I have some begonias rex that are dying. The older leaves start to turn black on the edges and then that black stuff keeps proliferating till all the leave is doomed and drop. The new ones are also affected. When they born, they are immediately attacked by that damned disiese and turn black or get defectuous and small. Is it a fungus? Or heat problems? Or even too much humidity? Can anyone help me? I am desesperated and dont know what to do...