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Rust (Under this popular name Fungi)

Under this popular name, Fungi of more than one group are included. The true Rusts, from which the name has been derived, belong to the Uredineae (see Puccinia), to the forms distinguished as Uredo, Caeoma, Trichobasis, AEcidium, and Lecythae, formerly considered true genera, but now regarded only as stages in the development of Puccinia and of allied genera. The conidia, or spores, in these forms are small, round or oval cells, thrown off from the ends of the threads of mycelium, either singly or in rows. They fall off readily, and the surfaces of the diseased plants, and of any body that is rubbed on them, becomes covered with the spores, like iron rust in colour and general appearance. They are, accordingly, known as True Rusts or as Red Rusts. Puccinia includes a very large number of species, some of which (e.g., P. graminis and allies) are called Mildews when occurring on Wheat and other cereals. The more common name for the species of Puccinia and of Phragmidium is Brands; but, occasionally, they are styled Black Rusts. For an account of all these forms, see Phragmidium and Puccinia.

White Rusts differ much from Uredineae, though by some they are associated with them, from the fact that they produce conidia, which break away from the mycelium, and which are grouped in patches, much as in some forms of Uredineae. Like these also they cover bodies rubbed against them with a powdery coating of conidia; but the latter are white, not rusty-red. The White Rusts belong to the genus Cystopus, and are nearly related to Peronospora (which see). There are few species in the genus; and only one, C. candidus, is really hurtful in gardens in this country. It grows on the leaves, stems, and flowers of many of the Cruciferae, causing distortions of these organs, and especially of the flowers, which become much swollen, and remain sterile. It may be found on Cabbage, Turnip, Radish, Horseradish, and many wild Cruciferae, e.g., Shepherd's Purse and Watercress. It is common in North America as well as in Europe. A microscopic examination of a section through one of the white spots, which are plentifully scattered over the diseased organs, shows a layer of mycelium, from which rise branches each of which bears on its tip a beadlike row of conidia. These fall off one after the other. When they fall into water, the contents break up into five or six zoospores, like those in Peronospora. These bodies escape by a hole, which opens at one end of the conidium, and move about for a time in the water by means of two hairs or cilia. Afterwards, they settle down, and push out a mycelium tube, which, on any suitable part of a plant fitted for the nourishment of the Fungus, makes its way in through a stoma, and reproduces the parasite. This process goes on during the summer; but, in autumn, "resting-spores" are formed in the tissues of the host-plants, to pass the winter in a quiescent state, and to propagate the species next summer, when food can be again obtained. These resting-spores are very similar in mode of production, and in appearance, to those of Peronospora. On each of two threads of mycelium, lying among the tissues of the host-plant, a branch is formed. One bears a nearly globular cell, the oogonium, the contents of which shrink a little, and lie loose in the cell, forming the oosphere. The other branch ends in a thickened cell, the antheridium, much smaller than the oogonium. It lies in contact with the latter, into which some of its contents pass through a narrow tube. The oosphere is thus fertilised, and becomes the oospore or resting-spore, distinguished by the possession of a thick, brown cell wall, which bears numerous warty ridges, and is well fitted to resist the cold of winter and other unfavourable influences. In spring, the contents of the resting-spore break up to form a large number of small cells, each capable of reproducing the Fungus in a suitable hostplant. These are set free by the bursting of the cell wall of the resting-spore.

Remedies. The only remedy available is, as far as possible, to limit the multiplication of the Fungus by the removal and burning of all plants, whether weeds or cultivated species, that show it, and by not sowing Cruciferae on any piece of ground for a year or two after diseased crops have been removed from it.