I have been foliar spraying in the dark for some time now with no problems of mold. I can see why some people have problems with this if they are not careful but with my mixture, spraying at night has always treated me better. I use penetrator, Pro grow, LK, and Purple maxx in my spray ( I also use bushmaster, and gravity as a foliar). I turn off the fan and spray them down, then turn on the fan. My humidity levels are 30%. I have had my levels in the summer turn to 80% and still no mold problems. I do not use sulfur. Using the Humboldt line while the lights are on is not a idea.