This section is from the book "The Gardener's Monthly And Horticulturist V28", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
In undertaking a subject of such wide scope and vital importance, I realize fully my unfitness to instruct such a body of older and more experi enced men, but as every one's experience is of some value, I trust you will not consider these ten minutes wasted.
In my observation, very few plants are attacked by insects or disease when in vigorous health. It is only when the vitality is impaired, or the growth checked by any cause, that they strike. Red spider rarely troubles plants when growing strongly, and even the mealy bug seems to pursue his ravages more vigorously when growth is slower during the winter months. As instances of this, we find that coleus are badly injured in midwinter by mealy bug, but outgrow their attacks in spring and summer. Bouvardia is another case in point, being one of the worst plants we have for the mealy bug, yet when spring comes, and plant vigor asserts itself, they seem to a great extent to disappear. Mildew attacks your roses when a ventilator is raised carelessly on a cold raw day, and the chilling air strikes down on the soft growth, checking the flowing sap and leaving the plant in a debilitated condition, which invites the fungus known under this name. A marked instance of this occurred in our place years ago. We had a house filled with hybrid roses in full leaf and just showing bud; the house was ventilated by old-fashioned square ventilators that slid up and down. One afternoon they were carelessly left open too long, and the plants under the openings were slightly frozen.
The frost apparently did but little injury, but in two days the plants that had been under the openings were completely covered with mildew, while the rest of the house was comparatively free from it. This showed conclusively, that the affected plants were made liable to the mildew by having their vitality checked by the slight frost. Of late years, one of the most annoying diseases attacking plants is that affecting the carnation, and it is undoubtedly caused by working our stock year after year at a high temperature, which weakens the general vitality, and the disease, be it a fungus or an insect, quickly follows. In the fall of 1883, we had a surplus of two varieties of carnations, and rather than throw them away we "heeled" them in a cold frame, putting straw mats on the glass in extreme weather. They wintered well, and in March we put in a few hundred cuttings of each. We marked them, and last winter they were the best plants we had, not one of them dying off, while we lost hundreds of the same kinds in our regular stock; and I firmly believe that if this plan was adopted of wintering carnations intended for propagation, that the "carnation disease" would disappear.
Another and perhaps more practicable way of avoiding the difficulty we have practiced for years; and that is, to propagate our young stock as early as possible in the winter, and, after they have become established, knocking them out of the pots and putting in shallow boxes in cold frames. This gives them some of the needed rest, and the good effect is very marked. This theory of weakened vitality being the cause, and not the consequence of most plant diseases is, perhaps, best borne out in the case of the "black rust, or verbena rust." It is a common mistake for growers to use for planting out, such plants of verbenas as have been propagated in mid-winter. These plants are usually held in the same pots long after they become pot-bound, and consequently are stunted, and perhaps diseased, when set out. Although they may appear to grow strongly at first, yet the taint is there, and when midsummer comes, with its protracted spells of heat and drouth, the vigor is gone completely, and the insect producing the disease we call "rust" appears in myriads.
The true plan is to use for planting, the last propagated plants in spring, these sustaining no check, grow right along until midsummer, when it is necessary to cut them severely back, and fork in a good dressing of manure as close to the plant as possible, followed up by a thorough soaking of water. This last, of course, if the ground is dry, which is almost invariably the case in August. Plants so handled grow vigorously, avoid the fatal check, and give healthy cuttings when needed in October. The "rust" that is found on heliotropes, bouvardias, etc., is probably the same thing, or in any case is produced by the same cause. This is particularly noticeable in heliotropes, as they become rusted at once if pot-bound. The insect producing "black rust" is invisible to the naked eye, but under the microscope somewhat resembles a cockroach in shape and general appearance. When plants are affected, a syringing twice a week with fir-tree oil is effective in checking it, but as in everything of this kind, prevention is the best remedy.
Although this is a little out of the florist's line, yet a valuable lesson may be drawn from the causes that produce it. The celery "rust" is occasioned by anything that injures the roots, either an excess of rain or a drouth - either cause kills the working roots, and the yellowing up or "rusting" of the leaves soon follows. In the open field this is beyond our control, but the hint given is invaluable in operations under glass, where watering is under our command. There is but little doubt that nine-tenths of the failures in rose growing for flowers in winter is traceable to the working roots of the plants being destroyed by being kept too wet or too dry.
Of course we all know that this insect can be destroyed by fumigation with tobacco, but in cases where cut flowers are grown, particularly roses, tobacco smoke will take the color out of the buds, and to a great extent lessen their value. The "fly" can be kept down by simply spreading tobacco stems about the house, and giving them a dash of water whenever you are watering. The slight fumes that are constantly arising from the tobacco will keep the green fly entirely under subjection. We kept a rose house, 312 feet long and 20 feet wide, entirely free from "fly" with a layer of tobacco stems, 10 inches wide and 2 inches deep, running the full length of the house. It is not safe to put the stems on the bed where plants are growing, as sometimes there are ingredients used in curing the tobacco which will cause injury to the plants. I have known several cases of this. The stems need renewing every six weeks.
 
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