Order 1. Erysiphaceæ, Mildews. The Fungi belonging to this order are epiphytic parasites, whose mycelium, somewhat resembling a cobweb, may be seen on the leaves and other green portions of plants (see Figs. [107], [108]). The hyphæ ramify in all directions upon the surface of their host, and emit haustoria which penetrate the epidermal cells, and thus derive the necessary nutriment. The Mildew-Fungi thus belong to the obligate parasites, and during their growth dwarf and destroy the portions of their host on which they live. The reproduction takes place in the first instance by abstriction of conidio-chains from the end of special branches (Fig. [108] c, a hypha is seen in the act of detaching a conidium). The conidia may germinate immediately, and thus quickly reproduce their species. When present in large numbers they appear as a white meal covering the surface of the plant on which the fungus is found. Later on appear the dark brown, spheroid ascocarps (Fig. [108] a) which, although small, are generally just visible to the naked eye as black specks.
Fig. 107.—Erysiphe cichoracearum: a mycelium-threads; b sterile hypha (“pollinodium”); c fertile hypha (ascogone or archicarp); d and e young ascocarps.
A characteristic feature of the Mildew-Fungi is the thin, pseudo-parenchymatous covering of the ascocarp, enclosing one (Podosphæra and Sphærotheca; compare Thelebolus among the Hemiasci) or a few asci (Fig. [108] c), which do not form any hymenium, but are irregularly placed. The cells of the ascocarp-envelope are often prolonged into hair-like appendages. The ascocarps are developed from the mycelium at places where two hyphæ cross each other (Fig. [107]). At these places two short and erect hyphæ are produced side by side. The one from the lower hypha (Fig. [107] c) assumes an ellipsoidal shape, and is known as the archicarp or ascogone, while the other (“pollinodium”) arches over the ascogone. From the latter one ascus may be at once developed (Sphærotheca, etc.), or after its division several asci may be produced, each developed from one division. The sterile hypha (termed “pollinodium,” since it was formerly, but erroneously, supposed to fertilise the ascogone) produces a number of branches, and forms the pseudo-parenchymatous envelope of one cell in thickness, enclosing the asci.
Fig. 108.—Erysiphe communis. A small portion of a leaf with this Fungus growing upon it (considerably magnified). The hyphæ b and d do not belong to this Fungus, but are reproductive organs of a pyrenomycetous Fungus parasitic upon it (Cicinnobolus).
Many plants, both cultivated and wild, are attacked by various species of Mildew. A common means of prevention against their attacks is to dust the diseased parts with sulphur.
Fig. 109.—Eurotium glaucum: α portion of mycelium lying horizontally; β vertically-placed conidiophore; the mycelium gives rise to another branch near α; the conidia are abstricted from short flask-shaped cells; b a ripe conidium; c, d germinating conidia; e spirally-twisted hypha, commencement of an ascocarp; f a stage later; g still later, the hypha at the base of the coil has given off branches which are applied to it; h, i sections of young ascocarps.
Sphærotheca pannosa occurs on the leaves of Roses, and on the fruit of Peaches and Apricots. S. castagnei on Humulus, Cucumis, etc.—Erysiphe tuckeri grows on the leaves and fruit of the Vine; it spins its hyphæ over the bunches of grapes, curtails their growth, and causes them to burst, and to become decayed and rotten (Grape-disease). The Fungus was first noticed in England in 1845, and later was found in all countries where grapes are grown. It is only known in the conidial form (“Oidium tuckeri”). Many other species of Erysiphe are found on herbaceous plants.—Microsphæra has appendages which are repeatedly forked at their extremities. M. grossulariæ on Ribes grossularia.—Uncinula has appendages with spirally-coiled extremities; on Salix and Acer.—Phyllactinia has a circle of bristle-like appendages with dilated bases. P. guttata on Corylus, Fraxinus, Fagus, etc.