Blue mold, or Peronopora hyoscyami, is an airborne fungus that is common in tobacco fields around the world. It can be mild, present in only a small number of plants, or it can be extreme, destroying an entire crop. The disease gets its name from the blue-like color that it can give to the veins near the damage, even though the spots it leaves tend to be white in color.
You can see blue mold damage to these leaves from a Central American tobacco field in the above video. Two of the spots have been caught before they could do big damage to the leaf, while the third has eaten away at the plant, doing considerable damage. None of these leaves will be used to make cigars—they will be discarded.
While blue mold has been found in Cuba, Connecticut, the Dominican Republic and Nicaragua, some of the worst outbreaks have occurred in Honduras. Nestor Plasencia, one of the world's preeminent growers of cigar tobacco, lost entire crops to the fungus years ago.
Corojo-seed wrapper, which originated on the Cuban farm El Corojo, proved very susceptible to blue mold damage and is no longer grown in Cuba. Hybrids developed by agronomists to be resistant to blue mold have resulted in mixed results, but the seed varieties grown today in much of the cigar world tends to have some level of defense against the disease, and modern-day outbreaks typically are mild and treatable.