Most types of plant-related diseases are caused by fungi.
The leaves of this plant have been infected by tar-spot fungus. Fungi can
infect all parts of the plant including leaves, stems, flowers, roots, and
fruit. The physical manifestations of fungal diseases of plants include
wilting, club root, root rot, wood rot, cankers, various types of mildews,
blights, lesions, and leaf spots. The effects of fungal diseases can be
devastating as evidenced by the potato blight that destroyed the Irish potato
harvest of 1845 and caused a widespread famine in Ireland. (Encarta
Encyclopedia)