Is that under a porch? Have you been getting a lot of rain? If so, that may be the beginning of rot but I’m guessing in the dark. What are the cure conditions? At what stage of maturity did you prime the leaf? What variety? What is the end use, cigars, cigarettes, etc? The reason I’m asking about the end use, I prime cigar leaf when they are mature, but let cigarette leaf go longer until they are ripe, rugose, and mostly yellow. What are the black looking dots on the back side of the leaf?
I prime cigar leaf at about this stage. This is a mature leaf.
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This is a mature to ripe leaf, for cigarettes I start priming at this stage or wait for even more yellow.
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edit: photos courtesy of Deluxestogie.