Plant Health Problems See Crucifers for a detailed discussion of the other common diseases of this host.
Diseases caused by Fungi:
Black root rot, Aphanomyces raphani.
Radish roots infected with this fungus disease show sunken dark streaks and blotches. As the disease progresses, the blotches may develop into a dry rot which tends to distort the shape of the growing root. The fungus lives in the soil.
To avoid the disease, practice crop rotation.
The turnip aphid, Lipaphis erysismi, green peach aphid, Myzus persicae, and cabbage aphid, Brevicoryne brassicae, have all been recorded from radish. Usually the plants grow so rapidly that they are ready to harvest before aphids seriously injure them and special controls have not been necessary. See Aphid fact sheet.
Cabbage maggot, Delia radicum.
This insect often injures radish, especially early in the season. The gray flies, which look like small houseflies, appear about the middle of May and the females lay eggs on the surface of the soil near the stem of the plant. These hatch into small white maggots, which tunnel in the roots and cause wilting and stunting of the plants. There are three generations each year and the insect overwinters as pupae in the soil. Early spring plantings are most likely to be affected, though late radish may be damaged by a later generation of maggots.
Covering plants with spun-bonded row cover or other fine-mesh material can keep the adult flies from being able to lay eggs on the plants, but be careful to rotate the planting away from the ground where plants in the cabbage family have been grown before and to bury the edges of the cover thoroughly. Beneficial nematodes that attack the maggots in the soil are also available. Diazinon is registered for control of cabbage maggots in Connecticut. To be effective, the chemical must be applied to the soil at the time of planting. Consult the label for dosage rates, safety precautions, and preharvest intervals. When wilting or other damage is visible, it is already too late.
Diamondback moth, Plutella xylostella.
This insect feeds upon radish. Although usually a minor pest, at times this insect mines in and eats small holes through the leaves. The larvae are pale green and only about a quarter inch long when full-grown. They are very active, and when disturbed, wriggle and drop to the ground. The pupa is enclosed in an openwork net attached to the leaves and overwinters in this stage. The adult is a small gray moth with light yellow rear margins on the forewings. It has a wingspread of five-eighths of an inch.
The diamondback moth is usually kept under control by its many effective natural enemies, except when insecticides have eliminated the enemies or in unusually hot weather. Among the compounds registered for control of this pest in Connecticut are formulations of Bacillus thuringiensis var kurstaki (Bt), although populations of caterpillars resistant to Bt are occasionally brought up from the southern U.S. on infested seedlings for transplant. These larvae may be sensitive to Bt var. aizawai. Consult the label for dosage rates, safety precautions, and preharvest intervals.
Garden springtail, Bourletiella hortensis.
This insect occasionally injures young radish plants. Minute dark purple, yellow-spotted insects eat small holes in the leaves. These insects have no wings but are equipped with forked, tail-like appendages by means of which they throw themselves into the air. They are usually found only on small plants near the surface of the soil. Control is not usually necessary.