Pathogen Biology

The genus Arceuthobium (Santalales: Viscaceae) is a group of small plants that are exclusively parasitic on conifers. The name is from Greek (arkeuthos + bios) and means juniper-life; the first species described was on juniper in Europe. There are about 42 species, mostly in North and Central America, where they occur only on members of the family Pinaceae. Eight species occur in Eurasia and Africa, where some are also parasitic on junipers (family Cupressaceae). Arceuthobium species tend to be host-specific, parasitizing only one or two principal hosts, but there are some exceptions; for example, A. globosum subsp. grandicaule parasitizes more than ten species of pine in Mexico. Some of the more important mistletoes in North America are

• A. abietinum, on true firs
• A. americanum, on lodgepole pine
• A. campylopodum, on ponderosa and Jeffrey pines
• A. douglasii, on Douglas-fir
• A. pusillum, on eastern spruces
• A. tsugense, on hemlocks
• A. vaginatum, on southwestern and Mexican pines

Endophytic system

The endophytic system, as the name implies, is the portion of the dwarf mistletoe that is inside the host plant. It consists of two components that provide anchorage and extract nutrients from the host—cortical strands and sinkers (Figure 12).

Cortical strands are fine strands of mistletoe tissue that ramify through the host phloem and give rise to shoots and sinkers. Older portions of the system may be pushed with the old phloem into the periderm (outer bark). In systemic infections, strands continue acropetal growth (towards the shoot apex) to the apical meristem of the host branch (shoot tip). There growth keeps up with branch elongation and new branches are also infected.

Sinkers are strands of mistletoe tissue that are embedded in the rays of the xylem (wood). They are initiated near the growing tips of the cortical strands and grow radially, towards the host vascular cambium. Here they displace cambium initials. When the cambium produces new xylem and phloem, the sinkers maintain pace via an intercalary meristem aligned with the cambium. In this way, without actively penetrating the xylem, the sinkers become embedded in the rays and avoid being torn apart by the host’s growth (Figure 13).

Figure 12

Figure 13

Local vs. systemic infection

There are two types of infections by dwarf mistletoes:

Local infection: The endophytic system does not advance far from the point of infection. Mistletoe shoots are produced only near the point of infection, and swelling is usually persistent (Figure 2). All species can produce this kind of infection.

Systemic infection: The endophytic system grows to the apical meristem of the infected shoot. It keeps pace with apical shoot growth and invades all branches subsequently produced from it (Figure7). Swelling may occur in early stages but it often becomes less pronounced or disappears over time. Mistletoes with systemic infection often produce very large brooms. Systemic infection is thought to be an apomorphic trait (i.e., species that cause systemic infection evolved from ancestors that caused local infection) and occurs regularly in only a few Arceuthobium species: A. americanum, A. douglasii, A. guatamalense, A. minutissimum, and A. pusillum. Systemic infection tends to be associated with other apomorphic traits such as high host specificity, small shoot size, flabellate (fan-shaped) vs. verticillate (whorled) branching, and formation of flower buds in late summer for early spring flowering.

Figure 2

Figure 7

Host susceptibility

For each dwarf mistletoe species, hosts are ranked in classes of susceptibility (Table). The classes are defined on the basis of percentage infection when within 6 m of a heavily infected host.

Table. Host susceptibility classes for dwarf mistletoes.

Class	Incidence of infection *
Principal	>= 90 %
Secondary	50-90 %
Occasional	5-50 %
Rare	<= 5 %
Immune	0 %

* Percentage of trees infected when within 6 m of heavily infected principal hosts in stands older than 30 years.

Parasitism

Dwarf mistletoes are obligate parasites, meaning they cannot survive (except as seed) without a living host. From the host they receive all of their water and inorganic nutrients, and most of their fixed carbon.

Dwarf mistletoe shoots (especially fruits) transpire much more water, on a surface-area basis, than does host foliage. The difference may increase up to 60-fold during periods of water stress, when the host limits its own water use. Most inorganic nutrients are in higher concentration in dwarf mistletoe shoots than in host tissues.

Although dwarf mistletoes can photosynthesize to a limited extent, they derive almost all their fixed carbon from their hosts. In contrast, the leafy, or true, mistletoes depend on their host mostly for water and minerals and less for photosynthates.

Dwarf mistletoes apparently establish themselves as a sink for fixed carbon by the production of hormones. Most attention to date has focused on cytokinins. This not only satisfies the need of the parasite, it also leads to lush development and often profuse branching of nearby host tissue. The resulting witches’ brooms are produced at the expense of other parts of the tree, especially the upper crown and perhaps the roots. This disturbance of host physiology profoundly affects growth and can eventually lead to mortality. Yet the witches’ brooms are often the last part of a tree to die.

Reproduction and dispersal

Although photosynthesis by shoots may contribute slightly to dwarf mistletoe nutrition, the primary function of shoots is reproduction. Dwarf mistletoes are dioecious; they produce male and female plants.

The mature fruit contains a single seed that is explosively discharged by one of the most effective hydrostatic mechanisms among flowering plants. During maturation, the fruit pedicel (Figure 1) bends so that the seed is often discharged at about 30 degrees above the horizontal, an angle that maximizes lateral distance for targets within 11 vertical meters (36 feet) below the source, and also allows the possibility of climbing the tree canopy. The initial velocity of the seed is about 27 m•sec^-1 (60 mph) (Figure 14). Maximum dispersal distance is about 16 m (52 ft), but dispersal distance of 10 m (33 ft) or less are more typical. The seed is coated with a sticky mucilage (viscin), so it adheres to needles that it strikes (Figure 15). It remains on the needle until rain wets the viscin, allowing the lubricated seed to slide down the needle and, if the needle is upright, make contact with the bark and needle base.

Figure 1

Figure 14

Figure 15

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