I strongly suspect this is Tuf Voyaging by George R.R. Martin.
Haviland Tuf has found and claimed a "seedship" from a preceding civilization. It was originally intended for ecological warfare, and because of that has an extensive genetic library of every species ever discovered.
The overpopulated world is S'uthlam, where they have a cultural and social imperative to have more children. He provides them with gene-engineered super-wheat and other crops to solve their food problems in return for repairs to his ship. He returns later to find that their constant population growth has taken what should have been a hundred-year improvement in food supply to a mere 13-year margin. He proposes increasingly radical (and less palatable) solutions, including massive fungi and aerial plants.
The food that will make the S'ulthamians infertile is called "manna" and is provided by Tuf on his 3rd visit to S'ultham.
'Yet, meanwhile, the manna shall have completed its real work, First Councillor Mune. The dust that collects upon the underside of each leaf is in actuality a symbiotic microorganism, vital to manna pollination, yet with certain other properties. Borne upon the wind, carried by vermin and human alike, it shall touch every cranny and nook upon the surface of your globe.'
'The dust,' she said. She had gotten it on her fingertips when she touched the manna plant...
Blackjack's growl was so low she felt it more than heard it.
Haviland Tuf folded his hands. 'One might consider manna dust as an organic prophylactic of sorts,' he said. 'Your biotechs will discover that it interferes powerfully, and permanently, with libido in the human male, and fertility in the human female. The mechanisms need not concern you.'
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