Answer: Sounds like little root maggots. The adult is a grey fly, half the size of a housefly, which lays white eggs at the base of plants. Legless white maggots, which hatch out of the eggs, tunnel into roots causing rotting of the roots and wilting of the plant.
To control, you'll need to ensure no adult flies can reach your radishes. A floating row cover such as Reemay set over the plants will exclude the flies.
Best wishes with your garden!
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