Hey Btt,
Pardon me for being critical when all others are showering you with accolades, but it appears to me that your plants are starving for nutes, at least wanting more than they are getting. The yellowing of the main stem fan leaves means that the plant is sucking all of the stored nutes from those leaves, and if it goes all the way to the top bud, and only on Day 51, then I read it as a deficiency. Now maybe the nutes are in there, and unavailable to the plant, likely due to a pH problem, it's hard to tell.
And it's probably too late in life to improve those plants much, but for future crops, you'll know what to look for, and can correct it sooner.
Have a great harvest, those plants will reward you many times.