Answer: It's normal for the older leaves to turn yellow. Once they fall off, new leaves will take their places, especially if you pinch the plants back a bit. If the yellowing is on both old and new leaves, I would suspect an insect invasion. Check the undersides of the leaves for signs of pests. Insecticidal soap works well against soft bodied insects such as aphids and whiteflies; Bt works against caterpillars. Once you have identified the pest, you can use the appropriate pesticide.
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