Several weeks ago, I put soil and sweet basil seeds into part of a plastic water bottle that I cut. I put one seed in each of the water bottles, and made a hole at the bottom of the bottles to allow excess water to flow out of the bottles. Both of the plants have been in the same exact environment since they germinated. They have never been outdoors. After germinating, both seedlings looked pretty similar until a couple of days ago. The leaves on one are becoming withered and contain white spots. It has gotten worse from one day to the next. Fearing that the container was too small for the roots, I transplanted the seedling into a large pot yesterday.
The first image is what it looked like when I was transplanting it. The other two images are what it looked like roughly 18 hours later. The leaves seem to be getting worse.
08/06/2019 Update: I have not had a chance to cut the pale parts of the leaves out, but I was able to take pictures of what the plant looks like now (24 hours after the last two pictures I uploaded) and I took pictures of the other plant that has been exposed to the same exact conditions (same amount of sun, same temperature, same amount of water, and soil from the same bag). I can't really tell if the problem is getting worse or not. Here are today's images of both the unhealthy looking plant, and the healthy looking one: