Dwarf Mistletoe: A Parasitic Plant on Black Spruce

***Arceuthobium pusillum* fruiting shoots on a black spruce branch. Some smaller shoots are on the left side and down below. These will be next year’s flowering shoots. This **dwarf mistletoe**** **was found in a Northern Poor Conifer Swamp, a type of Poor Fen, near Marble Lake in Lake County, Minnesota.**

This peculiar growth on a tree branch in the above photo is Arceuthobium pusillum (dwarf mistletoe), a parasitic plant that grows on black spruce (Picea mariana) trees. It is a flowering plant and a member of the same family (Viscaceae) as the familiar Christmas mistletoes, Phoradendron leucarpum and Viscum album. But at 2 to 3 mm in size, it is much smaller and unlikely to feature in any Christmas decorations.

Description

Arceuthobium pusillum is a minute perennial shrublet parasitic on the branches of bog conifers, primarily back spruce. Although photosynthetic to a limited extent, all of its other nutritional needs are derived from the host plant.

The stems of the Arceuthobium pusillum are green, orange, red, maroon, or brown, 2 to 3 cm long, and covered in small oppositely arranged, scale-like leaves.

The staminate (male) flowers are represented by three or four sepals, each with a sessile anther sac. There may be a prominent nectary in the center of the flower.

Pistillate (female) flowers have reduced sepals fused to the outside of the bicarpellate gynoecium. Mature pistillate flowers exude a pollination droplet that draws in the pollen grain.

Reproduction

Flower buds are formed in the fall. Flowering occurs in the spring. The minute flowers are either pistillate or staminate and borne on separate plants. Pollination is accomplished by insects and by wind.

The most frequent pollinating insects are flies (Syrphidae, Tachinidae), beetles (Lampyridae), and wasps (Aphidiidae, Ichneumonidae, Tenthredinidae, Vespidae). In studies where insects were excluded from access to the flowers, the seed set was lower.

The 2 to 3 mm long greenish or brownish fruits contain one seed. At maturity, pressure builds up in the fruit, causing the seed to be ejected up to 12 or more meters. The seeds are sticky, which helps them adhere to a potential growth site. If they land on a young live spruce twig and germinate, root-like growths (haustoria) push through the bark and into the cambium. It may take two or more years of growth under the bark of the host tree before the first mistletoe shoots appear.

The sticky seeds may also adhere to the feathers of birds, thus aiding in long-distance dispersal to new bogs with black spruce. After the seed is ejected, the mistletoe branch dies.

**Mistletoe witches’ broom on black spruce in Carlton County, MN.**

Witches’ brooms

The mistletoe alters the growth of the trees’ twigs by causing a loss of apical dominance. As a result, clusters of branchlets form, called “witches’ broom”. This alteration may also affect the growth form of larger side branches that develop from the witches’ broom. These branches are twisted and flattened into an oval shape.

Abundant growth of Arceuthobium pusillum on spruce can eventually kill the tree. But the witches’ brooms, whether living or dead, are shelter for many insects, spiders, birds, and small mammals.

**Many black spruce trees in this poor fen in **Cook County, MN**, have succumbed to dwarf mistletoe.**

Habitat

Arceuthobium pusillum is a parasitic plant that grows on black spruce trees in poor fens, intermediate fens, and coniferous forested peatlands. It is occasionally found on white spruce (Picea glauca) and red spruce (Picea rubens). It rarely occurs on tamarack (Larix laricina), white pine (Pinus strobus), jack pine (Pinus banksiana), red pine (Pinus resinosa), and balsam fir (Abies balsamea).

Arceuthobium pusillum has parasites, too

Caliciopsis arceuthobii is a fungus that infects the flowers of spring-flowering Arceuthobium species, including Arceuthobium pusillum. Its spores are spread by wind and by insects visiting the flowers. The fungal hyphae destroy the developing fruit.

Discovery of the species

Thoreau first described the brooms formed by Arceuthobium pusillum in 1858, although he did not know the cause of them. In his journal entry, he wrote,

“About the Ledum pond hole there is an abundance of that abnormal growth of the spruce–Instead of a regular free & open growth–you have a multitude putting out from the summit or side of the stem of slender branches crowded together & shooting up nearly perpendicularly–with dense fine wiry branchlets & pine needles which have an impoverished look–all together forming a broom-like mass–very much like a heath.”

Arceuthobium pusillum was not formally described until 1872 by C. H. Peck after he received correspondence about the plant and specimens from botanist Lucy B. Millington of Warrensburg, New York, in 1871. Her correspondence about her discovery of a tiny mistletoe on Abies nigra (an older name for Picea mariana) was published in the Bulletin of the Torrey Botanical Club. She wrote,

“I believe it to be a mistletoe. I found the first specimen in a small tree in the edge of a cold peat bog in Warrensburg, Warren Co., N.Y. In a few days I found more in a similar situation in Elizabethtown, Essex Co., N.Y. Later I found it halfway up the side of a small mountain. In every case the limbs of the trees infested were very much distorted. Every twig bristled with the little parasite…”

Range

The range of Arceuthobium pusillum is from Newfoundland to Saskatchewan and south into New Jersey, Pennsylvania, and the Great Lakes states, closely following the range of its primary host, black spruce.

Sources

Venerable Trees

This white pine is the largest and possibly the oldest tree on my property. It has a DBH (diameter at breast height or 1.3 meters) of 92 cm, and I estimate its age at about 180 years old.

The white pine probably began growing sometime around 1845. This was at the beginning of the white pine logging era in Minnesota. By 1900, it would have been large enough to cut for timber, but it was missed. The 1918 fires that swept through the area also missed it by just two short miles.

All through the 20th century, it continued to grow undisturbed. Over time, a forest of paper birch, aspen, balsam fir, and spruce grew under the pine. Beneath the trees, clubmosses, poverty oats, rough rice grass, purple melic grass, twinflower, and hairy goldenrod covered the ground. The forest had come back.

**The big white pine as seen from ground level.**

When other white pines on this 40-acre section were cut down for lumber in 1960, this huge white pine was not cut down. So were four other large white pines and five large red pines. I don’t know how or why this happened, but I am glad they were overlooked. What would this forest be without them?

One of the big white pine’s neighbors. This **white pine** **tree had a DBH of 82 cm in 2015. I estimate its age at 170 years.**

Logging of Minnesota’s white pine forests began in the 1830s, but it wasn’t until the 1870s, with the advent of railroads and new settlements, that it really took off. By 1900, Minnesota had produced almost 1.2 billion board feet of white pine timber. This was not to last, and by 1929, nearly all of Minnesota’s white pine forests were depleted, bringing white pine logging to an end. While not remnant trees from old-growth forests, these white and red pine trees are survivors from that time.

**One of the old red pines, DBH 56 cm, age estimate 121 years.**

White pines I planted in the early 2000s. These trees were grown from seeds produced by another old and enormous white pine on my land. They are part of my project to re-wild a former hayfield and pasture.

Missing the colors of mushrooms in the fall

This has been an exceptionally warm and dry summer and fall, affecting everything in the forests, fields, and wetlands. Fall colors have been rather drab this year. Wetlands are drying up. Many trees began dropping their leaves in August.

September and October are times when the forests are full of colorful fungi. I always look forward to going out and looking for mushrooms. But this year mushrooms of any kind are far and few between. A few puffballs have popped up, and some Amanitas tried to grow before succumbing to the heat.

This slideshow showcases some of the colorful fungi that should be here, but for lack of rain, are not.

About a year ago

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Its hard to believe when I walk in the woods now but last year by the second week of April almost all of the snow had melted, plants were beginning to grow in sunny patches under the trees, frogs were laying eggs, and insects from bees to bugs were active. But this year everywhere I look there is deep snow and cold temperatures. In the spruce and tamarack woods there is only the occasional call of the chickadees and nuthatches. Spring flowers are a long way off.