Blue mold is a downy mildew disease of tobacco that is caused by Peronospora tabacina, a fungus-like protist in the family Peronosporaceae. Blue mold can cause severe damage to susceptible tobacco plants in many parts of the world. Since it is an obligate parasite, it must have living tobacco tissue in order to grow.
Although tobacco remains the primary host for this disease, other hosts including pepper plants and eggplants can also be reservoirs.
The term blue mold may also be applied to Penicillium roqueforti, a mold used in producing the taste and characteristic blue-green inclusions in Roquefort and other blue cheeses.
A powdery mildew disease of hops has sometimes been erroneously called blue mold. It is caused by Sphaerotheca humuli, a fungus in the order Erysiphales. This hops disease became epidemic in the New York state hops industry in 1909, ultimately leading to the abandonment of the crop in the Northeastern USA.