Witch Knots
The name popularly bestowed on the curious clusters of twigs (looking very much like the nest of a rook, or some other large bird, when seen from a distance) that so frequently occur on the larger branches of Birch, Hornbeam, and certain other trees. The twigs are usually somewhat swollen, and both they and the sickly-looking leaves upon them are duller green than usual, and frequently show a slightly velvety surface. The causes of these deformities escaped detection till they were carefully investigated a few years ago with the microscope, and were found to differ in various trees. In Birch and Hornbeam, they are the work of minute Fungi, belonging to a very lowly group of Ascomycetes. The mycelium of the Fungi lives in the tissues of the host-plant's leaves and bark; and the velvety appearance mentioned above results from the outgrowth over the whole epidermis of the organs of reproduction. These latter consist of asci (see Pyrenomycetes) in each of which eight (or more) spores are formed, and from which they are scattered when mature. The asci in Exoascus (as the most important genus of Witch Knot Fungi is called) are quite exposed to casualties from without, instead of being inclosed in a perithecium, as in Pyrenomycetes. The number of asci and of spores is immense; but they are far too small individually to be detected, except when thin sections of the diseased part are examined with a lens of high magnifying power. The following species of Exoascus give rise to Witch Knots; E. turgidus, on Birch; E. Carpini, on Hornbeam; and E. Institiae, on Bullace. It may be noted that other species of the genus produce distortions of the leaves of various trees and shrubs (Poplars, Alder, Elm, Cherry, &c.), and that E. Pruni is the cause of the disease of the fruit in various species of Prunus known as "Bladder Plums."
In the Birch, a diseased condition, a good deal like a young Witch Knot, is often of frequent occurrence. It is the work of a species of Phytoptus or Gallmite (see Mites), and results from the stimulation, by the myriads of minute Mites, of the growing tissues in the buds, which are increased in size, but never develop properly. The buds in the axils of the leaf-scales, instead of remaining undeveloped, enlarge to a conspicuous size, or grow into stunted branches; and on this process being repeated year after year, the mass of buds, &c., may reach the size of a cricket ball, but is usually smaller. There is no real difficulty in distinguishing these Mite Galls from true Witch Knots caused by Fungi. The Silver Fir (Abies pectinata) occasionally bears Witch Knots, which are made up of a central swelling of the stem, or of a large limb. From this arise one or more branches, on which grow very many small branches crowded densely together. The needles are short, thick, brittle, and bright green. All parts of the Witch Knots are traversed by mycelium; and the leaves bear over their surfaces numerous small, yellow, membranous cups, filled with minute, yellow spores for reproduction. These are the characteristic "fruits" of a Fungus named AEcidium elatinum. This Fungus renders the stems, &c., brittle, and liable to injury from wind; and it also causes the leaves to fall early.
Treatment. The only method likely to be of use is to cut off and burn Witch Knots of all kinds, to prevent the diseases from being propagated by the spores.