[Ema]

I brought this plant this year from a diy store, I put it in the ground next to some French beans 2 months ago. It's had these white spots for at least a week and today they seemed to have spread to a few of the French bean leaves so I trimmed off the French bean leaves but I've left the effected leaves on the horseradish because its a new small plant.

From above
 

Underside


Any idea what it is and how to treat? Thanks

[BabbyAnn]

I'm inclined to think it is a tiny leaf miner or due to its spotty look (especially if nearby beans were affected), maybe sun scorch (through droplets of water)  Believe it or not, horseradish is a member of the brassica family so watch out for cabbage white caterpillars but generally they are incredibly hardy and don't think it will affect the roots that much which is what you are after.  You are very brave planting it direct in the soil - horseradish can quickly become a weed that is difficult to eradicate.

[Goosegirl]

I echo BabbyAnn on both ideas, plus it is VERY invasive!

[Ema]

Thanks the underside of the leaf spots are white and fluffy, the photots dont show this too well.

Do I need to strip the effected leaves off the horseradish? or can they be cut out? Killed with anything?

I know the roots get huge, Im hoping to dig the whole thing out this winter and move a bit of it elsewhere

[Goosegirl]

White and fluffy suggests some sort of fungus to me. Cut off the worst, spray the rest with a fungicide, and you may need to get a mattock out to move it, though it may not have established itself to that degree - YET!

[seaside]

Have never seen that on mine before. The biggest danger to my horseradish is me, as it's ever so easy to forget and mistake the horseradish for a dock initially when they first start shooting.
I'm taking a chance with it in open ground, but so far after 3 years of growing it, every thing is fine. I dig it all up thoroughly in the Autumn and then replace a root or two like new. Seems to work and one doesn't need a ton of the stuff.