Cause Albugo candida, a fungus-like microorganism, can cause disease on crucifers in general including arugula, radish, rapeseed (canola), horseradish, leafy brassicas, head and stem brassicas, and wild mustards. In the Pacific Northwest, white rust can be severe on radish seed crops and potentially in other types of cruciferous seed fields including arugula. 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 and 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 found cohabitating 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. Older pustules may degrade. Severely affected leaves may defoliate. 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 and seed pedicels may terminate, resulting in formations known as staghorns or stagheads, where little to no seeds develop. Seed yield and quality can be severely reduced.
Cultural control
- Plant only seed grown in white rust-free plantings.
- Avoid overhead irrigation.
- Practice a 3-year rotation between susceptible crops.
- Select field sites away from established crucifeous crop fields.
- Control wild mustards around cruciferous seed fields.
- Plow under infected plants or volunteers and incorporate plant debris into soil soon after harvest.
Chemical control Seed treatment minimizes white rust spread through contaminated seed.
- Captan 4F (Group M4) at 2.2 fl oz/100 lb seed plus a dye is labeled for other diseases of broccoli, Brussels sprouts, cabbage, cauliflower, radish, and turnip seed and may help with white rust, so it can be used on these seed crops in Oregon.
- Coronet fungicide seed treatment (Group 7 + 11) at 5.1 to 6.2 fl oz/100 lb seed (commercial seed treaters only, not registered for use on radish) plus a dye is registered for other pathogens and may help with white rust, so it can be used on Brassica seed crops in Oregon.
- 42-S Thiram (Group M3) at 8 fl oz/100 lb seed plus a dye is labeled for other diseases of broccoli, Brussels sprouts, cabbage, cauliflower, collards, kale, kohlrabi, mustard, radish, and turnip seed and may help with white rust, so it can be used on these seed crops in Oregon. See label for reentry restrictions.
Use systemic fungicides at planting. To minimize development of resistant fungi, do not make foliar applications for other diseases if Ridomil is used at planting. Do not use in the greenhouse. Ridomil is not registered in British Columbia.
- Ridomil Gold Copper (Group 4 + M1). See label for rates. Do not apply within 7 days of harvest. 48-hr reentry.
Spray applications after planting, especially if dews or rainy weather are in the forecast.
- Badge SC (Group M1) at 1 to 2.25 pints/A on 10-day intervals. Is a poor stand-alone product. Preharvest interval is 0 days. 48-hr reentry.
- Ridomil Gold Copper (Group 4 + M1) at 1 to 2 lb/A on 14-day intervals. Preharvest interval is 7 days. 48-hr reentry.
- Strobilurin fungicides (Group 11) are labeled for use. Do not make more than one (1) application of a Group 11 fungicide before alternating to a labeled fungicide with a different mode of action.
- Cabrio EG at 8 to 16 oz/A on 7- to 14-day intervals is labeled for use on arugula as well as radish, rutabaga, turnip, leafy greens brassicas, and head and stem brassicas, so it can be used on seed production of these crops in Oregon. Studies by OSU showed a significant reduction in the percentage of diseased radish vegetable plants when Cabrio was used compared to nontreated plants (62% vs 2%) under late onset of disease. Preharvest interval is 0 days. 12-hr reentry.
- Quadris Flowable at 6 to 15.5 fl oz/A on 7- to 14-day intervals is labeled for use on arugula as well as radish, rutabaga, turnip, leafy greens brassicas, and head and stem brassicas, so it can be used on seed production of these crops in Oregon. May be applied day of harvest. 4-hr reentry.
- Satori at 6 to 20 fl oz/A on 7- to 14-day intervals is labeled for use on arugula as well as radish, rutabaga, turnip, leafy greens brassicas, and head and stem brassicas, so it can be used on seed production of these crops in Oregon.. Preharvest interval is 0 days. 4-hr reentry.
References Johnson, D.A. 1990. White Rust of Seed Radish. WSU Extension Service EB1570.
Ocamb, C.M., Buckland, K., and Bates, T.A. 2019. Evaluation of materials for foliar diseases on radish in Oregon, 2018. Plant Dis. Manage. Rep.: Rep. No. 13:V094.
Scheck, H.J., and Koike, S.T. 1999. First occurrence of white rust of arugula, caused by Albugo candida. Plant Disease 83:877. https://doi.org/10.1094/PDIS.1999.83.9.877D