I'm growing a mint plant indoors from a cutting I took some weeks ago. I rooted the cutting in water and after it had roots I planted it in the container you see in the pic. The cutting was growing relatively well, in fact I took 3 more cuttings from its top which I'm currently rooting in water again (in fact in the pic you can see my cut at the topmost part of the stalk, just above the highest leaves. Those leaves are new, they grew in the 3 days after I cut the stalk).
The problem is that the lower leaves of the plant seem to be decaying and turning white or brown. They are translucent to light and look shrivelled and wilted. The bottom of the stem is turning black. The top of the stem still pushes out new little leaves, but after a day or two they also get spotty and withered and die off. Then the plant grows new tiny leaves somewhere else and those also die until there's no new nodes for it to sprout from. I had this happen to another cutting and eventually it completely died off.
Environmental conditions: I currently have this mint under a grow light programmed to run for 12 hours a day. Temperature is 26°C and relative humidity is 68%. I think I may have over-watered it a bit, I kept the pot pretty waterlogged for a few days after cutting. The pot is a food container in which I poked holes for drainage.
Remedial actions attempted: I've stopped watering the mint since yesterday, and 24 hours ago I put a small computer fan near the mint set to very gently blow on it constantly because another post I read here said that airflow helps stop fungal diseases - however so far it seems to have made things worse.
After taking that pic I sprayed it with a solution of dish soap, baking soda, vegetable oil and water (recommended by some websites for addressing mold and pests).
Any ideas what's killing my mint, and what remedial actions I can take to save this plant or the ones I grow after it?
