Two of my younger additions have developed rusty spots on the lower sets of leaves( I have since culled one ). What is the cause. I am using liquid miracle grow at a rate of 14 drops per half gallon( 2 litres ) of water. I do not have a ph tester but the two sisters I had started just two weeks before are doing great with the same untested water, light cycle ( 14/10 ), 400 hps, 25 deg C and 45 % humidity. I do not know the strain I am growing but I am sure they that they are an Indica. Headned