The diseases to which the Rose is liable are generally owing either to the presence of various Cryptogamic plants, or to the attacks of certain insects whose larvæ are supported at the expense of the plant. Among Cryptogamic parasites which have been observed upon rose-bushes, and which infest chiefly the Provence and other rough-leaved roses, the following are the most troublesome:
Rust.—The rust, when examined by a magnifier, is found to consist of minute yellow spots, each of which is a fungus, Lecythea Rosæ. It is common and injurious to roses, as it frequently covers all the leaves. The most effectual mode of preventing its spreading is to cut off with care and burn all the infected branches, which will sometimes render necessary the destruction of the whole plant.
Mildew.—The minute fungus which produces mildew is called by botanists Sphærotheca pannosa. It appears like a gray mould on the smaller stems and blistered leaves. It is a very troublesome enemy to the Rose, and will sometimes put at defiance every application for its destruction. The most effectual is smoking with sulphur, dusting with dry flowers of sulphur, or syringing with sulphur water. The former should only be practiced by a skillful hand, as too much sulphur-smoke will sometimes entirely kill the plant.
Mould is due to a minute gray fungus, Peronospora sparsa, and manifests its presence by the appearance of irregular pale brownish spots upon the upper surface of the leaf. Upon the under surface of these spots the mould will be found.
Other species of fungi attack the Rose, but they are not sufficiently troublesome to the cultivator to need enumeration here.
The insects which infest the Rose are quite numerous, and their attacks are more or less injurious. The majority of those which are found on the plant in the state of perfect insects are comparatively harmless. The most injurious are those whose larvæ feed on the leaves and pith of the trunk and limbs, and thus destroy the plant; while the perfect insect, like the Green-fly, will simply stop the growth and impair the health of the tree, by fastening upon the green and tender bark of the young shoots, and devouring the sap. It is highly desirable that amateur cultivators should devote more time to the study of Entomology, for upon an intimate acquaintance with the habits of these minute depredators depends, in a greater degree than is generally supposed, the success of cultivation. Our own leisure is so limited, that we have been able to devote very little time to this subject; and we can find no work that treats in detail the insects that attack the Rose. We simply give some account of the most troublesome ones drawn mainly from Harris’ Insects Injurious to Vegetation.
Green-Fly, or Plant-Louse.—Aphis Rosæ.—This very common insect is a scourge to roses, from the facility of its reproduction, and its numerous progeny sometimes entirely cover the leaves, the young sprouts, and the flower buds. Devouring the sap, they are very injurious, and, when numerous, sometimes destroy the plant, while they soil every part on which they collect. The most common species is of a pale green, but there is a variety of a dingy yellow. Many are destroyed by small birds, but they have other enemies, as the larvæ of the Coccinellas, or Lady-birds, and other insects destroy large numbers. The first eggs of the Green-fly are deposited in the autumn, at the base of the buds, and are hatched in the early part of the following spring. Generation after generation is then rapidly produced, numbering sometimes eight or ten before autumn. These are produced alive, and without the intervention of the male. Reaumur estimated that a single Aphis might produce six thousand millions in one summer. The first hatching can be prevented by washing the plant with soft soap and water, or with whale-oil soap, before the buds commence swelling. When the plant is infested with them, it can be washed with tobacco-water and then rinsed in clean water. If in a house, fumigation with tobacco is better. An English writer recommends washing in a decoction of an ounce of quassia to a quart of water, as a very effective and safe remedy. Fumigation is, however, the must thoroughly searching remedy, and can be easily applied to plants in the open air, by means of an empty barrel inverted over the plant, and a pan of burning tobacco.
Gall-Flies.—Several species of Cynips, or Gall-flies, attack the rose, their punctures, made for the purpose of depositing their eggs, being followed by variously formed excrescences containing the larvæ. The Bédéguars, formed by the puncture of the Cynips Rosæ, were formerly employed in medicine as astringents. Harris enumerates the American species as follows:
Cynips bicolor.—“Round, prickly galls, of a reddish color, and rather larger than a pea, may often be seen on rose-bushes. Each of them contains a single grub, and this in due time turns to a gall-fly. Its head and thorax are black, and rough with numerous little pits; its hind-body is polished, and, with the legs, of a brownish-red color. It is a large insect compared with the size of its gall, measuring nearly one-fifth of an inch in length, while the diameter of its gall, not including the prickles, rarely exceeds three-tenths of an inch.”
Cynips dichlocerus, “or the gall-fly with two-colored antennæ, is of a brownish-red or cinnamon color, with four little longitudinal grooves on the top of the thorax, the lower part of the antennæ red, and the remainder black. It varies in being darker sometimes, and measures from one-eighth to three-sixteenths of an inch in length. Great numbers of these gall-flies are bred in the irregular woody galls, or long excrescences, of the stems of rose-bushes.”