Pretty serious I think. I'd get rid of the affected leaves for certain, don't put them on the compost heap - its a fungus and will live on debris and come back again. If once the leaves are removed, the plants appear healthy and grow new unaffected leaves, then just carry on, but if more appear I'd bin the plants. There's nothing you can do that I know of to treat them with.
You aren't too late to start again with brussels that are resistant. There's one called Bridge which you can get from D.T.Brown or Fothergills.
