I have a very healthy WW plant 4 wks into flowering. Some black marks are showing up on a couple leaves in no particular pattern. (Lights are close (13"-15"), cooled and not emitting any heat on plant. pH is 5.8 so I need to raise it. Humidity never gets above 35-55% or so.) I read that black marks may be a fungus. (Leaves in right photo are at top of plant and are closer to the HPS. Left photo are in middle of plant.) Ideas?