Cause Albugo candida, a fungus-like microorganism, can cause disease on all crucifers including horseradish, arugula, rapeseed (canola), radish, leafy brassicas, head and stem brassicas, and wild mustards. The fungus overwinters as oospores in plant debris or as mycelium in perennial plants. The oospores can germinate and infect young plants, leading to pustules on the plant. 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 well 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 The fungus may attack any part of the plant: stem, leaves, or flowers. Systemic infections enlarge and distort infected parts. Infected flower parts often lose normal pigmentation and produce chlorophyll. The raised, white, spore-containing pustules, isolated or grouped together to form large patches, are the diagnostic symptom. Pustules have a powdery consistency and are produced on leaves or stems.
Cultural control
- Destroy horseradish debris, especially prior to harvest, which can reduce spread to crowns.
- Plant only roots from uninfested fields.
- Control wild mustards.
- Avoid overhead irrigation.
- Practice a 3-year rotation between susceptible crops.
- Scout for and destroy infected horseradish plants during the spring and early summer.
- Destroy cull piles.
Chemical control
- Strobilurin formulations (Group 11) are labeled for use. Do not apply more than one (1) foliar application of any 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 radish plants with white rust when Cabrio was applied 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 the day of harvest. 4-hr reentry.
- Satori at 6 to 20 fl oz/A on 7- to 14-day intervals. Preharvest interval is 0 days. 4-hr reentry.
References Babadoost, M. 1990. White Rusts of Vegetables. University of Illinois Extension Service PDR No. 960.
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.