Hi, if your plant stems are hard and woody then you can eliminate 95% of diseases. You're using hydro., is the plant root systems o.k. ? does the res. smell ? If yes, you have pythium, or root rot. If your roots are healthy, stems hard then yes, maybe a rodent or cat has decided to sharpen their claws on your babies. White powder normally indicates powdery mildew, but you have no obvious fungus attack. Maybe its just debris from the attack ? Don't forget that if your stems have suffered physical injury, then the healing process will harden stems up. You refer to knuckles at the plant base. This could be entirely normal, or the plant has been bent over and damaged, and the knuckle is the result of stem healing.
In the absence of any other symptoms, I would simply use an anti-fungal preparation to avoid the damaged stems picking up disease. You could also bind the stems with string, cloth or similar to help them heal, if the injury is bad.