Disease Management

Development of management approaches for dwarf mistletoes can be considered one of the first success stories of forest pathology in North America. Early western pathologists provided principles and approaches for control of dwarf mistletoes through forest management. However, it gradually became clear that, although control of dwarf mistletoes was simple in theory, it was sometimes complex and difficult in practice.

Features of dwarf mistletoes that facilitate their management include the following:

1. Dwarf mistletoes are obligate parasites. They cannot survive without a living host. Once the branch or tree dies or is cut, the parasite dies.
2. Dispersal distance is limited and spread is slow. Explosive seed dispersal is only up to about 60 ft from a tall, isolated tree. In single-storied stands, spread is usually about 2 ft per year. This creates possibilities for protecting trees by separation from infected trees.
3. The life cycle is long. From dispersal to production of a new generation of mature fruit typically takes 6–8 years. Disease intensification (multiplication of infections and increase in severity) is therefore fairly slow in infested areas.
4. Dwarf mistletoes tend to be host specific. Mixed stands and changes in composition therefore can create a disadvantage for the mistletoes.
5. They are relatively easy to detect. Unlike most pathogens, dwarf mistletoes are entirely above ground, partly exposed on the surface of the host, visible without a microscope, and usually cause distinctive symptoms.

Dwarf mistletoe rating

Hawksworth’s 6-class dwarf mistletoe rating (DMR) system is used almost universally to rate severity of dwarf mistletoe (Figure 21). Many disease parameters and management recommendations are provided in terms of DMR. Actually a 7-class system since it ranges from 0 (uninfected) to 6, it is based on rating each third of the crown on a scale from 0–2, then summing for the tree rating. In recent applications, stem infections are only considered if there are no branch infections at all, in which case the tree is scored “1”. Stem infections usually contribute little to effective spread.

In a check of rating accuracy by cutting and examining tree crowns, ratings were accurate about 75% of the time. Ratings were more reliable for small than for large trees. Severely infected trees tended to be underrated because of failure to detect infected branches in the upper crown. Lightly infected trees tended to be overrated because of a tendency to lower the boundary between the lower and middle third. These rating errors tended to cancel each other out, although overall there was a slight underestimate. Binoculars should be used to enhance detection. A common mistake is to stand too close to the tree, which can obscure symptoms and signs as well as cause perspective errors in dividing the crown into thirds.

Stand DMR as a single measure of disease intensity is calculated by averaging the DMR of all principal host trees in the stand, including uninfected trees. The percent of host trees that are visibly infected provides a measure of mistletoe incidence; the average DMR of infected trees only is designated dwarf mistletoe index (DMI) and complements percent infected with a relative measure of mistletoe severity on those trees.

Management approaches

Chemical and biological controls appear to be inadequate in most cases. Herbicides that kill the parasite, including the endophytic system, usually damage the host as well. Ethephon formulations (active ingredient 2-chloroethyl phosphoric acid, which releases ethylene) have been used successfully to cause abscission of shoots (Figure 22). They do not affect the endophytic system, however, so application must be repeated every few years to prevent inoculum production. A variety of interesting insects and pathogens attack dwarf mistletoes, but none are yet developed sufficiently for practical application (Figure 23).

Figure 21	Figure 22
Figure 23

In forest settings, then, silviculture is almost the sole toolkit for dwarf mistletoe management. Detailed considerations cannot be presented here, but following are some approaches that are used:

1. Choose uninfested borders for treatments. Regardless of stage of stand development or the management approach, treated areas should be bordered as much as possible by areas that will not be sources of inoculum (Figure 24). This includes nonsusceptible or uninfested stands, roads, forest openings, etc.
2. Size matters. To avoid re-infestation of treated areas that have infested stands on the border, treated areas must be large enough that spread into them from the borders is insignificant, or at least acceptable, during the life of the stand (Figure 25). The proportion of a treated area affected by mistletoe from the border decreases as the treated area becomes larger. In this situation, 20 acres is considered a minimum and 40 acres is recommended. As patches increase beyond 40 acres, the advantage of increasing size becomes less. Irregularly shaped or long, narrow patches must be larger to have a similar area protected.
3. Favor nonhosts. Whether planting, spacing, thinning, selecting trees to provide seed for the next generation, etc., encourage and favor tree species that are not hosts of the mistletoe in the stand (Figure 26).
4. Grace period for seedlings. Because of their small size as targets and their short exposure to inoculum, seedlings generally can be considered safe from infection until they are 10 years old or 3 feet tall, whichever comes first. Infection of smaller or younger trees does occur, but it is generally rare. This gives some time before infected overstories must be treated after regeneration is established.
5. Even-aged management. Even-aged management is regulation of age structure through removal and regeneration treatments such that trees are of similar age. Where silviculturally appropriate, even-aged management, especially at the stage of regeneration, offers the best opportunity to establish a mistletoe-free stand. This means cutting or killing the stand, either all at once (e.g., clearcut) or in stages by seed-tree method (cutting of all trees except for a few desirable individuals that provide seed for the next generation) or shelterwood method (cutting of all trees in a series of two or more over a relatively short period of time, establishing even-aged reproduction under the partial shelter of the previous generation). In seed-tree or shelterwood methods, residual overstory trees must be removed before the regeneration is out of its grace period (Figure 27).
6. Sanitation. Sanitation, the removal or killing of infected trees to protect other trees, is important in many kinds of stands at various developmental stages. “Sanitation cutting” (or simply sanitation) has been distinguished from “sanitation thinning.” Sanitation cutting is the attempted removal of all visibly infected trees, though it usually is combined with thinning goals also. In sanitation thinning, the emphasis is on spacing, and only the most severely diseased trees may be removed. Because of the long period between infection and appearance of symptoms (latency) and difficulty of locating every visibly infected tree, multiple entries (5 to 10 years apart) are usually necessary to sanitize a stand or minimize disease severity.
7. Prescribed fire. Species that regenerate well after fire, such as lodgepole pine, can be treated with prescribed, stand-replacing fire to establish a new stand in the absence of inoculum. This can be an effective and economical means of stand replacement, and it is entirely consistent with the natural disturbance regime. It can be used in heavily infested stands that have little economic value, or after merchantable trees are harvested. Stands should be inspected following the fire and any surviving trees felled. Even low-severity fire (such as a prescribed burn to reduce fuel loads) may be beneficial because infection centers tend to torch and lower branches, most likely to be infected, are often killed or pruned by scorch (Figure 28).
8. Pruning. Pruning may have several objectives. Pruning of large brooms, which are generally in the lower crown, can allow trees to recover vigor and substantially prolong their life (Figure 29). It is most often used in developed recreation sites. Another objective of pruning is sanitation (i.e., sanitation pruning). In this case the removals are intended to reduce the population and impact of dwarf mistletoes as well as future inoculum. Pruning can also be used to reduce hazard of personal injury or property damage from the fall of a large broom (especially on species with brittle wood, such as larch).

Figure 24	Figure 25	Figure 26
Figure 27	Figure 28	Figure 29

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