Dwarf Mistletoe

Dwarf mistletoes are parasitic plants that infect conifers. This page covers the basic biology, taxonomy, and signs and symptoms of dwarf mistletoes. At the end is a photo gallery, tables of species, and references. Other pages cover damage from and ecology of dwarf mistletoes.

Hosts

Dwarf mistletoes infect only conifers. In North America, in fact, they only infect one family, Pinaceae. In western North America, many of the most important tree species are hosts. Among the 8 Old World species, three infect Juniperus spp. (Cupressaceae) ​[5]​.

Pathogen

The dwarf mistletoes are in the genus Arceuthobium in the order Santalales. Nickrent places them in family Viscaceae while the Angiosperm Phylogeny Group 2016 retains it in Santalaceae pending further data ​[2, 7]​. They are all “leafless” (there are minute scale leaves). They are “dwarf” in comparison to true mistletoes (Phoradendron and Viscum spp.). The shoots range up to about 20 cm long for the larger species. The shoots are nonwoody, last only 2-7 yr, and then can regenerate. Like Phoradendron and most Viscum spp., dwarf mistletoes are dioecious.

A very important feature is the mode of dispersal. The fruit literally explodes like a cannon, shooting its seed at an initial velocity of 97 km hr^-1 (60 mph) 10 meters or so, up to a maximum of 16 m ​[5]​. You don’t want to be taking a close look at it when that happens!

For years we were settled on about 42 species in the genus. After developing an extensive molecular phylogeny, Nickrent proposed reducing the number of species to 26 ​[8]​. He applied this general concept in the chapter on Viscaceae in the updated Flora of North America ​[7]​, resulting in only 7 species recognized in the USA and Canada. Many of the former species are considered subspecies of A. campylopodum. All 8 Old World species survived ​[8]​. Tables of species are toward the end of the page.

Environment

Arceuthobium spp. are not as dependent on particular conditions for reproduction, dispersal, and infection like many fungi are. However, forest stand conditions and disturbance regime can dramatically affect their spread rate, intensification, and long-term prospects. See the ecology page for details.

Distribution

Dwarf mistletoes occur in much of the northern hemisphere, but the greatest diversity is in western North America. Here they occur from boreal Canada and Alaska, south to Guatemala, Belize, and Honduras. Only one species, A. pusillum, occurs in eastern North America, from eastern Saskatchewan to Newfoundland, and south to northern New Jersey and adjacent Pennsylvania. One species, A. bicarinatum, is endemic to the Caribbean island Hispaniola.

Eight species occur outside the Americas ​[5, 8]​. See a table of species near the bottom of the page for their distribution.

Life Cycle

Let’s start with a seed flying through the air. Depending on where it lands, a lot of things can happen. If you’re the baby embryo, landing on the ground is bad – your journey on this earth is already finished. Or you land on a non-host, and everything goes well until you penetrate and find you don’t have the keys to the house. Again, game over.

Where you really want to land is on a compatible host, on an upward-pointing needle on a branch segment that is up to about 5 years old (although A. americanum can penetrate the thin bark of Pinus contorta on much older branches). The sticky viscin coat on the seed holds on. But when there is rain or fog, the viscin swells and turns from adhesive to lubricant, and you slide down the needle to the bark surface. Then you hold on tight. If you land on a downward-pointing needle, you fall and get another chance to land on a branch . . . or the ground.

Some tropical species (including southwestern dwarf mistletoe, which reaches up into the USA) will germinate right away. Most temperate species wait for spring. You send out your radicle. When it encounters an obstruction, such as a needle base, it develops a “holdfast” that grips tight. In the middle it sends down a penetration wedge that penetrates the bark by mechanical pressure. This is when you’re glad you’re on a young branch, otherwise you might run out of energy before you get in.

Now comes a period of setting up house inside the host. Establishing an endophytic system (see Anatomy below) provides food and water from the host. Eventually a swelling usually will develop, then 2-5 years after infection, the first shoots appear. After another year or two, flowers appear. Pollen is dispersed by wind or insects to another plant.

The species vary quite a bit in phenology (what time of year they do certain things). Some flower early in the growing season and some rather late. The fruit takes 13-17 months to mature, so seed dispersal occurs the following year, later than flowering. The complete cycle takes at least 6-8 years.

Signs and Symptoms

Being a plant with macroscopic shoots, one would think dwarf mistletoes are easier to detect than are most other diseases. That is not always the case. First consider shoots. It takes at least two years after infection before shoots appear (during that period they are “latent” infections). Second, especially during a drought, some species may drop shoots and they can be very scarce, although the plants are very much alive inside the host. Finally, they are “dwarf”, but some are very small, even shorter than the needles of their host, and very difficult to see except in branches near the ground:

• Arceuthobium pusillum, eastern dwarf mistletoe, shoots average 1 cm long
• A. douglasii, Douglas-fir dwarf mistletoe, averages 2 cm long

Luckily there are other signs and symptoms (see slide carousels below and on the damage page for illustrations):

• When shoots drop, they leave behind basal cups where they were borne on the bark. These are inconspicuous and can only be detected when close.
• Swellings occur at the site of initial infection in most cases. The swellings are fusiform, tapering down at each end. Infections can result in persistent swelling that becomes very large and gradually kills bark (a canker). These cankers can be infection courts for wood-decay fungi.
• The most striking symptom is a witches’ broom, an area of abnormally frequent branching resulting in a cluster of branches. These range from small to large and from loose to tight and dense.
• Resinosis (resin exudation) often occurs around infections
• Dead tops are common in some cases when numerous large brooms in the lower crown divert food away from the top.
• Heavy infection can lead to thin foliage and chlorosis.
• Seed production and seed viability are reduced.
• As intensification of the disease results in more branches infected, growth is reduced and premature mortality may result.

Anatomy

After penetration under the holdfast (see Life Cycle above), the penetration wedge grows down as far as the cambium ​[3]​. Then the endophytic system begins to form (endophytic means inside the plant). First, tissue from the penetration wedge continues to grow radially down to just beyond the cambium. It does not actively penetrate the xylem; it waits for the xylem to grow out around it, year by year. This structure becomes the first sinker (the first sinker is also called the ‘primary haustorium’) ​[1]​. Sinkers become embedded in the rays. One can age mistletoe infections precisely by counting annual growth rings from the tip of the primary haustorium (oldest sinker) out to the cambium.

But wait – if the cambium adds a new layer of wood, and the sinker is inside the wood and the rest of the mistletoe outside, why doesn’t this stretch and tear the sinker away from the rest? The mistletoe has a very sophisticated mechanism to deal with this. Where the sinker penetrates the host cambium, it develops a little stretching and growing zone. This can be called an intercalary meristem. Intercalary means it is inside, along the length. It’s growth is synchronized precisely with that of the cambium.

Another kind of organ grows out parallel to the cambium. These are cortical strands. These are mostly longitudinal but also circumferential. They ramify through the phloem, where they begin to appropriate organic nutrients.

The cortical strands send down additional sinkers as they grow. The cortical strands plus the sinkers comprise the endophytic system. Now the mistletoe is set for life.

Localized vs. systemic infections

Localized infections remain in a limited section of the host branch ​[3]​. A fusiform swelling usually forms there, extending about as far as the endophytic system. All dwarf mistletoes can form localized infections.

Some dwarf mistletoes can become systemic (within a branch system). The endophytic system grows acropetally (toward the branch tip) faster than basipetally. Swelling is generally restricted to the area of initial infection. Eventually they reach just behind the apical meristem at the branch tip. In a more intimate form of synchronization, they keep up with tip growth and and develop into new branches as they are produced. This systemic infection is similar to Elytroderma needle cast of ponderosa pine. The mistletoe can then produce shoots anywhere along the systemically infected branch system.

Systemic infections are considered to represent a more advanced evolutionary state, and are generally associated with small shoots. In North America, they are consistently formed only by A. americanum, A. douglasii, A. pusillum, and A. guatemalense. Brooms are generally formed in both kinds of infection.

Physiology

The dwarf mistletoes have no significant leaves, but have some chlorophyll in their stems and can photosynthesize there. And why shouldn’t they? These parasites evolved from photosynthetic, free-living ancestors, so the ability is already there. Those stems are up there in the canopy, so why not take advantage of that extra energy, given that the photosynthetic machinery is operative.

However, they have less chlorophyll than true mistletoes, corresponding with an often sickly green or yellow color. They take more photosynthate from the host than do true mistletoes. They are considered on the brink of being holoparasites ​[7]​. And indeed, in old stem infections it is not uncommon for the pathogen to survive for decades or more while producing no shoots, which is certainly holoparasitism. This is one reason they are more damaging than true mistletoes when there are many infections on a tree.

Mechanisms of tree damage (next section) are related in part to allocation of resources. The biomass of the mistletoe plant itself may be a minor drain to the tree (although the endophytic systems of systemically infecting mistletoes can be much larger than the shoots); disruption of tree physiology may be a bigger effect. High hormone levels in the mistletoe cause photosynthate and other nutrients to be shunted to infected branches ​[6]​. Host tissues near the infection may receive much of this bounty, but the tree is damaged because nutrients do not go to the growing top and roots where they are needed most. Witches’ brooms develop luxuriantly while the upper crown thins and dies. It is not uncommon to observe that infected branches are the last part of the crown to die.

An additional mechanism of damage relates to water relations. Dwarf mistletoes are typically less efficient at water use and transpire at a rate several times that of their host, with even greater differential under conditions of water stress ​[5]​. During a drought, this additional water demand may result in decreased growth or even death of other parts of the tree.

Summary

• Dwarf mistletoes infect only conifers, in fact, only one family (Pinaceae) in North America.
• They are all “leafless”, that is, the leaves are minute scales.
• They have chlorophyll, but depend on host for most carbohydrates (as well as all water and inorganic nutrients).
• They are “dwarf” in comparison to true mistletoes (Phoradendron and Viscum spp.). The shoots range up to about 20 cm long for the larger species.
• The part inside the host plant is the endophytic system, consisting of
  • cortical strands, ramifying through the phloem
  • sinkers following rays into the xylem
• Shoots are nonwoody, last a few years, and then can regenerate.
• Like Phoradendron and most Viscum spp., they are dioecious (separate male and female plants).
• A very important feature is the mode of dispersal: the berries explode, shooting sticky seeds at an initial velocity of 97 km hr^-1 (60 mph) 10 meters or more.
• Tend to be host specific, which is important in management.
• They are among the most damaging forest pathogens in western North America.

Heavy dwarf mistletoe infection in a stand of Pinus contorta. It is causing growth loss, mortality, and accumulation of fuel with ladder fuels. Near Tincup, Gunnison National Forest, Colorado, USA.

Stem infection with prolific shoot development of Arceuthobium americanum on Pinus contorta, Gunnison National Forest, Colorado, USA.

Female Arceuthobium americanum on Pinus contorta, mid-August, Gunnison National Forest, Colorado, USA. Note the basal cups left from earlier shoots

Male Arceuthobium americanum on Pinus contorta, mid-August, Gunnison National Forest, Colorado, USA.

Heavy dwarf mistletoe infection in a stand of Pinus contorta. It is causing growth loss, mortality, and accumulation of fuel with ladder fuels. Picea engelmannii is unaffected. Near Tincup, Gunnison National Forest, Colorado, USA.

Arceuthobium douglasii in Pseudotsuga douglasii (Douglas-fir). Rio Grande National Forest, Colorado, USA.

Arceuthobium douglasii in Pseudotsuga douglasii (Douglas-fir). Rio Grande National Forest, Colorado, USA.

Arceuthobium douglasii in Pseudotsuga douglasii (Douglas-fir). Rio Grande National Forest, Colorado, USA.

Arceuthobium douglasii in Pseudotsuga douglasii (Douglas-fir). Rio Grande National Forest, Colorado, USA.

Typical large basal broom caused by Arceuthobium douglasii in Pseudotsuga douglasii (Douglas-fir). Many additional brooms are above it. Rio Grande National Forest, Colorado, USA.

Species

Nickrent ​[7, 8]​ used for taxonomy; other data mostly from Hawksworth & Wiens ​[5]​ and Ciesla et al. ​[4]​.

Dwarf mistletoes of USA and Canada

Dwarf mistletoes outside the Americas

Related pages include:

• Dwarf mistletoe ecology
• Dwarf mistletoe damage