Almost all of the time, the problem is cultural, not a disease. They definitely need more light. Light through a window isn't enough. Mine like a four ft, four bulb led shop light. The more they can photosynthesize, the better they can recover from stress. Also, if it IS a disease, you can remove the affected leaves and see if that takes care of the problem. That was why I was asking how many leaves they had.
Good luck!