Forest and Shade Tree Pathology
There are many flowering plants that parasitize trees. Most of them parasitize via root-root contacts. Two important orders are Santalales and Scrophulariales. Although the botanical aspects have been studied (anatomy, physiology, evolution, etc.), not much is really known about the impact to trees in terms of damage and growth loss.
Most of these root parasites are in the tropics and subtropics, perhaps explaining in part the lack of study of them.
Some have no chlorophyll and are entirely dependent on the host for food and water. Others are green and can produce some or all of their own food. Some are even capable of going to reproduction without attaching to a host. The green ones can be very tricky: there is no obvious indication that they are root parasites.
Most root parasites do not seem to be very host-specific.
In the order Scrophulariales are two notable vascular plants that parasitize the roots of trees:
Mistletoes are a group of vascular, flowering plants that parasitize stems of trees and shrubs, flowering and coniferous. For some reason, they are far more damaging than the root parasites.
All mistletoes in North America (and most elsewhere) are in the family Viscaceae (formerly part of Loranthaceae). Flower characters are not important to us, except to note that the flowers are usually dioecious and the fruit is a berry with one seed.
The two important genera in North America are Phoradendron and Arceuthobium:
|Common name||Dwarf mistletoes||(American) true or "leafy" mistletoes|
|Shoots||usu. <10-20 cm, non-woody, last few years, regenerate||up to meter or more, woody, persistent (8-15 yr) can regenerate|
|Leaves||absent or greatly reduced||usually leafy, evergreen|
|Seed dispersal||explosive berries shoot sticky seeds, occasionally birds?||birds eat fruit, deposit sticky seeds elsewhere|
|Hosts||conifers||hardwoods, some conifers|
The classic European mistletoe is Viscum album. It was introduced to California on purpose by someone who thought it was pretty (and it is; it has nice white berries and evergreen leaves). It has spread out somewhat and infects various trees in a small area. It is probably too late to eradicate it.
Viscum album of Europe is the subject of much folklore, legends. Mistletoe was thought to be sent to earth by the gods, using a bird that was considered to be a messenger of the gods, the mistletoe thrush. In fact, this bird migrates from Africa to Europe in late winter. From January to March, it feeds almost exclusively on mistletoe berries. Then it evacuates the seeds intact. It serves as an effective vector. The distribution and abundance of mistletoe is thought to be determined in part by the migration patterns of the mistletoe thrush.
Because it was sent by the gods, the plant was thought to have spiritual power. One aspect is medicinal. Through the middle ages and beyond, V. album has been used as medicine for all sorts of ailments. Amulets were worn to ward off illness. Leaves and teas were eaten. As pharmacology developed, research was done that suggested it could be useful in reducing high blood pressure and as a diuretic. Other benefits were claimed. Trials had mixed results though. Medicinal power was also believed in Japan, especially when the mistletoe grew on the sacred willow tree. Even Navajos used certain Arceuthobium species as medicines.
Several cultures seem to associate mistletoe with fertility. In both England and Japan, women were advised to eat mistletoe leaves to promote conception. In England, an old practice was to feed mistletoe shoots to the first cow to give birth each year; that would increase fruitfulness of the herd and protect their health. In Japan, a few mistletoe leaves were crumbled and sown with crop seeds to promote fertility. Southwestern Indians made a tea out of Phoradendron juniperinum in order to relax muscles for childbirth.
In parts of Europe, other sorts of spiritual power were associated with mistletoe, Viscum album. To the Druids, the oak tree represented God, and the mistletoe in the oak represented human dependence on God. In Germany, mistletoe could be brought into a haunted house. It was thought that ghosts and other evil spirits would be forced to come out of hiding and answer any questions the peasants had. Generally, it was thought to bring good fortune, so it was hung over doorways, especially in midwinter. It seems reasonable that people would hang an evergreen plant indoors in winter just for the summery feeling it gives. The power is just a bonus. We don't know how the custom of kissing under mistletoe originated, but one connection is that Vikings associated mistletoe with their goddess of love, Frigga. It makes sense that if mistletoe is hanging up around Christmas time and has the power of good fortune and fertility, that getting two lovers together would be evidence of its power. Supposedly a man should remove one berry from the plant for each kiss he steals from the woman beneath it. When the berries are gone, so is the opportunity for kissing!
In America, the mystical power and symbolism has been vested in Phoradendron flavescens. This is sometimes sold as Christmas mistletoe; most of it collected in Texas.
The seeds have sticky stuff on them and stick to where they fall. If on a thin-barked stem or branch, the germinating seed produces a radicle that grows until it meets a bud or leaf base. It then flattens out and sticks tight to the surface (holdfast). It then penetrates the bark. The penetrating organ can be called a primary haustorium.
It grows down as far as the cambium. Then, circumferential growth begins. The organs that grow out parallel to the cambium are called cortical strands (=cortical haustoria). These are mostly longitudinal.
The cortical strands touch the cambium here and there. Where this happens, sinkers are initiated. They grow just past the cambium and stop (they would have a pretty hard time growing farther into sound wood anyway). They end up in the xylem, but there is not much to see the first year. They basically just sit there, and each year the xylem grows out around them. The mistletoe has a little stretching and growing zone at the point where the sinkers go through the cambium. This can be called an intercalary meristem. Intercalary means it is inside, along the length.
So the sinkers set up house with the water-conducting cells in the xylem. In the meantime, the cortical strands get set up in the phloem. Now the mistletoe is set for life. The cortical strands plus the sinkers can be called the endophytic system.
In some dwarf mistletoes, the endophytic system can develop right behind the apical meristem, keeping up with it, and developing into new branches as they are produced. This is called systemic infection, just like with Elytroderma needle cast of ponderosa pine. With mistletoe, it can then produce shoots anywhere along the systemically infected branch system.
Mistletoes have some chlorophyll and engage in some photosynthesis, so they are not totally dependent on the host for everything. In the language of parasitic plants, this is known as hemiparasitism as opposed to holoparasitism, where the parasite gets all water, minerals, and fixed carbon from the host.
Even the dwarf mistletoes, which have practically no leaves, have green 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 it would be stupid (teleologically speaking) not to take advantage of that extra energy, given that the photosynthetic machinery is operative.
Birds disseminate true mistletoe after eating berries. Cedar waxwings and euphonias are most important, but various others can do it. Euphonia in particular is said to be adapted to Phoradendron. The seed usually goes through with sticky coat intact.
True mistletoes can cause drought stress. As a tree is dying, there is often more mistletoe foliage than host foliage. Mistletoe foliage may keep branches alive and be the last to die.
True mistletoes are generally not as important or damaging as dwarf mistletoes, but are quite common in many areas. Eastern mistletoe, P. flavescens, has a wide host range.
In much of the West, dwarf mistletoes are considered the most economically damaging group of diseases. There are some areas where root diseases are probably more important, but mistletoes are important in many forest types, on most major tree species, over vast areas.
Most important forms of damage include:
It has been estimated that dwarf mistletoes cause direct volume loss of 3.3 billion board feet (a measure of wood volume) per year in the Western United States (growth loss + mortality). That's:
Significant growth impact is seldom seen until at least the lower 1/3 to 1/2 of the crown is heavily infected. Light infection throughout the crown or heavy infection in just the lower crown usually cause little or no damage, except perhaps when large brooms are produced or stem infections cause problems.
Brooms seem to have a special effect on growth. A large broom or several smaller ones can devastate the rest of the crown, leading to severe crown thinning.
Spread and intensification is much faster from overstory to understory than in an even-aged stand. At left is infection over time in an understory with an infected overstory:
Similarly, reproduction adjacent to an infected stand is infected faster than an even-aged stand. At right, you can see that infection tails off as distance from the residual stand increases.
Spread is faster with open canopy than with closed canopy.
Lodgepole pine gets an average rating of 5 within about 70 years after 1st infection (see below under managment for the rating system).
There has been interest in natural enemies of mistletoes as biological controls, but there is no evidence that any have sufficient impact on populations. Some fungal diseases (rust, anthracnose), some lepidopterans and bugs feed on them, small mammals (squirrels) eat bark from mistletoe swellings, greatly preferring it to normal bark, probably has the most significant impact.
Aside: Incidentally, mistletoes themselves, dwarf and true, can be a major source of winter forage for deer and elk. Ranchers often use Phoradendron for cattle during tough winters. However, there is no evidence that this acts as a control of mistletoe.
Remember the generalization that obligate parasites do better on vigorous hosts? With mistletoes, it isn't that simple. The parasite grows more vigorously on a vigorous host, producing more pollen or seeds, but other features of a vigorous host decrease success of the mistletoes. Think about an infection on the lower crown. It needs to have its progeny climb the tree to be successful. But a vigorous tree has a fuller crown with greater retention of needles. Now on the one hand this increases the chance of catching seeds, but on the other hand seeds will have a hard time shooting very high. Just as important, the live crown will be climbing faster than the mistetoe may be able to keep up. If the tree grows in height significantly faster than the mistletoe climbs, infection will never reach the upper or middle crown and may even be lost.
Tree density can have a strong influence on spread rate (distance through the stand over time). Clearly the spread rate will be 0 when tree spacing exceeds the maximum dispersal distance (about 16 m or 52 ft). As density increases, more seeds will successfully make the jump and spread rate increases. However, as density increases further, seeds are intercepted before they get very far, so spread rate decreases again (Hawksworth 1961b). As a practical matter, within the range of densities usually found in forests, mistletoes spread more rapidly in open than in dense stands (Parmeter 1978).
At right is a hypothetical rate of dwarf mistletoe spread in relation to stand density for ponderosa pine 30-40 years old in the Southwest (from Hawksworth 1961b). The curve would be shifted to left for younger stands and to the right for older stands.
Specificity obviously restricts the ability of mistletoes to spread and intensify in mixed stands.
Logging influences many of the factors already discussed that in turn influence mistletoes. Past cutting practices that did not heed the long-known methods of mistletoe control created many of today's problems. High grading and selective logging created open, multistoried stands with reduced species complexity. The worst situation is heavy cutting that left the worthless, heavily infected trees from the previous trees standing to infect the regeneration. That was fairly common in the old days. On the other hand, extensive clearcutting in other areas, where complete, has eradicated mistletoes from large areas.
Fire can have various effects. Where it kills infected trees of the susceptible species,it has done the same as clearcuts: eradication. It is an effective natural control agent, especially for systems like lodgepole pine where fire is important. There are extensive areas of lodgepole that today are relatively free of mistletoe because of extensive and intense fires in the past century. Areas that did not burn as much have severe infestations. In a natural stand, frequent, low-intensity ground fires were common. Foci of mistletoe infection in such stands, however, had dead trees, brooms, resinous branches, etc., and the fire could flare up and essentially sanitize such infested spots. Infected trees are more likely to burn. Brooms trap dead needles and are physically more flammable, act like fire ladder. In lodgepole pine ecosystems there is a fascinating relationship between fire, mountain pine beetle, and mistletoe.
Important in management of dwarf mistletoe are surveys to quantify the extent of the problem in various stands. A widely used rating system is Hawksworth's 6-class system. Pretty simple:
Here are facts that anyone considering management of dwarf mistletoes should clearly understand. These are features that make the dwarf mistletoes especially amenable to control:
Management of forests to reduce dwarf mistletoes is at once simple and complex. Simple in that we have known for a long time how to eradicate or greatly reduce mistletoes from a stand.
Remove infected trees. Clearcut in a large block (not narrow strips or small patches; must be 20 acres to avoid edge effects), right down to the ground. No infected residual, small trees should be left (cleancuts instead of clearcuts). This eradicates the mistletoe. Place sale boundaries in uninfested areas or against natural barriers such as roads or meadows. This prevents it from creeping in from the treatment boundary.
In the real world it is more complex. For one thing, economics is a consideration. We know how to reduce mistletoes to 0, but we might get by with a low level of disease that costs less and has a better benefit/cost ratio. That means we have to study those intermediate approaches and be able to predict the outcome.
In today's social, scientific and political climate, disease concerns and even timber management are not necessarily the most important considerations in forest management. Esthetics and wildlife concerns may tend to dictate management schemes. If you think about the ways to control mistletoes, you can imagine that mistletoe control may conflict with those other interests. For instance, forest managers in some cases today have a hard time justifying clearcuts. And when they can justify them, it is going to be very difficult to use a road or meadow as a treatment boundary because of esthetic considerations. Also, mistletoes promote forest diversity (disease foci); animals nest in brooms, etc. So approaches to disease management have to get more sophisticated.
Now, in many cases a shelterwood can be considered. This is where most trees are harvested, but a scattered overstory is left to provide regeneration and nowadays esthetic appeal. If trees that appear ±free of mistletoe can be selected for leaving, there is little cause for concern. If trees with some infections must be left, there is still a good prognosis if they are removed before regeneration is 3 ft tall or 10 years old (smaller trees are rarely infected).
In an uneven-aged stand that is not too severely infested, a good approach is what is called sanitation thinning. This means removing infected overstory trees, essentially converting it to a single story, then remove infected understory trees while precommercially thinning. To update this and promote stand diversity, it might be a good idea to identify uninfected overstory trees and leave them uncut.
For a severely infested stands, even if not otherwise ready for harvest, stand replacement is recommended.
Resistance. This is another approach we've known about for a long time. Favor resistant species during any management activity.
Pruning. On high-value areas (recreation sites, around buildings), pruning can be effective. Remove infected branches, especially brooms. Can lead to dramatic recovery in crown vigor and longevity.
Ethephon. Also for high-value areas. Ethephon releases ethylene, a plant hormone, which causes abscission of mistetoe shoots. It does not kill the endophytic system, however, so it will resprout. Prevents new infections. Need to repeat every 3-6 years.
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