Uh-oh, what's happening to the leaves?

Last week the seedlings were doing really well, but during this week, the leaves seem to have developed dark patches.  I need to do some googling to find out what browny red patches on beetroot leaves means, but the plants look healthy enough, so I'm not too bothered just yet.

7 week old alto f1 hybrid beetroot seedlings with worrying patches on the leaves

I did scrape a little soil away from one plant to see if any root was starting, but there was nothing obvious.  Must be more patient!