noun
1. : a fungus of the genus Penicillium ; especially : a mold that produces blue or blue-green surface growths on bread and other foods — compare green mold
2. also blue mold rot : a plant disease caused by a blue-mold fungus
3. : a serious fungous disease of tobacco seedlings caused by a fungus ( Peronospora tabacina ) and characterized by yellowish spots and a bluish gray mildew on the underside of the leaves — called also downy mildew