White Rust (fungus – Albugo candida): White, raised spore masses appear on the underside of leaves. Even though the disease is confined usually to the leaves, it may affect other parts of the plant. The host epidermis is ruptured and powdery masses of spores are produced. Spots may range from minute dots to one-half inch or more in diameter. With mild temperature and high humidity, the disease spreads quickly. Extreme hot or cold temperatures and dry weather will slow the disease progress. The following practices should control white rust: destroy crop residue, and cultivate fields prior to planting to destroy all volunteer plants.
Cercospora Leafspot (fungi – Cercospora cruciferarum and C. atrogrisea): The fungus attacks the cotyledons, leaves, and petioles of the host plant. Lesions are circular with gray, brown or slightly off-white centers with slightly darkened margins. Severe infection can cause yellowing of foliage and eventual defoliation. Young seedlings are killed whereas later infection causes a reduction in size of the edible portion. The fungus is soilborne and can live in several weeds or old volunteer plants. Stromata are formed in the leaves and these overwinter in the fallen foliage. When weather conditions favor fungal development, growth resumes. Spores are formed from this tissue and they are carried by wind or splashed onto the foliage by rain. The disease organism is favored by cool temperatures (55oF to 65oF) and free moisture on the leaves. Deep plowing or removal of infected plant parts will help break the life cycle of the organism. All volunteer members of the cabbage family and cruciferous weeds should be destroyed in and around the area where radishes are to be grown.
Bacterial Leaf Spot (bacterium – Xanthomonas campestris pv. campestris): The bacterium attacks the leaves and petioles causing small tan to white spots with narrow, yellowish, watersoaked zones on the leaves. The spots on the leaf petioles are black, sunken and elongated. Severe infection results in defoliation and, in extremely severe cases, death may occur. The causal organism is carried over in the crop residue and in infected seed. Once a plant is infected, further spread is by insects, rain, etc. During warm spring days, lesions are visible four to five days after infection. In cooler periods development is slower. The bacterium will grow between temperatures of 41oF 94oF, but is favored by temperatures between 80oF 86oF. Field sanitation is important in preventing infection. Rotation will also reduce the possibility of the disease becoming a problem. Use high quality seed. Chemicals are not recommended.
Black Root (fungus – Aphanomyces raphani): The fungus attacks radishes at any stage of plant development. Early infection results in seedling death. Some escapes will occur which, although infected, continue to develop. This often is visible as a deep crater type lesion. Field sanitation and rotation will help reduce the occurrence of black root. Cool wet soil favors development. Planting on a raised bed in well drained soil will prevent losses. Fungicides are not currently recommended for the control of black root.