When turnips become wilted, stunted or die for no discernible reason, it may be that the turnips are infested with root maggots. You may be disgusted to find hordes of squirming white maggots when you dig an affected turnip to discover the root of the problem. Although root maggots are unsettling, it is easy to remove them from a turnip patch. However, this season's infested crops should be discarded. They have likely been munched on even if extensive damage is not visible, and there is no way to solve the problem while the turnips are still in the ground.
Pull up all of the infested turnips, even turnips that have not begun to show above-ground symptoms of root maggot feeding. Burn or throw away all of the uprooted turnips. Do not compost them.
Release insect parasitic nematodes into your garden. Water the turnip patch so that the soil is moist at least to the depth that your turnips were growing, then sprinkle the package of dehydrated nematodes over the soil according to the manufacturer's instructions. Water the soil again, to the same depth as before.
Soak the soil with a pesticide prescribed for use on root maggots, such as one that contains imidacloprid. Follow the manufacturer's instructions for application rates for the pesticide that you choose.
Spread a 2-inch layer of mulch over the next year's turnips. This will inhibit root maggot flies from laying their eggs in the soil.
Cover the planting area with a planting row cover to keep the maggot flies from laying eggs on the soil. Row covers can be purchased from most gardening centers. Lightweight and breathable, they can simply be draped over the planting area and secured at their edges with stakes or nails.
Move the next season's turnip crop to another site far away from the infected site. This will foil any root maggots currently in the soil. You should still apply a row cover or a layer of mulch to prevent root maggot flies from laying their eggs in that location.
Dust the planting area with diatomaceous earth in early May. Diatomaceous earth will dry out any fly maggot eggs laid on your turnip patch. However, the diatomaceous earth will need to be re-applied after every rain until June when root maggot flies are no longer laying eggs.