Cause Albugo candida, a fungus-like microorganism, can cause disease on broccoli, cabbage, radish, rapeseed, and wild mustards. Crucifers in general are potential hosts, as well as spinach and horseradish. In the Pacific Northwest, white rust can be severe on crucifer seed crops. The pathogen overwinters as oospores in stagheads (galls formed on infected seed heads) or plant debris as well as mycelium in infected hosts. Pieces of staghorns are often found as seed contaminants and can be the primary source of infection in a field. The oospores can germinate and infect young plants, leading to pustules on leaves. Sporangia are produced in pustules, can be moved by wind, rain, or insects, and can germinate, giving rise to zoospores. Zoospores are motile; they swim for a short distance and then invade the plant by germinating through the stomata. Sporangia require some drying in order to germinate but disease development is favored by moist conditions and temperatures between 50°F and 77°F. Dew, fog, or periods of extended rainfall and cool temperatures are ideal for zoospore activity. White rust is often =associated with downy mildew.
Symptoms First symptoms may appear as small, light green spots, which later turn white and finally result in blister-like, raised, white pustules, usually on the lower leaf surface. Pustules can develop on the upper or lower leaf surfaces or on stems and consist of masses of sporangia. Systemic infections can occur, causing the aboveground portion of young seedlings to be distorted and appear abnormally shaped. Infections of the flower parts may cause a bizarre outgrowth on the seed stalk. Seed pedicels may terminate and form staghorns without seeds developing. Seed yield and quality are severely reduced.
Cultural control
- Plant only seed grown in white rust-free plantings.
- Avoid sprinkler irrigation.
- Practice a 3-year rotation between susceptible crops.
- Select fields away from established radish fields.
- Control wild mustards around the field.
- Plow under infected plants or volunteers and incorporate plant debris into soil soon after harvest.
Chemical control Seed treatment minimizes spread through contaminated seed.
- 42-S Thiram at 6.4 fl oz/100 lb seed plus a dye. See label for reentry restrictions.
Spray applications after planting, especially if dews or rainy weather are forecast.
- Phosphorous acid-based products (Group P7) are labeled for use.
- Fosphite at 1 to 3 quarts/A at 2- to 3-week intervals. 4-hr reentry.
- Fungi-Phite at 1 to 3 quarts/A at 1- to 3-week intervals. 4-hr reentry.