Fungi are responsible for by far the greatest number and diversity of plant diseases. All crop plants are apparently attacked by some variety of fungus; often a dozen or more different fungi induce disease on a single species. The distance to which fungus spores may be carried by air currents, for example, is very great. Because of this and because of their astonishing productivity, fungi are responsible for a much larger number of the rapidly spreading, hence epidemic diseases, than are viruses or bacteria. It is the sporadic nature of this disease that brings about the greatest hardships on the individual farmer.
There is the correlation between the mode of dissemination of the fungus and its relation to the host. In general leaf, stem, and fruit diseases are caused by airborne or insect-carried fungi, root diseases by soil inhabiting species. Some vascular wilts are caused by soil fungi, some by fungi possessing insect vectors.
Among the most common and widespread diseases of plants caused by fungi are the various downy mildews (of grape, onion, tobacco, etc), the powdery mildews (of grape, cherry, apple, peach, rose, lilac); the smuts (of maize, wheat, onion); the rusts (of wheat, oats, beans, asparagus, snapdragon, hollyhock); apple scab; brown rot of stone fruits; and various leaf spots, blights and wilts. These diseases are responsible for millions of dollars' worth of damage annually, to growers all over the world.
Certain fungi are strictly local in their effect, producing lesions on leaf, stem or root system, though their localism may at times be an expression of host resistance. Other fungi are selective with respect to particular tissues, as exemplified by the vascular wilt organisms, which are confined to the water-conducting tissues; or the chestnut blight fungus, injuring the cambium layer. Still others are indiscriminate, establishing themselves at various points, and at times destroying the entire plant.