DWARF MISTLETOES

LAKE S. GILL, JESS L. BEDWELL.

The dwarf mistletoes are serious pests of western coniferous forests. The losses they inflict in volume of timber and quality of lumber have never been accurately evaluated but are believed to be exceeded only by the damage done by heart rots.

The dwarf mistletoes belong to the genus Arceuthobium (it is also called Razoumofskya), a group of the family Loranthaceae, of which all mistletoes and some other parasitic plants are members. Among their next of kin, in the genus Phoradendron, are the familiar Christmas mistletoes, which attack mostly deciduous trees and junipers. In North America the junipers and their relatives are immune to dwarf mistletoes, although the generic name, Arceuthobium, is derived from Greek words meaning "juniper living," because juniper is the most common host in the Mediterranean region, where these plants were first described. They have also been reported from China and constitute a problem in forest management in the Himalaya Mountains of India. The dwarf species attack only conifers and are not used for decorative or symbolic purposes.

Five species are recognized in North America. One of them, A. pusillum, is found only from the Great Lakes region east, mainly on spruce. The other four, typically western, range from Canada and Alaska to Mexico. Of these four, one (A. americanum) is confined to the ranges of lodgepole and jack pine, another (A. douglasii) to the range of inland Douglas-fir. Of the others, A. vaginatum is restricted to three-needled pines, notably Pinus ponderosa var. scopulorum in the southwestern United States and Mexico, and A. campylopodum attacks pine, spruce, fir, hemlock, and larch from Alaska to Arizona and, probably, Mexico.

The dwarf mistletoes have been reported in Arizona, California, Colorado, Connecticut, Idaho, Maine, Massachusetts, Michigan, Minnesota, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, Oregon, Pennsylvania, Rhode Island, Texas ( the northwestern part) , Utah, Vermont, Washington, Wisconsin, and Wyoming. Although the list indicates widespread occurrence from east to west, it should be pointed out that none has been found in the island of ponderosa pine covering the Black Hills of South Dakota or in the great Douglas-fir forests on the west side of the Cascade Range north of the Umpqua-Willamette Divide. It is also notable that they do not attack the high-producing forests of the South. In Texas they are restricted to isolated mountain ranges in the northwestern part of the State, where the timber values are negligible.

THE DWARF MISTLETOES are leafless, flowering plants. They are dioecious that is, the staminate, or male, flowers are borne on separate plants from those producing the seed. The root system of these parasites has developed into an absorption system, which can invade and maintain itself in both the wood and the bark of its host.

From the host it derives nutrients and water. The absorption system has been known to live for many years within the tissues of the host plant without producing aerial shoots. The shoots are segmented stems, which may or may not branch. In A. pusillum they attain a height of about an inch and are unbranched. In A. campylopodum and A. vaginatum they may become several inches long and are usually branched. The primary (if not the sole) function of the shoots is to produce flowers and fruits. Most of the Phoradendrons, on the other hand, appear capable of producing most of their own food and are believed to rob their host primarily of water and dissolved minerals.

In all species except A. pusillum, the fruits mature the second season after pollination. They are berrylike structures that vary in color from light green to blue green or even brown. In size and shape they resemble a grain of wheat.

The outside casing, or skin, of the fruit is a tough and elastic sac. At maturity the sac contains the seed and a hygroscopic material called viscin. As the viscin absorbs water, pressure against the elastic wall of the casing is increased. When the seed is ripe the casing is ruptured from its base, leaving one end of the sac open. Simultaneously, the wall of the casing contracts, and the seed is forcibly ejected into the air. The stems, or pedicels, supporting the ripe fruit curl downward in such a way that the base of the fruit points skyward at the time of the explosion and the expelled seed then follows a trajectory like a mortar shell. Seed that are shot from 20 or More feet above ground and allowed to follow their course without obstruction will usually travel from 20 to 40 feet horizontally sometimes more than 60 feet.

The seed carries with it a small amount of the sticky viscin, which serves the double purpose of holding it fast to the medium on which it alights and of gathering and holding moisture for the protection of the primary root upon germination. With proper conditions of moisture and temperature, mistletoe seed will germinate on practically any substratum, but only those that happen to be on the young, tender branchlets of suitable host plants can survive. The primary rootlet then forces its way into the tender bark and from there establishes an absorption system inside the host; after 2 years or more it may produce many crops of aerial shoots.

The explosive nature of the seed dispersal tends to intensify the mistletoe on a tree once it is infected and leads also to a slow but steady encroachment of the parasite into the forest once it is established on a single tree. In that respect it differs fundamentally from the leafy or Christmas mistletoes, which are spread only by birds, with the result that trees are seldom infected until they are large enough to provide attractive roosting places. The distribution of the dwarf mistletoes indicates that they, too, may be carried long distances, presumably by birds.