[BabbyAnn]

where there is risk of club root, the alternate to mustard is fodder radish

[Salmo]

Fodder radish is a brassica. Does it not get clubroot?

It needs to be sown in August to get a good growth frosts knock it down. Its frost suseptability is about the same as mustard.

[BabbyAnn]

according to this website info : http://www.allotment-garden.org/fertilizer/green-manures.php sow Aug-Sep but does not overwinter 

I have read somewhere that fodder radish is resistant to clubroot, but as a brassica it should of course be incorporated into the crop rotation

[Salmo]

Maybe it does not develop clubroot symptoms because it is only growing for a short time?