Albugo candida (Many races have been identified)
White rust affects every known cruciferous crop. However, this disease is most common on radishes, horseradish, mustard and turnips. Symptoms manifest as chlorotic or necrotic spots on upper leaf surfaces. Later pustules form on abaxial leaf surfaces, small stems and floral parts. Pustules rupture the host epidermis and expose a white, chalky dust of sporangia in small, zonate areas. Occasionally affected portions of leaves are swollen and distorted. On radishes, A. candida causes clubroot-like swellings on the roots. On flower stalks, distorted stems and flowers result in a staghead appearance.
Conditions for Disease Development
Oospores serve as primary inoculum for this disease and can survive for many years in soil or as a contaminant of seeds. Infection is generally favored by cool [13-18°C (55- 64°F)], wet weather in the form of prolonged dews or fog. Sporangia are produced in the pustules and are spread by wind, rain or insects to neighboring plants.
Use Albugo candida-free seed. Incorporate crop debris and eradicate cruciferous weeds. Where practical, implement long rations to non-host species. Apply fungicides to help control this disease.