Mn and Mg or seaweed extract go one weekly with the blight spray.Should be foliar feeding the things every other blight spray so I can't see it's P or K deficiency.
I haven't put any on that field since early 17 pre beet.If I remember correctly you use a fair bit of the liquid factory waste sludge stuff. Could it be something in that???? I've seen similar in beet where an excessive amount of shed washings along with detergents and disinfectants have been spread on ground.
Just a thought.