Strange to lose brassicas and onions as well - more than one cause?
Did you use some manure or other amendment on both of them?
Yellowing turnip or brassica leaves, we will always suspect and look for clubroot first. Cabbage maggot on the roots is another possibility.
Our ag people advise not to use manure from animals that were fed with diseased material, as it can be spread that way.
https://www.faa.gov.nl.ca/agrifoods/...pdf/turnip.pdf