am reading the story, Alberto goes to the family farmhouse, once owned by his great-grandmother. We see him there as a child. As a young man the children of the town, who have eaten the pumpkin candy start to mutilate the adults. At the end of the story, we are back to where he is standing in front of the farmhouse.

I have included a line-by-line review below:
Red - changes needed
Blue - suggestive changes and comments

***Gravel growled and dust clouds puffed behind the Volvo as it braked to a stop in front of the dilapidated farm house. (one word) A door clicked open, a shoe crunched gravel, and the car twitched as the door slammed shut.
***As the nearby city grew, they had sold their farm, except for the small plot containing the house, to developers.
***She had had a small garden where she grew her herbs and pumpkins.
***Pointing to the biggest one, he said (comma) “This one looks good, Grandma.”
***“Grandma (comma) why do you talk to the pumpkins?”
***“To be kind to them. If I don’t, they won’t be kind to us. I talk to them everyday. (two words) That’s why they grow delicious.”
*** “The stew is ready. Would you like some.” (question mark not a period)
***Laughing, she urged. (comma, not a period) “Then, eat more. Here, eat.”
***He blinked. He saw the farm house (one word) in front of him, and a tear rolled down his face.
***Grandma Zeppa had told him the pumpkins would grow bad unless she spoke the words. He had learnt learned the words.

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