Tare Mould.—The under surface of the leaves of tares, and sometimes also of peas, is liable to attack from an allied species of mould (Peronospora Viciæ). In the spring of 1846 it appeared amongst vetches in some districts to such an extent as at one time to threaten the destruction of the crops; but a succession of dry weather at once abridged its power and limited its mischief. Mouldy vetches and mouldy peas are, especially in moist seasons, evils to which the agriculturist knows his crops to be subject; he may not know, however, that this kind of mould (fig. [266]) is of so near a kin to that which has acquired such wide-spread fame in connection with the potato. Another species of fungus attacks the garden pea in damp seasons, forming small depressed brownish spots on the leaves and pods; but this is quite distinct from the mould, though probably not less injurious.
The fertile threads are produced in dense clusters, each many times branched, and bearing elliptic acrospores obtuse at their apices, and of a violaceous tint (fig. [266]). The oospores are beautifully reticulated and of a yellowish-brown colour ([Plate X.] fig. 212).
Trefoil and some other allied plants are attacked by another species, characterized by Dr. de Bary as Peronospora trifoliorum, which we have found rather plentifully in some localities on lucern.
The Parsnip Mould (Peronospora nivea, Ung.) is found on many umbelliferous plants; but its attacks upon the parsnip are most to be deplored, because it injures and ultimately destroys an article of human food. The plants infested with this parasite are first attacked in the leaves, but afterwards the roots become spotted and diseased in a similar manner to the potatoes attacked by its congener. The disease has not hitherto been so general with the former as the latter; but in some districts it has been far from uncommon.
The fertile threads are collected in bundles, erect, and not so much branched as in many other species. The acrospores are subglobose or ovoid, and papillate at their apices. This species is sometimes called P. umbelliferarum, and sometimes P. macrospora. Generally speaking the average humidity of a season but little affects the production of parasitic fungi. In a dry season, like that of 1864, we found as many species, and these as flourishing and numerous in individuals, as in a proverbially wet year. Such is not the case, however, with the moulds under notice, or such fungi as are reproduced through the medium of zoospores: these are undoubtedly less common in a very dry season; but it must be remembered that a single shower is sufficient for the development of zoospores, and occasional showers or heavy dews will speed them on their course of destruction as readily almost as continuous moisture. The large fungi, on the contrary, become very limited in numbers when the weather is unusually dry.
Spinach Mould.—Spinach is likewise liable to suffer from the establishment of a mould upon the under surface of the leaves: unfortunately this is not unfrequent, and has been known in England certainly for the last fifty or sixty years, since it was figured by Sowerby in his “British Fungi” as many years since. We have lately seen a bed of spinach, utterly destroyed by this fungus; whilst on another, not twenty yards apart, not a spotted leaf could be found. This mould is the Peronospora effusa of botanists; it occurs also on some species of goosefoot ([Plate X.] fig. 215), and probably on knotgrass. To the naked eye it appears in pale purplish-grey patches, which, when examined microscopically, are found to consist of dense bundles of branched threads, bearing ellipsoid acrospores, the membranes of which have a violaceous tint. The oogonia produced upon the mycelium vary considerably in size. The oospores are of the character delineated in our plate ([Plate X.] fig. 214).
Hitherto all the species of mould to which we have had occasion to refer have been found infesting plants more or less employed as food; but there remain one or two other species to which we must make special reference. One of these affects the most universal of favourites amongst flowers: this is the rose mould. Attention was directed to this mould, and it was described for the first time under the name of Peronospora sparsa, in the columns of the Gardeners’ Chronicle, in 1862. It occurred on a quantity of potted rose-plants in a conservatory. Irregular pale brownish discoloured spots appeared on the upper surface of the leaves; these extended rapidly, and in a short time the leaves withered and shrivelled up, and ultimately the whole plant perished. A delicate greyish mould was to be seen by the aid of a lens, scattered over the under surface of the leaves. By the microscope, the branched threads, having the tips furnished with subelliptic spores, were revealed, and an ally of the potato mould found revelling amongst the roses.
During the winter of 1863-4, we found the leaves of several species of dock occupied by a mould which appears to be a very low form of Peronospora. Its presence was indicated by brownish orbicular spots, on which the fertile threads occurred in small bundles. These threads were generally simple, but occasionally forked, bearing rather large elliptical acrospores attached obliquely to the tips of the threads (fig. 269). In consequence of this peculiarity, we have named the species, which does not appear to have been noticed before, Peronospora obliqua. It is clearly very distinct from another species found on dock leaves by Corda.
Of the remaining British species, one (P. Arenariæ) is found on the leaves of the three-veined sandwort (fig. [268]); another attacks the red corn-poppy, a third is found on the common nettle, one on the brooklime, another on the wood-anemone (fig. [267]), and another on the figwort.
Doubtless all the species in this genus are possessed of the third means of reproduction, by zoospores, as discovered in the potato mould, not only from the acrospores, but also from the oospores.