Answer: There's a disease call White Rust that can attack horseradish (and radish). This fungal disease affects both leaves and roots, and can overwinter in the soil. Many weeds are also hosts, so removing the affected horseradish plants and cleaning up weed debris will go a long way in helping you control the disease. The fungal spores are spread by wind and often infect leaves when air temperatures are 68F and free water is present. Fungicides are not very effective in the control of White Rust.
I'd plant horseradish in a different part of the garden, and use new plants or fresh seeds, to avoid carrying the disease to the new plot.
Q&A Library Searching Tips