Everything was growing great, until the lettuce head at the far right back corner of the garden slowly withered and died. I was confused, I thought maybe because of its location it didn't have enough light or something, and that theory was somewhat supported when the three nearest lettuces to the one that had died started to wither down too. I moved them closer to the sun but with no results.
This morning, one of the newly-dying ones was dead enough for me to give up hope and uproot it for an autopsy. The base was covered in a cottony white mold, which I researched and found something which has symptoms that perfectly match what happens.
Home Depot sold me lettuce that was infected with White Mold/ Lettuce Drop.
Not only is my lettuce probably doomed, but the disease is infectious to several (many?) other crops in my garden. Thanks a freaking lot, Home Depot, my garden is now f**ked.