Are you getting small rust spots anywhere as well?
Could be a magnesium deficiency.
But you will see such deficiency results if the roots are wet/cold or if the soil is to acidic/cold.
Im pretty sure sure shot is right with that mag deficiency. I had hard water where im at and the calcium in the water was locking out the mag. Get some epsom salt and you should be fine
i use 1 teaspoon epsom salt every 3erd water if done weekly er every5 days, and 1 fourth apple cider vinegar, i add these to one gallon of water, use the fish tank drops for your waters chlorine , works great for me.