Ph meters will never give an accurate reading with a "slurry" test, your run off is ALWAYS higher when you add water or water with nutes, the whole reason for that is that this is the run off, meaning that anything in excess such as salts, residual nutrients, and even less oxygenated water,( which can and will raise you ph and ppm's). Your plant is seven to eight weeks, senescence is actually normal, in fact you see this in fall/ Autumn with trees. I know you do not know me, I can only tell you, you have nothing to worry about.