Chapter II: THEORIES.—A variety of theories exist as to the causes of this

disease. Some are mere imaginations; some are only ingenious; and some so near to what I suppose to be the truth, that it is hardly possible to imagine how the discovery was not made.

The injury is done in the fall, but is not seen till spring or summer, or even the next fall. Thus, six months or a year intervene between the _cause_ and the _effect_—a sufficient reason for the difficulty of detecting the origin of the evil.

1. Some have alleged that the rays of the sun, passing through vapors which arise about the trees, concentrate upon the branches, and destroy them by the literal energy of fire. Were this true, the young and tender shoots would suffer first and most; all pear-trees would suffer alike; moist and hot summers would be affected with blight; herbaceous plants would suffer more than ligneous: all of which results are contrary to facts.

2. Some have supposed the soil to contain deleterious substances, or to be wanting in properties necessary to health. But in either case such a cause of the blight appears untrue, when we consider that trees suffer in all soils, rich or poor; that, in the same soil, one tree is blighted and the next tree escapes; that they will flourish for twenty years and then blight; that a tree partially diseased recovers, and thrives for ten or more years without recurrence of blight.

3. It has been attributed to violent and sudden changes of temperature in the air and of moisture in the earth; to sudden change from sward to high tillage; and the result is stated to be an “overplus” of sap, or a “surfeit.” All these causes occur every year; but the blight does not every year follow them. Changes of temperature, and violent changes in the condition of the soil, may be _allied_ with the true cause. But when _only_ these things exist, no blight follows.

4. Others have attributed the disease to over-stimulation by high manuring, or constant tillage; and it has been said that covering the roots with stones and rubbish, or laying the orchard down to grass, would prevent the evil. Facts warrant no such conclusions. Pear-trees in Gibson County, Indiana, on a clay soil, with blue slaty subsoil, were affected this year more severely than any of which we have heard. Pears in southern parts of this State, on red clay, where the ground had long been neglected, suffered as much as along the rich bottom lands of the Wabash about Vincennes. If there was any difference it was in favor of the richest land. About Mooresville, Morgan County, Indiana, pears have been generally affected, and those in grass lands _as much_ as those in open soils. Aside from these facts, it is well known that pear-trees do not blight in those seasons when they make the rankest growth more than in others. They will thrive rampantly for years, no evil arising from their luxuriance, and then suddenly die of blight.

5. It has been supposed by a few to be the effect of _age_, the disease beginning on old varieties, and propagated upon new varieties by contagion. Were this the true cause, we should expect it to be most frequently developed in those pear regions where old varieties most abound. But this disease seems to be so little known in England, that Loudon, in his elaborate _Encyclopedia of Gardening_, does not even mention it. Mr. Manning’s statement will be given further on, to the same purport.

6. Insect theory: The confidence with which eastern cultivators pronounce the cause to be an _insect_, has in part served to cover up singular discrepancies in the separate statements in respect to the ravages, and even the species of this destroyer. The _Genesee Farmer_ of July, 1843, says, “the cause of the disease was for many years a matter of dispute, and is so still by some persons; but the majority are now fully convinced that it is the work of an insect (_scolytus pyri_).” T. W. Harris, in his work on insects, speaks of the minuteness and obscure habits of this insect, as “reasons why it has eluded the researches of those persons who disbelieve in its existence as the cause of the blasting of the limbs of the _pear-tree_.” Dr. Harris evidently supposed, until so late as 1843, that this insect infested only the _pear-tree_; for he says, “the discovery of the blight-beetle in the limbs of the apple-tree, is a new fact in natural history; but it is easily accounted for, because this tree belongs not only to the same natural group, but also to the same genus as the pear-tree. It is not, therefore, surprising, that both the pear and the apple-tree should occasionally be attacked by the same insect.” [See an article in the _Massachusetts Ploughman_, summer of 1843, quoted in _Genesee Farmer_, July, 1843.]

This insect is said to eat through the _alburnum_, the hard wood, and even a part of the pith, and to destroy the branch by separation of part from part, as a saw would. On these facts, which there is no room to question, we make two remarks.

1st. That the blight thus produced is _limited_, and probably sectional or local. No account has met my eye which leads me to suppose that any considerable injury has been done by it. Mr. Manning, of Salem, Mass., in the second edition of his “Book of Flowers,” states that he has never “_had any trees affected by it_”—the blight. Yet his garden and nursery has existed for twenty years, and contained immense numbers of trees.

2d. It is very plain that neither Mr. Lowell, originally, nor Dr. Harris, nor any who describe the blight as caused by the blight-beetle, had any notion of that disease which passes by the same name in the middle and western States. The blight of the _scolytus pyri_ is a mere _girdling_ of the branches—a mechanical separation of parts; and no mention is made of the most striking facts incident to the great blight—the viscid unctuous sap; the bursting of the bark, through which it issues; and its poisonous effects on the young shoots upon which it drops.

We do not doubt the insect-blight; but we are sure that it is not _our_ blight. We feel very confident, also, that this blight, which from its devastations may be called the great blight, has been felt in New England, in connection with the insect-blight, and confounded with it, and the effects of two different causes happening to appear in conjunction, have been attributed to one, and the least influential cause. The writer in Fessenden’s _American Gardener_ (Mr. Lowell?) says of the blight, “it is sometimes so rapid in its progress, that in a few hours from its first appearance the whole tree will appear to be mortally diseased.” This is not insect-blight; for did the blight-beetle eat so _suddenly_ around the whole _trunk_? Now here is a striking appearance of the great blight, confounded with the minor blight, as we think will appear in the sequel.

This theory has stood in the way of a discovery of the true cause of the great blight; for every cultivator has gone in search of insects; they have been found in great plenty, and in great variety of species, and their harmless presence accused with all the mischief of the season. A writer in the _Farmer’s Advocate_, Jamestown, N. C., discerned the fire-blight, and traced it to “small, _red_, pellucid insects, briskly moving from place to place on the branches.” This is not the _scolytus pyri_ of Prof. Peck and Dr. Harris.

Dr. Mosher, of Cincinnati, in a letter published in the _Farmer and Gardener_ for June, 1844, describes a third insect—“_very minute brown-colored aphides_, snugly secreted in the axilla of every leaf on several small branches; … most of them were busily engaged with their proboscis inserted through the tender cuticle of this part of the _petiole_ of the leaf, feasting upon the _vital juices_ of the tree. The leaves being thus deprived of the necessary sap for nourishment and elaboration soon perished, … while all that part of the branch and trunk below, dependent upon the elaborated sap of the deadened leaves above, shrunk, turned black, and dried up,” p. 261.

Lindley, in his work on _Horticulture_, p. 42-46, has detailed experiments illustrating vegetable _perspiration_, from which we may form an idea of the amount of fluid which these “very-minute brown-colored aphides” would have to drink. A sunflower, three and a half feet high, perspired in a very warm day thirty ounces—nearly two pounds; on another day, twenty ounces. Taking the old rule, “a pint a pound,” nearly a quart of fluid was exhaled by a sunflower in twelve hours; and the vessels were still inflated with a fresh supply drawn from the roots. Admitting that the leaves of a fruit-tree have a less current of sap than a sunflower or a grape-vine, yet in the months of May and June, the amount of sap to be exhausted by these very minute brown aphides, would be so great, that if they drank it so suddenly as to cause a tree to die in a day, they would surely augment in bulk enough to be discovered without a lens. If some one had accounted for the low water in the Mississippi, in the summer of 1843, by saying that buffaloes had drank up all the upper Missouri, and cut off the supply, we should be at a loss which most to pity, the faith of the narrator, or the probable condition of the buffaloes after their feat of imbibition.

But the most curious results _follow_ these feats of suction. The limbs and trunk _below_ shrink and turn black, for want of that elaborated sap extracted by the aphides. And yet every year we perform artificially this very operation in _ringing_ or _decortication_ of branches, for the purpose of accelerating maturation or improving the fruit. Every year the _saw_ takes off a third, a half, and sometimes more, of a living tree; and the effect is to produce new shoots, not death. Is an operation which can be safely performed by man, _deadly_ when performed by an insect? Dr. Masher did not detect the insects without extreme search, and then only in colonies, on healthy branches. Do whole trees wither in a day by the mere suction of such insects? Had they been supposed to _poison_ the fluids, the theory would be less exceptionable, since poisons in minute quantities may be very malignant.

While we admit a limited mischief of insects, they can never be the cause of the prevalent blight of the middle and western States—such a blight as prevailed in and around Cincinnati in the summer of 1844—nor of that blight which prevailed in 1832. The _blight-beetle_, after most careful search and dissection, has not been found, nor any trace or passage of it. Dr. Mosher’s insect may be set aside without further remark.

I think that further observation will confirm the following conclusions:

1. Insects are frequently found feeding in various ways upon blighted trees, or on trees which afterward become so.

2. Trees are fatally blighted on which no insects are discerned feeding—neither aphides nor _scolytus pyri_.

3. Multitudes of trees have such insects on them as are in other cases supposed to cause the blight, without a sign of blight following. This has been the case in our own garden.