It's a phosphorus (P) deficiency. Not usually caused by lack of P, but by cool temps. Cool temps in the root zone cause P to be locked up and not available for transport into the roots. Some plants (strains/cultivated varieties (cultivars)) are more susceptible to this than others, which is why you see it more in some plants than others, even when they share the same envirinment. Cool temps above ground can have an effect as well.
It can be caused by pH being out of an accepted range as well, when P bonds to other ions in the electrolyte, those ions depending on which side of the scale you are off. If this is the case, you will see other deficiencies as well, and is a way to identify the source of the problem.