A complex involving a powdery mildew Podosphaera phytoptophila and an eryiophyid mite Eriophyes [Aceria] celtis

Hackberry witches’ broom occurs throughout the range of hackberry (Celtis occidentalis) in the Great Plains ....

The most obvious symptom of this complex is the occurrence of multiple brooms consisting of numerous short twigs that arise close together, often at a conspicuous swelling or knot on a branch. These are not seen at branch tips, as the brooms do not occur on branches less than one year old. Buds of shoots within a broom are usually larger than normal and have more open bud scales. Witches'-brooms on hackberry are more unsightly than harmful to the tree. Brooms can cause branches to break more readily and therefore expose wood to decay fungi. Extensive brooming can reduce the vigor and stunt growth of the tree; however, trees are seldom seriously injured. Severity of attack can vary greatly among trees in close proximity to one another, but reasons for this variation are unknown.

The cause of witches'-broom of hackberry is not completely understood, but it is attributed to two agents acting in concert, a powdery mildew and an eriophyid mite. During spring and early summer, the powdery mildew fungus can sometimes be found growing on succulent stems, petioles, buds and sometimes the lower surfaces of leaves. Infection and spread may be caused by spores or mycelium within buds. Mites in all stages of development can be found throughout the year and are most numerous in late summer. They occur and overwinter beneath bud scales.

Eriophyes sp.

"Witch-broom" on branches and twigs

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.