The Slime-Fungi (Myxomycetes) are responsible for a few diseases of economic importance. The best known example is the finger-and-toe or club root of crucifers (turnip, swede, cab bage, etc.) due to the organism Plasmodiophora brassicae. This disease is remarkable for the large size of the hypertrophies pro duced on the roots of the affected plants.
The group of the Lower Fungi (Phycomycetes) includes two important disease-producing families. The more important of these is the Peronospora family or downy mildews which cause important diseases of a great number of crops, potato, vine, hop, lettuce, several grasses and many others. The downy mildew of the potato (Phytophthora infestans) produces the most important disease of all, the potato blight, to which reference has already been made. Many seedling diseases ("damping off") also belong here. The most important member of the second family is the organism Synchytrium endobioticum, the cause of the black scab or wart disease of potato.
Within the group of the Ascomycetes there are numerous im portant parasites. Species of .Sclerotinia produce the brown-rot or "mummy" diseases of apple, plum and similar fruits, being among the most destructive orchard parasites. The family of the Erysiphaceae comprises the powdery mildews. These fungi cause the abundant formation on the plant of a powdery mass of summer spores, whence the name of the group. Crops of economic im portance which are attacked by powdery mildews include cereals, clover, vine, hop, gooseberry, strawberry, apple, rose and oak. Species of Exoascus produce the leaf-curl disease of peaches and almonds and a number of interesting if economically unim portant diseases such as witches' brooms on cherry and birch. To the group of Ascomycetes also belong such important diseases as ergot of grasses (Claviceps purpurea), apple canker (Nectria galligena), wheat scab (Gibberella saubinetii), black knot of plum (Plowrightia morbosa), chestnut blight (Endothia parasitica), and apple scab (V enturia inaequalis). The group Basidiomycetes
includes three families of parasites of very great economic impor tance: the smuts, the rusts and the bracket fungi.
The smuts (Ustilagineae) are a peculiar group of parasites which live almost wholly within the host plant and usually direct their attack to the developing flower and fruit. The latter in the typical case is converted into a black powdery mass of spores which constitute the "smutted" head so characteristic of this type of disease. The most important examples are met with among the grasses—viz., the loose and covered smuts of barley, wheat, oats, rye, rice, maize, etc.
The rusts (Uredineae) include perhaps the most important of all plant parasites. The cereal rusts (Puccinia graminis and allied forms) are the best known and economically the most serious, but important parasites occur on apple, plum, coffee, Weymouth pine, and many others, especially herbaceous plants. A striking feature in the life history of some of these rusts is that they pass from one host species to another. Thus some forms of the wheat rust parasite have a stage on the barberry; the Weymouth pine blister rust passes to various species of currant. The elimination of the so-called "alternate host" offers a means of control for certain of these rusts. Hence the "barberry eradication campaign" which is being actively prosecuted in certain wheat-growing countries.
The bracket fungi (Polyporaceae) are a group which attack living trees and timber. These are generally wound parasites which gain entrance through broken branches or other injuries, destroy the wood of the tree, and from time to time produce the well-known bracket-like fructifications on the surface. Species of Polyporus and Fomes attack a large variety of growing trees.
Diseases due to Imperfect Fungi make up an enormous and miscellaneous list, including such as the following: many fruit rots (Botrytis, Penicillium), wilts (Fusarium), stripe disease of cereals (Helminthosporium), and an almost infinite number of leaf, stem and fruit spots (Phoma, Septoria, Gloeosporium, etc.).