Yellowing leaves doesn't mean they have to be removed. Nutrient deficiency can cause yellowing. Overwatering will cause it. And yes, a disease of some sort can also do it. If pest attack is heavy then you may have contracted something. Having no idea of your location, soil conditions, method of growing, fertilization, watering, I can't really, in good faith, diagnose your problem. If you could provide a bit more information then I'm sure you'll get answers of a more accurate nature. Best of luck.