Do you have any other spotting on the leaves or undersides of your plants?? Do you have thrips in your garden.. When making a post would be important to write if you have issues such as pests going on to trouble shoot.. I still believe looks like black mold.. Or mildew of some sort did u try and run your fingers along it does it seem slimy or mushy? I've seen it before on my buddies grow mildew/mold grew along stems and stalks around upper region of plants In top canopy because of no air flow.. Low humidity doesn't always mean you won't have fungus mold or mildew that meter is only reading your wall or wherever you have it sitting out the sensor in your canopy you will see a huge difference in the repeating trust me
