Issue with lower leaves wilting

[hunter]

I'm starting to notice that my otherwise healthy seedlings are getting wilting bottom leaves. Below is a lead I pulled today from one of the plants. Not sure what could cause it - or what it is.

I water when the potting soil is fairly dry. Minimum fertilizer (light doses of MG weekly). Fan on to create a breeze 24/7. The only difference is that this year when I potted up to 4" pots, I used MG Organics Potting Soil instead of the soilless mix I typically use.

Any help would be appreciated!