Disease Cycle and Epidemiology

Disease Cycle

The dwarf mistletoe life cycle commonly takes 6 to 8 years (Figure 16). As the seed germinates on host shoots, the young radicle contacts the host bark, often beside a needle fascicle, and forms a disk-like holdfast that enlarges and grips the bark tightly. From it, a wedge develops and penetrates the bark. Penetration continues to the cambium and stops. From this penetration peg, cortical strands begin to grow in the bark, initiating development of the endophytic system (see Pathogen Biology page).

Figure 16

Infection generally occurs in host shoots that are one to five years old. That is because such shoots are most likely to have needles that intercept seeds, but also because the bark is thin enough to be penetrated before the germinating seed depletes its resources.

About 3 to 5 years after infection, after the endophytic system is developed, the mistletoe plant begins to produce aerial shoots (see Symptoms and Signs page). Shoots typically live for 5 to 7 years and may produce several crops of flowers before they die and abscise. The plant stays alive inside the host, however, and typically produces new shoots repeatedly for many years. Flowering occurs either in early summer or late summer/fall. The mature seed is explosively dispersed (see Pathogen Biology).

Many seeds are intercepted within the originating crown but some are intercepted by the crown of adjacent susceptible trees. Most seeds fail to reach a site suitable for infection; a rare few seeds are successfully transported by animal vectors to establish new infestations.

Effects of stand conditions and history

Fire may be the most important factor affecting the distribution and abundance of dwarf mistletoes in most forest types of the western United States. The fire regime in general and most recent fire in particular determines whether fire reduces or increases mistletoe. A burn that kills infected trees reduces the population of dwarf mistletoe, at least in the short term. In most cases, trees re-colonize the site much more quickly than does the dwarf mistletoe. Large, continuous, stand-replacing fires substantially reduce dwarf mistletoe populations across the landscape over long periods (Figure 17) and may eliminate local populations and result in new stands that are disease-free to maturity. Patchy burns also reduce dwarf mistletoe populations, but scattered, infected trees that escape the fire provide inoculum for early infection of new regeneration. Because infected trees may survive even extensive burns, fire can also increase mistletoe in the long term.

Historic practices can have strong influences on dwarf mistletoes. As might be inferred from the discussion of fire above, fire suppression and exclusion can lead to increase in the distribution and severity of dwarf mistletoe (Figure 18). Harvest practices that emulate fire, such as clearcuts, can have the opposite effect. Other silvicultural approaches, when carefully designed with mistletoe in mind, can have long-lasting effects in reducing severity (see Disease Management page). On the other hand, partial harvesting that does not address mistletoe, even when it is silviculturally appropriate in healthy stands, can have profound effects by creating the multistory structure that favors intensification and spread of mistletoe. Several large-scale surveys, repeated during the 20th century, suggest that there was a gradual increase in incidence of dwarf mistletoe in the areas studied (Figure 19).

Stand structure is important in determining the rate of spread and intensification of dwarf mistletoe. When small trees are growing under large, infected trees, dispersal distance can be greater than in even-aged stands. The small trees can be infected in their upper crowns at a young age, which leads to severe disease and early mortality.

Composition certainly can affect the spread and intensification of mistletoes. In stands with both host and nonhost species, host trees are usually more isolated from other host trees and nonhost trees can act as barriers to seed dispersal—both conditions tend to reduce mistletoe spread.

Host vigor has mixed effects. Dwarf mistletoe plants on vigorous hosts, especially in open stands, grow larger and produce more seed than those on less vigorous hosts. Dense crowns of vigorous hosts tend to intercept more seeds and thus infection may be more likely. On the other hand, the host grows in height faster, and vertical spread of the parasite is impeded by dense foliage. Thus, vigorous trees can sometimes keep infection in the lower crown and escape serious damage.

Figure 17	Figure 18
Figure 19	Figure 20

Sources of inoculum

When reduction of dwarf mistletoe is a goal in a managed forest, it is useful to consider sources of inoculum in four categories (Figure 20). These sources should be considered before any management actions in infested stands:

1. The most potentially damaging inoculum typically comes from residual infected trees over developing regeneration (see Stand structure, above). This situation could arise naturally from partial killing of an overstory, but also from partial cutting that leaves infected, mature trees in place.
2. Infected, advanced regeneration can cause similar damage, although dispersal distance from such trees is less. These are often noncommercial trees that are left after treatment of the overstory. Even if the overstory is completely sanitized, the understory must be treated also if it contains infected trees.
3. Inoculum from residual, infected stands at the edges of disease-free stands can also be significant. Carefully selecting boundaries or size of treatment units can reduce the effect of such inoculum (see Disease management page).
4. A variety of animals, particularly birds, have been implicated in the dispersal of the sticky seeds of dwarf mistletoes. This probably accounts for the occurrence of the disease in isolated stands and disease centers in large, regenerated, disease-free stands. However, most workers feel that such inoculum is rare enough to be epidemiologically insignificant within the time frame of forest management.

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