Blue mould of commercial tobacco is caused by Peronospora tabacina, a fungus-like organism that is highly destructive to tobacco seed beds, transplants and production fields in the humid farming zones. Once established, blue mold is fairly easy to identify, although symptoms vary with plant age. On beds of seedlings with leaves up to 4 cm in diameter, blue mold is first seen as circular, yellow areas of diseased seedlings. Plants in the center of the affected area may have distinctly cupped leaves. Some of these leaves should exhibit a gray or bluish downy mold on the lower surface; hence the name blue mold. The upper surfaces of infected leaves will remain almost normal in appearance for 1-2 days before the plants begin to die and turn light brown. Diseased leaves often become twisted so that the lower surfaces turn upward.