So, I have a friend in NYC who just acquired a begonia for her south facing window. It produces buds, but the buds do not open. After a while, the unopened buds just drop off.

Aside from a few aphids that crawled over from an infested pansy, the begonia looks healthy. She re-potted the plant after she brought it home, but the new pot is a zinc planter without drainage holes. Now, I am aware this may cause problems but the bud dropping seems to be a pre-existing condition.

She has dissected a few buds, and didn't find parasites inside. To me bud dropping seems to be a systemic cultural problem. But I couldn't offer anything useful. Any ideas, guys?