I have two Mandarin plants that I grew from seeds. They've been growing pretty well over the last month of two. They are both in pots with potting soil, watered regularly with holes in the bottom to let excess water come out, and I have plant nutrient sticks in each pot which I replace every 60 days. I have two CFL light bulbs with reflectors that I use to supplement the light for them because I feel they don't get sufficient sunlight where I am. I'm been doing this for the past month, they're on a timer and provide 15 hrs of light a day. They've been doing really well, especially after I started using the lights. One of the issues that just recently appeared in the last 3 days is a dry spot on one of the leaves. There are only 5 leaves, hence the concern. It looks orange, has a crisp edge and looks like it dried out. The orange/drying is also even throughout the patch. I have attached a picture. I'm not sure what is wrong as I haven't changed anything. The other plant with the exact same conditions has no issues.