It's a cultural thing. Comes from watering, either too much or not enough or both. If you look in the leaf axils, I'll bet you'll see some new leaves coming on. These'll eventually yellow completely and fall off anyway. I'm near enough to the WV border, and I have it on my g'house basils. Not a big issue.

It looks like it's sending out roots; was that part of the stem resting against the soil?

hawkeyext, So, I couldn’t make it out to the farms but did visit some great Hydro stores in the Seattle area (Sodo Hydro and Lucky Greenhouse and Lighting). I am hopefully going to make it out to some farms this weekend or next week, schedule dependent. In talking with the team at Sodo Hydro, they recommended mixing up the sprays if the pests are hard to get rid of. It sounds like you may have already eradicated them so this might not apply any more but this is still a good place for a knowledge dump. The products that they suggested where: Organocide 3-in-1 Garden Spary Captain Jacks Deadbug Brew Micro-Life Biological Mosquito Control (BMC) What they stressed is during an infestation it is good to spray with a mix of different organic compounds. This would normally be achieved by using compost tea and Neem Oil outside; however, most people don’t want to spray compost tea around an indoor area. The main things outside of Neem to use during an infestation are Sesame Oil, Edible Fish Oil, Lemon Juice and beneficial bacteria (good luck if you want to mix them yourself). To my understanding the Captain Jacks adds bacteria to the plant that goes after insects. I would be cautious to anyone looking to spray plants. I always recommend looking up the type of plant and how it reacts to the pest control substance. Even though the compounds are organic, that doesn’t mean a plant or fruit won’t have a bad reaction to it! Also, be sure to give sometime between spray and harvest. I normally just do this as a precaution even though a lot of organic sprays are fine for human consumption (hence why we use them). Now the BMC was more so to go after the flies that are in root system. I would say that I would use this as a last resort and just try to cover up holes as best as possible, but they said it would do the trick to weed and critters out of the water system. I know there are a couple powders that can be used; however, due to the vertical nature I don’t think those would prove as usuful.

Remove the leaves & discard them in the trash, don't compost them.