It depends. How old is this plant and where is this leaf located on it? If it is a mature plant and this leaf is at the bottom, it could just be an older leaf that is no longer needed and the plant is shedding it. If this is the case then just cut it off.
If this is a young plant, then my guess would be magnesium deficiency. This can be corrected by feeding or spraying the plant with 1 tablespoon of Epsom salts to 1 gallon of water.
If this leaf is at the top or middle of the plant, then it might be mites. Look with a magnifying glass at the underside of the leaves.
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~ Patti ~
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