A maggot infestation in carrots and radishes is not caused, as is often assumed, by soil-dwelling pests. It is down to certain species of flies that lay their eggs in the plants. These produce larvae, which then eat into the plants. You can effectively prevent this by using a pest protection net, which almost completely stops flies from laying eggs. Putting a pest protection net over the plants does not adversely affect their growth in any way, on the contrary, it promotes plant growth. Use this natural means of strengthening plants from planting or forcing seedlings.