The spicy root -- usually a bright ruby red -- is the appeal of annual radishes (Raphanus sativus). As a member of the cabbage family, the radish is susceptible to a variety of pests. Because you eat the entire root, you're probably not keen on using chemical pesticides near your radishes. Instead, use gardening practices to control pests.
Radishes are vulnerable to several types of insect pests, including cutworm, aphids, flea beetles and root maggots. All but the last damage the plants by eating the leaves, stems and sap. Root maggots tunnel into the radish roots, causing root rot and sometimes killing the plant. Radishes are also susceptible to attack from several types of fungus, including those that cause club root, downy mildew, black rot and leaf spot.
Using floating fabric row covers to keep insects away from the plants will help prevent infestations. Spacing plants so there's good air circulation between them can be effective at preventing insect infestations and the growth of fungus by eliminating the dark, moist conditions that pests prefer. Watering early in the day so moisture doesn't remain on the leaves overnight will help, too.
To avoid repeated infestations by pests, rotate your crops and don't grow radishes in the same place more than once during a time period that corresponds to the length of the life cycle of a particular pest, which may be as long as seven years. Removing weeds and debris from the planting bed can help to control insect pests by removing some of their cover and food sources, and it may prevent an infestation of insects or fungal spores in the soil.
You can reduce the numbers of insect pests by attracting beneficial insects that eat the harmful insects. Surrounding young plants with collars made from plastic cups or cardboard bath tissue rolls can protect them from attacks by cutworms, and turning the soil before you plant will expose cutworms that are living underground so birds can more easily find and eat them. You can control the growth of some fungi by making sure your pH is around 7.