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A good soil, a good situation, free air and full sun, joined with good manuring, good pruning, and good subsequent culture, will prevent more diseases than the most skilful practitioner would ever be able to cure. There are certain diseases, however, to which roses, under the best circumstances, are more or less liable. Of these, the most common, and perhaps the worst, is mildew. It consists in the formation on the leaves and stems of û sort of minute fungus, sometimes presenting the appearance of a white frost. Though often thought to be the result of dampness, it frequently appears in the dryest weather. Many of the Bourbon roses, and those of the Hybrid Perpétuais nearest akin to the Bourbons, are peculiarly liable to it. In the greenhouse, the best remedy is sulphur, melted and evaporated at a heat not high enough to cause it to burn. In the open air, the flour of sulphur may be sifted over the diseased plants. English florists use a remedy against mildew and other kinds of fungus, which is highly recommended, but of which I cannot speak from trial. It consists in syringing the plants affected with a solution of two ounces of blue vitriol dissolved in a largo stable bucket of water.
The worst enemies of the rose belong to the insect world. Of these there are four, which, in this part of the country, cause far more mischief than all the rest combined. The first is the aphis, or green fly; the second is the rose-slug, or larva of the saw-fly; the third is the leaf-hopper, sometimes called the thrip; and the fourth is the small beetle, popularly called the rose-bug. The first three are vulnerable, and can be got rid of by using the right means. The slug is a small, green, semi-transparent grub, which appears on the leaves of the rose about the middle of June, eats away their vital part, and leaves nothing but a brown skeleton, till at length the whole bush looks as if burned. The aphis clings to the ends of young shoots, and sucks out their sap. It is prolific beyond belief, and a single one will soon increase to thousands. Both are quickly killed by a solution of whale-oil soap, or a strong decoction of tobacco, which should be applied with a syringe in the morning or evening, as the application of any liquid to the leaves of a plant under the hot sun is always injurious. The same remedy will kill the leaf-hopper, which, being much more agile than the others, is best assailed on a cold day, when its activity is to some degree chilled out of it. Both sides of the leaves should be syringed, and the plant thoroughly saturated with the soap or tobacco-water.
Two thorough and well-timed applications will suffice to destroy the year's crop of slugs.
The rose-bug is endowed with a constitution which defies tobacco and soap; and, though innumerable remedies have been proposed, we know no better plan than to pick them off the bushes by hand, or, watching a time when they are chilled with cold, to shake them off upon a cloth laid on the ground beneath. In either case, sure work should be made of them by scalding or crushing them to death.
The following account of the rose-bug and the slug is from Dr. Harris's work on "Insects Injurious to Vegetation:"—
"The saw-fly of the rose, which, as it does not seem to have been described before, may be called Selandria Rosae, from its favorite plant, so nearly resembles the slug-worm saw-fly as not to be distinguished therefrom except by a practised observer. It is also very much like Selandria barda, Vitis, and pygmaea, but has not the red thorax of these three closely-allied species. It is of a deep and shining black color. The first two pairs of legs are brownish-gray, or dirty white, except the thighs, which are almost entirely black. The hind legs are black, with whitish knees. The wings are smoky and transparent, with dark-brown veins, and a brown spot near the middle of the edge of the first pair. The body of the male is a little more than three-twentieths of an inch long; that of the female, one-fifth of an inch or more; and the wings expand nearly or quite two-fifths of an inch. These saw-flies come out of the ground at various times between the 20th of May and the middle of June, during which period they pair, and lay their eggs. The females do not fly much, and may be seen, during most of the day, resting on the leaves; and, when touched, they draw up their legs, and fall to the ground. The males are now active, fly from one rose-bush to another, and hover around their sluggish partners. The latter, when about to lay their eggs, turn a little on one side, unsheathe their saws, and thrust them obliquely into the skin of the leaf, depositing in each incision thus made a single egg. The young begin to hatch in ten days or a fortnight after the eggs are laid. They may sometimes be found on the leaves as early as the 1st of June, but do not usually appear in considerable numbers till the 20th of the same month. How long they are in coming to maturity, I have not particularly observed; but the period of their existence in the caterpillar state probably does not exceed three weeks. They somewhat resemble young slug-worms in form, but are not quite so convex. They have a small, round, yellowish head, with a black dot on each side of it; and are provided with twenty-two short legs. The body is green above, paler at the sides, and yellowish beneath; and it is soft and almost transparent, like jelly. The skin of the back is transversely wrinkled, and covered with minute elevated points; and there are two small, triple-pointed warts on the edge of the first ring, immediately behind the head.
"The gelatinous and sluggish creatures eat the upper surface of the leaf in large, irregular patches, leaving the veins and the skin beneath untouched; and they are sometimes so thick, that not a leaf on the bushes is spared by them, and the whole foliage looks as if it had been scorched by fire, and drops off soon afterwards. They cast their skins several times, leaving them extended and fastened on the leaves: after the last moulting, they lose their semi-transparent and greenish color, and acquire an opaque yellowish hue. They then leave the rose-bushes; some of them slowly creeping down the stem, and others rolling up and dropping off, especially when the bushes are shaken by the wind. Having reached the ground, they burrow to the depth of an inch or more in the earth, where each one makes for itself a small oval cell of grains of earth, cemented with a little gummy silk. Having finished their transformations, and turned to flies within their cells, they come out of the ground early in August, and lay their eggs for a second brood of young. These, in turn, perform their appointed work of destruction in the autumn: they then go into the ground, make their earthen cells, remain therein throughout the winter, and appear in the winged form in the following spring and summer. During several years past, these pernicious vermin have infested the rose-bushes in the vicinity of Boston, and have proved so injurious to them as to have elicited the attention of the Massachusetts Horticultural Society, by whom a premium of one hundred dollars, for the most successful mode of destroying these insects, was offered in the summer of 1840. In the year 1832, I first observed them in the gardens in Cambridge, and then made myself acquainted with their transformations. At that time they had not reached Milton, my former place of residence; and they did not appear in that place till six or seven years later. They now seem to be gradually extending in all directions; and an effectual method for preserving our roses from their attacks has become very desirable to all persons who set any value on this beautiful ornament of our gardens and shrubberies. Showering or syringing the bushes, with a liquor made by mixing with water the juice expressed from tobacco by tobacconists, has been recommended: but some caution is necessary in making this mixture of a proper strength; for, if too strong, it is injurious to plants; and the experiment does not seem, as yet, to have been conducted with sufficient care to insure safety and success. Dusting lime over the plants, when wet with dew, has been tried, and found of some use; but this and all other remedies will probably yield in efficacy to Mr. Haggerston's mixture of whale-oil soap and water, in the proportion of two pounds of the soap to fifteen gallons of water.
"Particular directions, drawn up by Mr. Haggerston himself, for the preparation and use of this simple and cheap application, may be found in the 'Boston Courier' for the 25th of June, 1841, and also in most of our agricultural and horticultural journals of the same time. The utility of this mixture has already been repeatedly mentioned in this treatise, and it may be applied in other cases with advantage. Mr. Haggerston finds that it effectually destroys many kinds of insects; and he particularly mentions plant-lice, red spiders, canker-worms, and a little jumping insect, which has lately been found quite as hurtful to rose-bushes as the slugs or young of the saw-fly. The little insect alluded to has been mistaken for a Thrips, or vinc-frettor: it is, however, a leaf-hopper, or species of Tettiyonia, and is described in