See:
Seed Crop, Crucifers (Brassica, Eruca, and Raphanus spp.) - White Rust
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. 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. Sporangia 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.
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.
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 the field.
- 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.
- Dynasty (Group 11) at 0.1 to 0.38 fl oz/100 lb seed plus a dye is labeled for other diseases of arugula and may help with white rust, so it can be used on arugula seed in Oregon. 4-hr reentry.
Spray applications after planting, especially if dews or rainy weather are forecasted.
- Badge SC (Group M1) at 1 to 2.25 pints/A on 10-day intervals. 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. Studies by OSU showed a significant reduction in the percentage of diseased 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. May be applied day of harvest. 4-hr reentry.
- Reason 500 SC at 5.5 to 8.2 fl oz/A. Do not apply within 2 days of harvest. 12-hr reentry.
References 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