The yellowing appears to be from the leaf edges back to the veins with some healthy green colour still remaining as a narrow halo around the veins.

The only thing I can think of is Magnesium defficiency.

Does anyone else have any other explanations that fit.

I am not a microbiologist but the symptoms you describe sound more like Phyththora cinnamoni than a nutrient problem. Perhaps a little more information on what soil or other medium is in the pots and whether you used a fungicide such as Fongarid before planting may help get closer to the problem.

Mac