This is only going on with one of my three plants (Pineapple Express). It started as only a few tiny spots on the lower leaves which got more yellow, and it seems that it's spreading to the top leaves. My grow box is a cupboard that is 2.5x2.5x1 and is lined w/ mylar and has 4 23 watt (100 watt equivalent) 5000k lights, @ about 6" above the plant, placing them closer seemed to cause some heat/light damage. They are in one gallon pots, and I transplanted them last week into Fox Farms Ocean Soil, and have given them only one feeding Friday with Fox Farms Big Grow, at 1/4 strength. The PH is holding at 6.4-6.5. There's a lot of things that seem to have this symptom, so I don't know which it is. Is it pests? I don't see anything on this plant, and again, this is the only plant that has this problem, and the only thing that gets into this cupboard seems to be regular house flies, maybe a couple fruit flies. I included pics below. The first is a pic of the lower leaves, the second shows the beginnings of the yellow dots on the upper leaves, if you look close. Any help would be appreciated...thanks in advance.
Lots of ways... check for spider mite threads If you can control humidty kick it up as hight as you can, they like it hot and dry. So give them cool and wet. Also a spray of 1 part isopropyl alcohol and 3 parts water will start killing them, be sure to spray the underside of the leaves. Like I said there are many products to finish them job... lots of threads on this subject