White rust, caused by several species of Albugo, occurs on a wide range of crops including beet, brussels sprout, cabbage, cauliflower, collard, horseradish, kale, lettuce, mustard, parsnip, and radish. In addition, many common weeds and herbaceous ornamentals are attacked.
White rust, sometimes called white blister, is easily recognized by the chalk-white, cheesy, raised spore masses that occur most commonly on the underside of the leaf. The floral parts of radish, cabbage, and cauliflower seed plants are grossly deformed and sterile. Occasionally, swollen galls form in the petioles and stems of some plants and even in the roots of radish, horseradish, and a few other plants. The leaves of systemically infected horseradish plants are usually smaller than normal and may curl inward. The first, and often overlooked, symptom of white rust is the appearance of small, irregular yellow areas on the upper leaf surface. Once spores are released, these yellowish areas die and become reddish brown to brown and may be difficult to distinguish from other diseases.
The pathogen survives from one season to the next as thick-walled spores (oospores) either in the soil, plant debris, or within overwintering and systemically infected crop and weed hosts. Just a few infected volunteer plants can provide primary infection spores to one or more fields. Severe outbreaks commonly occur in the spring and fall months during prolonged periods of cool, dewy nights and slightly warmer days.
Crop rotation and clean tillage (for example, plowing) help reduce the risk of disease by reducing the amount of primary inoculum (spores) in the immediate area. However, plowing may increase soil loss through erosion. As mentioned above, there are several species of Albugo that attack crop and weed plants. However, most of the crop plants affected by white rust are only susceptible to Albugo candida. Because this species is subdivided into several races that are highly specific in the host that they infect, rotating among some of the hosts listed above is beneficial. Control all susceptible weeds and volunteer plants using cultural and chemical controls. Apply recommended fungicides when conditions favor disease.