Rust, Smut, Mildew, & Mould: An Introduction to the Study of Microscopic Fungi

by M. C. Cooke

266.—Pea Mould.
Peronospora Viciæ.

267.—Anemone Mould.
Peronospora pygmæa.

The potato mould has been judiciously named Peronospora infestans, or, as it was at first called, Botrytis infestans; but on a revision of the genera Botrytis and Peronospora, it was transferred to the latter genus, in which it remains. Three names were given to it, within a short period of each other, by different mycologists, in ignorance of its having already received a name. The one we have adopted appears to have the priority, at least of publication, and was given by Dr. Montagne. That of Botrytis devastatrix was given by Madame Libert, and Botrytis fallax by M. Desmazières. The principal feature in this species seems to consist in the branches becoming alternately thickened and constricted, so as to resemble a moniliform string or necklace of little bladders or vesicles. The branches are also more erect than in the allied species, and the spores are solitary on the tips or from the sides of the branches, and not in pairs or clusters, and the tips are simple, and not bifid or trifid, as in most of its allies. It need scarcely be remarked, that a high power of the microscope is necessary to make out the distinctive features of the different members of this genus, and that to the naked eye they only appear as a minute whitish mould. As already stated, this little fungus makes its first appearance on the under surface of the leaves, especially the lower ones, of the potato plant, and afterwards attacks the stem, and ultimately the tuber. For examination it is better to select the leaves soon after the fungus makes its appearance.

Turnip Mould.—Since the advent of the potato murrain a similar disease has been witnessed, though more limited in its extent, amongst Swedish turnips, commencing in little waved irregular lines following the course of the vessels, around which spots are formed by the deposition of dark granules in the same manner as in the potato. In this instance, the leaves apparently are first attacked in a similar manner by a species of mould or Peronospora allied to the one already described, but which has been long known as parasitic upon cruciferous plants, to which the turnip belongs. This species, termed Peronospora parasitica, is white in all stages of its growth. It is much more branched, and the branches are comparatively shorter than in the potato mould, and the tips of the branches are bifid (fig. 262). The acrospores are very large and globose, features also which distinguish this mould from the last. A short time since we were called to witness a bed of splendid cauliflowers, which had, up to that time, been the pride of their cultivator; but, alas! their glory was threatened with speedy annihilation, for in nearly every instance the lower leaves had become more or less covered on their upper surface with yellow spots, and beneath glaucous with the mould we have been describing. The diseased leaves were all immediately removed, but we fear without success, although no positive information has since reached us. The almost unnatural vigorous green of the leaves, prior to the appearance of the mould, is not at all an uncommon occurrence: this phenomenon has been noticed in the ears of corn, in which every grain was soon afterwards filled with spores of bunt.

This species was at one time believed to be parasitic on the “white rust,” from which circumstance its specific name was derived. More precise examination proves that it sometimes occurs where no white rust is present, and therefore its parasitism is imaginary. The suckers in this species are large and penetrate the cells, often entirely filling them. This feature is sufficient to distinguish the mycelium of the mould from that of the “rust.” The turnip mould occurs on many cruciferous plants, and especially on the shepherd’s-purse, as well as upon the different varieties of cabbage and its more immediate allies.

Onion Mould.—Another disease, produced by fungi of the same genus, makes its appearance upon young onion plants in the spring. The mould is called Peronospora Schleideniana, and has many features in common with those already described. In this instance the threads are greyish and erect, with alternate branches, not divided by transverse septa, and the spores are obovate, attenuated towards their base (fig. 263). This mould, in some years, is very common and destructive, by preventing the young plants which are attacked from coming to perfection. It is not confined to the onion, but appears on other allied species of Allium (to which the onion belongs). The threads form large patches or blotches on the leaves, and sometimes cover them entirely. It very much resembles the turnip mould, from which the form of the spores considerably differs. This is the same species as that described by Caspary, and afterwards by Berkeley, under the name of P. destructor.

Lettuce Mould.—A very similar mould (Peronospora gangliformis) is sometimes very common in spring on the under surface of the leaves of the cultivated lettuce, appearing in definite white mouldy spots. By reference to the figure of a portion of a thread magnified (fig. 265), it will be seen that the peculiar form of the tips of the branchlets evidences the distinctness of this species.

The oospores are small, globose, and of a yellowish-tawny colour. This mould is by no means confined to lettuces, but has also been found on species of ragwort, sow-thistle, nipplewort, endive, and other composite plants; and has from time to time received numerous names, which it is unnecessary to enumerate.