Is this a lower leaf? I know the leaves age over time and will slowly lose strength and die. I'm talking about only the lowest leaves though. How much are you fertilizing right now?

Thanks for replying. It is not the lower leaves. I know how those yellow, but this leaf here was like in the middle of the plant and even some of the top leaves are doing it. At this point I have only fertilized it twice this season. Any idea what else it could be

Fertilizing only twice is not very much. I fertilize mine 3 times a week with a water-soluble and that is in addition to time-release fertilizer I have in the soil. I would up the fertilizing considerably and see if the leaves change.

Could be lack of iron, as I get leaves very much like this when I first take them out of the basement. Brugs benefit from extra iron, and keeping the ph below 5.8. Try some Milorganite, I put a handful or two or 3 on all of my brugs, mostly for the iron. This works 99%. I feed my brugs with triple 20 (that is 20-20-20)twice a month, at 3 level tablespoons to 5 gallons. Miracle Gro sells 4lbs for 9.99 at my local shop.
This provides about 500ppm nitrogen.
Another possibilty is magnesium, this is in the 1% range. Use Epsom salts at 1 level tablespoon to 5 gallons. This is magnesium sulfate, and provides both magnesium and sulfur.

could be sun burn too.