Eriophyes sp.

This gall, a typical witches-broom, (PI. XIX, Fig. 1) represents a more or less serious disturbance of growth at the nodes. An excessive number (2 or 3 generally) of abnormal (wood reduced, pith increased) branches are produced from the same bud, followed by the development of an indefinite number of buds, all closely sessile in a mass at the node between the "gall" branches (PL XIII, Fig. 1). The subsequent infection of the basal buds of the new branches, the buds nearest to the original node attacked, accounts in great part for the characteristic irregular massing of the branches. If a young "broom" be stripped of its bark, (PL XIII, Fig. la), this relation of the primary and secondary branches is made evident. Often, however, in later years, buds located at the base of the primary "gall" branches will develop a shoot. After a number of years the mass of branches becomes so large as to be very conspicuous and unsightly. The author has investigated new branches growing on old galls without finding any mites. It seems probable that the condition of things grows worse after the primary infection, whether or not the mites are present. The gall proper is altogether confined to the nodes affected, in which region two prominent facts stand out in relation to the histology of the parts affected: (1) The bases of the gall branches have suffered an inhibition of their differentiation; (2) The cortex shows definite hyperplasia. These facts are shown in PL XIII, Fig. Ic, which illustrates the longitudinal section of the part indicated at c, in Fig. la, which is a longitudinal, median section through a primary "gall" branch and the normal twig, from which it has grown out.

Kellerman and Swingle (12, 13) have associated a fungus (Sphaerotheca phytoptophila Kell. and Sw.) with this gall. No mycelium was seen in the affected tissues; indeed none would be expected belonging to the fungus named, since its position among the Erysiphaceas would indicate it to be wholly superficial, the haustoria only affecting epidermal cells. Other well known witches-brooms, particularly those of Europe, have been shown to be caused by mites only.

- Bertram Wells: (1916) The comparative morphology of the zoocecidia of Celtis occidentalis^©

Reference: https://www.biodiversitylibrary.org/item/21732#page/262/mode/1up