black root rot fungus: fungus causing black root rot in apples
fungus: an organism of the kingdom Fungi lacking chlorophyll and feeding on organic matter; ranging from unicellular or multicellular organisms to spore-bearing syncytia
respiratory syncytial virus: a paramyxovirus that forms syncytia in tissue culture and that is responsible for severe respiratory diseases such as bronchiolitis and bronchial pneumonia (especially in children)