This section is from the book "The Gardener's Monthly And Horticulturist V22", by Thomas Meehan. See also: Four-Season Harvest: Organic Vegetables from Your Home Garden All Year Long.
The following excellent exposition of this disease was recently made before the Illinois State Horticultural Society.
A wide-spread disease of pear-leaves in this country and in Europe is caused by a mite to which Schenten, a German naturalist, gave the name Typhlodromus pyri. This was twenty-one years ago. Ignorant of this information, the writer during the last season rediscovered the cause of the disease, and, it is believed, first announced its occurrence in our country. There is scarcely a question as to the identity of the disease and its cause on the opposite sides of the Atlantic, and granting this identity, we may conclude that it is another one of the horticultural scourges that have been imported from across the water, for which we have returned the phylloxera and possibly shall send over the Colorado potato-beetle. Nothing of the life-history of this pear-leaf mite has heretofore been published, except the discussions which have arisen as to whether the form usually found is a young or mature animal. It has but four feet, while most mites have eight. But the young larvŠ of others have six, as far as made out. Is this an exception? Schenten called this a larva, and Doctor Packard, in the Guide to the Study of Insects, adopts the idea.
According to the former, the mature form has eight legs and widely different mouth-parts, but the only proof of the genetic connection of the two is that they were found associated. As this eight-legged form certainly belongs to a group whose members are mostly parasitic in other insects it is probable that if any relation exists between the two kinds it is of this nature. But having found the mite in its autumn and winter condition, I am able to add an item to the controversy opposed to the change of form indicated. And this last discovery, carrying with it the possible basis for a remedy, is my excuse for introducing this account.
When young leaves appear in the spring or during the summer, reddish spots an eighth of an inch or more are seen scattered more or less numerously over their surface, especially conspicuous on the upper side. At a later time these spots turn brown by the death of the parts, after which they are more easily discovered beneath. "With a good magnifier a minute hole can be distinguished near the centre of each spot through the lower side epidermis, and the spots are somewhat thickened. This is about all that can be ascertained with a hand magnifier, for if we •dissect the spots nothing can ordinarily be found but the spongy cell-tissues brown with disease or death. But if one of these spots is carefully •opened and magnified fifty to one hundred times numerous peach-colored slowly-moving things are discovered. These are the depredators caught, 'if not in the act, with the evidence of their misdoings in the same field of view. The size is much less than that of any true insect known: the length from extremity to extremity being but .0055 of an inch and the width not more than .0017. It would take nearly two hundred of them placed end to end to measure an inch, and six hundred could march elbow to elbow within that space.
We cannot wonder, therefore, that more has not been known about them or that •they were so long entirely unknown. They make up, however, what they lack in square measure by the multiplication table. Dozens, perhaps •scores, occur in a single spot, and dozens of spots may be found on a single leaf. The two pairs of legs are directed forward and the little thing clumsily drags its body along not unlike the larvse of the May beetles, usually known as white grub-worms. Its progress is excessively -slow. The perilous trip down the foot-stalk of the leaf to the buds in autumn must be an immense undertaking. Some of them do not make it, for, whether from simple procrastination or a dread of attempting the great journey to an unknown country, numbers remain and fall with the leaf to the ground. Possibly this is the way that slow dissemination takes place from tree to tree in an orchard, yet it hardly seems possible that, though carried by the winds to the very foot of a tree, they could climb the trunk to the limbs.
The chance would be better an the nursery, where the leaves are very near the ground.
The fact referred to above as new, not having "heretofore been publicly announced, is that these minute creatures do creep from their galls in the leaves in autumn and pass the winter within the leaf-scales of the buds. Hundreds •of them may be found there now of the size and: form previously mentioned, and by keeping them warm for some time they may be seen crawling as lively as nature ever permits them to move. Neither eggs, except perhaps within the body of the females, nor larvŠ have been observed, but they almost surely exist within the leaf-galls. Probably my own investigations have been made too late in the season.
My story is longer than it should be, but there must be a suggestion added as to the treatment or remedy. Is the disease preventable or curable? Human beings sometimes, more is the pity, have a skin disease popularly known as the itch. Is it preventable or curable? It, too, is caused by a mite, not distinctly related to the little thing of which we speak. No one believes this human parasite originates spontaneously under the skin of the hand: so we may rest assured that when pear trees are thus affected - catch the itch - they themselves are not the incubators of the mite-species which causes it. The mite comes from abroad, is disseminated in scions and very gradually spreads from tree to tree located near each other. Its marks in spring and summer are conspicuous enough. Is not the road to extermination evident enough? Let war be made by cutting back the one-year-old wood of all effected trees in winter and burning the removed portions. Then in spring-time remove every young shoot which shows the need of it, and likewise destroy it. Let this be kept up during the summer and we may be sure that the next season will show us healthy trees in this respect. Most care should be taken with nurseries, and especially in the selection of buds and scions for propagation.
Seedling stocks may be contaminated: in one case they were known to be. If such have the buds entirely cut away and burned, and for further safety the roots dipped into strong potash solution, no mites can escape. The pruning advised may sometimes be severe, but no large limbs need be removed, only last year's growth, bearing the buds, and we may proceed with the understanding that it is to be done once, and once only if the work is thorough and general throughout the orchard or nursery, provided that some one else's orchard or nursery does not closely adjoin that operated upon and new importation of the mite is not made.
Believing that no good reason exists for the generic separation of this little creature from its kindred previously described, Andrew Murray classes it among the species of the genus Phy-toptus. This is almost certainly correct, and we write to close with - Phytoptus pyri.