This essay is the more valuable because of the absence of the empiricism just complained of:—
"It is a black, puffy, irregular swelling on the twigs and smaller limbs of plum and cherry trees, and, in one instance that came under my personal observation of peach trees, making its first appearance in the latitude of New York early in June, and attaining its full growth by the end of July. Usually a tree, that is attacked in this manner, is affected worse and worse every year, until it is finally killed; and wherever one tree of a group is affected, the malady usually spreads to them all in process of time. In 1865 whole cherry orchards were destroyed in Western New York by this disease, and I have myself seen many groups of wild plum trees in Illinois that were gradually perishing by it; but in Southern Ohio, as I am told, the Black-knot is never met with. In the Eastern States it has been observed from time immemorial, and various contradictory opinions have been broached as to its real nature and origin.
"In 1865 I watched the Black-knot carefully through all its stages, from its earliest commencement to its complete maturity, experimenting at the same time on numerous specimens collected week after week, so as to ascertain what insects bred in it. The practical conclusion I have arrived at, is simply this:—If the diseased twigs are all cut off and destroyed early in July in the latitude of New York, or a little earlier or later according to the latitude, taking care to cut a few inches below the affected part, the Black-knot can be checked and probably entirely eradicated; but if this operation is delayed till August, it will be of no benefit whatever. Hence we can easily account for a circumstance which has puzzled many men wonderfully, viz.: That cutting off and burning the diseased twigs is pronounced by some to be a sovereign remedy, and by others to be a delusive humbug. Those that do this early enough, find it effectual; those that delay it till too late, find it of no use.
"This perhaps will be sufficient for some few impatient souls, who take everything upon trust that they see in print, and care nothing about the rationale of a mode of treatment, so long as it be practically available. But for the benefit of that large class of intelligent agriculturists, who have been deluded by too many quack prescriptions to place much faith in any man's ipse dixit, and who in any case like to understand the principle of a remedy before they apply it, I subjoin a full account of all that is at present known on this subject, and of the different theories respecting it entertained by different writers.
"Three radically different theories have been broached as to the nature and origin of Black-knot: 1st, that it is a mere disease of the tree, like the cancer or the gout in the human race, which is the view maintained by Dr. Fitch, the State Entomologist of New York; 2d, that it is what naturalists term a "gall," produced by some unknown insect depositing its eggs in the twig—just as the well-known "oak-apples" are produced by a Gall-fly, (Cynips), depositing its egg in the bud of the oak—which is the opinion that I myself formerly held and maintained, before I had fully examined into the subject; (Proceedings Ent. Soc., Phil., III, p.p. 613-618;) and 3d, that it is what botanists term an epiphytous fungus, growing on the tree as a mushroom or toad-stool grows on the ground, which is the opinion of the botanist Schweinitz, and which has recently been re-asserted by Mr. Glover, the Entomologist of the Bureau of Agriculture at Washington, (Ag'l Rep., 1863, p. 572.) This last is the opinion which, upon full inquiry, I have now adopted.
"Before discussing these theories, the facts arrived at by myself in the summer of 1865, must first be briefly noticed. It should be premised that the old, dry Black-knot remains on the tree for many years, and that the place to look for the new Black-knot is on such trees as have been already attacked and are loaded with old Black-knot, without being as yet completely killed by it.
"1st. By the middle of June the new Black-knot is pretty well developed, and may then be readily distinguished from the old by its dull, opaque, brown-black color, while the old is coal-black and more or less glossy. When cut into, it is found to be fleshy inside, like an apple, but not juicy, and of a pale greenish-yellow color, with fibres radiating from the axis of the twig, while the old Black-knot is internally hard and woody, and of a reddish-brown or rust-red color. The brown-black color of the external surface is retained till the last week in July, when the surface of the new Black-knot becomes gradually covered all over with little, coal-black, hemispherical plates, appearing when viewed through a pocket glass, about the size of the head of a pin, each of these is a distinct fungus, named long ago by Schweinitz 'Sphæria morbosa.' Even on the old Black-knot this fungus may be readily seen, at any time of the year, covering its entire surface. So far I have added little to the information already published on this subject, except by the specification of dates. But in addition to these facts, I discovered that about the last of July or the first week in August, there grows from each fungus on the surface of the Black-knot a little cylindrical filament about one-eighth of an inch long, which no doubt bears the seed or "spores," as they are technically termed, of the fungus, and that these filaments very shortly afterwards fall off and disappear, leaving behind them the hemispherical plates, which alone had been hitherto noticed by the botanists. In another Epiphytous fungus, which grows commonly and abundantly in Illinois on the Red Cedar, but which differs from the Black-knot in being attached to the twig by a very short stalk or peduncle, and in being roundish and externally of a reddish-brown color instead of elongate and black, there is a precisely similar phenomenon; except that the plates and filaments are very much larger, and that each filament, when it falls off, leaves a ragged scar behind it. In a single specimen of Black-knot noticed August 6th, I discovered that the filaments not only covered the entire surface of the Black-knot itself, except where a few of them had already fallen off, but that they were thinly studded over the twig for an inch or two above and below the swollen black part; thus proving that the fungus sometimes extends rather further than on a cursory view it would appear to do. Towards the middle of August, the new Black-knot, having perfected its seed, gradually dries up and becomes internally of a reddish-brown color. In other words, like so many other annual plants, it dies shortly after it has perfected its seed, just as a stalk of wheat or of corn dies shortly after the grain is ripe.
"2d. During the months of June and July I collected from time to time very numerous specimens of Black-knot, some of which I cut into to see what larvæ they contained, and some I preserved to see what perfect insects could be bred from them. Besides seven specimens of the common "Curculio," which many persons had previously bred from Black-knot, I bred for the first time therefrom no less than five distinct species of insects, none of which can be considered as gall-makers, but not a single true gall-maker; and I can confirm Dr. Fitch's assertion, that some specimens are wholly free from larvæ of any kind when cut into.
"We will now take up in order the three different theories respecting the nature and origin of Black-knot, which, as already stated, have been maintained by different writers.
"1st. That Black-knot is a mere disease like the cancer.—Dr. Fitch, who maintains this opinion, allows that the black granules found on the Black-knot are a true fungus, 'that the surface of these excrescences, when mature, is always covered with this plant,' and that 'this plant never grows, or at least has never been found, in any other situation.' (Address N.Y. State Ag'l Soc., 1860, p. 21.) * * *