The majority of Rust-Fungi hibernate in the teleutospore-form. Many species are able to hibernate in the uredospore-form (Coleosporium senecionis). Others pass the winter in the æcidio-form, and develope æcidia on new hosts (Uromyces pisi, on Euphorbia cyparissias; Phragmidium subcorticium, on Rosa; Æcidium elatinum, on Abies alba). In Chrysomyxa abietis, the mycelium, developed from the basidiospores, survives the winter.
Among the Rust-Fungi, with several forms of reproduction, there are about sixty whose development can only be completed by an alternation of hosts, that is, on one host only uredo-and teleutospores are produced, while the further development of the germinating basidiospores, and the formation of the æcidia and spermogonia from its mycelium, can only take place on a second quite distinct and definite host (heterœcious or metoxenous Fungi). Those Fungi which have all their forms of reproduction on the same host are termed autœcious or autoxenous. It is not, however, always necessary that the heterœcious Rust-Fungi should regularly change their hosts; for example, Puccinia graminis can hibernate in the uredo-form on the wild Grasses, and in the spring can distribute itself again in the same form.
As a consequence of the alternation of hosts the various forms of development were considered as independent genera (Uredo, Æcidium, Rœstelia, Cæoma, Peridermium), until De Bary and Oersted established, about the same time (1865), the mutual connection of some forms, and paved the way for the right conception of these Fungi.
Fig. 146.—Puccinia graminis.
As an example of one of the most highly developed species, Puccinia graminis, the “Rust of Wheat,” holds a prominent position. Its uredospores and teleutospores are produced (Fig. [146]) on Grasses (on cereals, especially Wheat, Rye, Oats, and many wild Grasses), while the æcidia and spermogonia are confined to the Berberidaceæ. The teleutospores, developed on the Grasses, hibernate on the dried portions of their host, and in the succeeding year each of the two cells of the teleutospore may develop a basidium with four basidiospores (Fig. [146] D, c). The basidiospores are distributed by the wind, germinate quickly, and only proceed to further development on Berberis or Mahonia. The germ-tube bores through the epidermis of the Barberry-leaf, and forms a mycelium in its interior, its presence being indicated by reddish-yellow spots on the leaf. After 6–10 days the flask-shaped spermogonia appear (Fig. [147] B; C, a; conidia in Fig. [147] D) and a few days later the cup-shaped æcidia (Fig. [147] A; C, c, d, e). The former are generally on the upper, and the latter on the under side of the leaf. The orange-coloured æcidiospores scatter like dust, and germinate only on Grasses; the germination takes place in about two days when placed on any green part of a Grass. The germ-tube enters the Grass-leaf through a stoma; a mycelium is developed in the leaf, giving rise to a small, oval, rust-coloured spot (Fig. [146] A); in about 6–9 days the epidermis is ruptured over the red spot, and numerous reddish-yellow uredospores, formed on the mycelium, are set free. The uredospores (Fig. [146] B) are scattered by the wind, and can germinate should they fall on the green portions of other Grasses: they then emit 2–4 germ-tubes through the equatorially-placed germ-pores. The germ-tubes enter a leaf through a stoma, a new mycelium is then developed, and in about eight days a fresh production of uredospores takes place, which germinate as before. The uredospore-mycelium very soon produces, in addition, the brown teleutospores, which give a brown colour to the rust-coloured spots, the familiar uredospores on the cereals being quite suppressed towards the close of the summer (Fig. [146] C, D). The “Rust of Wheat” hibernates on some wild Grasses in the uredospore-form.
Fig. 147.—Æcidium berberidis. A Portion of lower surface of leaf of Barberry, with cluster-cups (æcidia). B A small portion of leaf, with spermogonia, from above. C Transverse section of leaf on the upper side, in the palisade parenchyma are three spermogonia (a b); on the lower side an unripe æcidium (c d) and two ripe æcidia (d, e, f); f chain of æcidiospores. D Hyphæ, forming conidia.
Genera. Puccinia (Fig. [146], [147]) has bicellular teleutospores, each having a germ-pore, and the æcidia when present have an indented peridium; some species, as exceptions, have 1–3-celled teleutospores. Many species are HETERŒCIOUS, for example, P. graminis, described above; P. rubigo, which also infests various Grasses, but whose æcidia appear on Anchusa; the masses of teleutospores are small; they contain paraphyses, and are for a long time covered by the epidermis. P. coronata, on Oats and Rye Grass; its æcidia on Rhamnus; the teleutospores are surmounted by a crown—“coronate processes.” P. phragmitis, on Reeds; æcidia on species of Rumex and Rheum. P. moliniæ, on Molinia cœrulea; the æcidia on Orchids. P. poarum, on Meadow-Grass; æcidia on Tussilago. Various Puccinias growing on species of Carex have their æcidia on Urtica, Lysimachia, Cirsium, Pedicularis, etc.—Of those AUTŒCIOUS species, which have all their generations on the same host, may be noted:—P. galii, P. menthæ, P. violæ, P. epilobii, P. asparagi, which grow on the hosts from which they have taken their specific names.—As representative of a group which have spermogonia, uredo-and teleutospores on the same host, but on different individuals, P. suaveolens, on the Field-Thistle, may be mentioned. The spermogonia have a strong odour.—A peculiar group (Leptopuccinia) has only teleutospores, which germinate immediately, and whilst still attached to their living host. To this group belong P. arenariæ, on a number of Caryophyllaceæ; and P. malvacearum, on various Malvaceæ, introduced in 1873 from South America to Europe, where it soon proved very destructive to Hollyhocks.
Uromyces (Fig. [149]) differs only from Puccinia in always having unicellular teleutospores. Among this genus both heterœcious and autœcious species are found. To the first group belong U. pisi, whose æcidia are found on Euphorbia cyparissias, and U. dactylidis, whose æcidia appear on Ranunculus; to the second group belong U. betæ, U. phaseoli, U. trifolii.