Dwarf mistletoes of ponderosa pine

10 Pages·2003·3.3 MB·English

by  Beatty, Jerome S

Historic, Archive Document
Do not assume content reflects current
scientific knowledge, policies, or practices.

Forest Insect
sae DYW er:Ws
Leaflet 49'
Revised September 2003
U.S. Department of Agriculture ¢ Forest Service
Dwarf Mistletoes of Ponderosa Pine __
Jerome S. Beatty’ and Robert L. Mathiasen*
The dwarf mistletoes (Arceuthobium Southwestern dwarf mistletoe (A.
Spp.) are important parasites found vaginatum subsp. cryptopodum)
throughout most of the range of pon- occurs on the Rocky Mountain vari-
derosa pine (Pinus ponderosa). ety of ponderosa pine (Pinus pon-
derosa var. scopulorum) from
Ponderosa pine 1s parasitized by
Arizona and New Mexico into central
two widespread species.
Utah and northern
Colorado. Western dwarf
mistletoe (A. campylopo-
dum) affects the Pacific
coast variety of ponderosa
pine (P ponderosa var. pon-
derosa) from southern
California to northern
Washington and western
Idaho (Figure 1).
Although both dwarf
mistletoes occur primarily
on ponderosa pine, they
| also commonly infect some
related pines (Table 1).
Southwestern dwarf
mistletoe is equally damag-
| ing to Arizona pine (Pinus
arizonica) and Apache pine
(Pinus engelmannii) in the
Figure 1. Distribution of
OY southwestem ) southwestern and western
bm dwarf mistletoe ; dwarf mistletoes (shaded
CAA Pliers: Ss areas) and ponderosa pine
Ld Meiovaa' = (thick blue line).
'This Insect and Disease Leaflet combines Leaflet #19 “Dwarf mistletoes of Ponderosa Pine in
the Southwest” and #40, “Western dwarf mistletoe on Ponderosa Pine.” * Jerome Beatty is Deputy
Director, USDA Forest Service, Forest Health Protection, Washington, DC; * Robert Mathiasen is
Associate Professor, School of Forestry, Northern Arizona University, Flagstaff, AZ.
I
Southwest. Similarly, western dwarf Dwarf mistletoes do not have
mistletoe frequently infects Jeffrey large leaves; instead their leaves are
pine (Pinus jeffreyi) and Coulter pine small squamate scales found at the
(Pinus coulteri) in California. nodes of shoots. The average height
of aerial shoots for these mistletoes is
Both mistletoes will also attack
about 4 to 6 inches (10-15 cm).
some introduced pines; those hosts
are listed in Table 1. Aerial shoots arise from a network
Table 1. Hosts ofs outhwestern and western dwarf mistletoe.
Susceptibility Class | Southwestern Dwarf Mistletoe | Western Dwarf Mistletoe
(Arceuthobium vaginatum) (Arceuthobium campylopodum)
Principal hosts Rocky Mountain ponderosa pine | Pacific coast ponderosa pine
Arizona pine Jeffrey pine
Apache pine
Secondary hosts’ Coulter pine
Knobcone pi
Occasional hosts? Rocky Mountain bristlecone pine | Grey pine
Rocky Mountain lodgepole pine Rocky Mountain lodgepole pine
Sierra lodgepole pine
Rare hosts’ Limber pine Sugar pine
Southwestern white pine
‘Trees frequently attacked when they are associated with infected principal hosts
*Trees occasionally attacked when associated with infected principal host
*Trees rarely attacked, even when associated with infected principal hosts
Life History
of root-like absorbing strands
a
imbedded in host tissues. This net-
The external (aerial) shoots of west-
work, called the endophytic system,
ern dwarf mistletoe are olive green to
consists of cortical strands growing
brown or yellow-brown (Figure 2).
within the bark and sinkers within the
The aerial shoots of southwestern
wood. The endophytic system
dwarf mistletoe are orange to reddish-
obtains nutrients and water from the
brown to almost black (Figure 3).
host tree.
Figure 2. Female shoots of western dwarf
Table 2. Flowering and seed dispersal periods of the
two ponderosa pine dwarf mistletoes.
Characteristic Southwestern Dwarf Mistletoe | Western Dwarf Mistletoe
(Arceuthobium vaginatum) (Arceuthobium campylopodum)
Flowering period August - October
Seed dispersal period | July - August September - November
Germination Seeds germinate immediately Seeds overwinter and germinate
after seed dispersal fhe following April - May
The endophytic system usually in length. Seed dispersal is one of
lives as long as adjacent host tissues the most interesting characteristics of
are alive. The mistletoe is dependent dwarf mistletoes. Seeds are dis-
upon its host trees for water and charged explosively from ripe fruits
nutrients, and most of its carbohy- in late summer for southwestern
drates. Although the aerial shoots dwarf mistletoe and early fall for
contain chlorophyll that allows them western dwarf mistletoe (Table 2).
to produce small amounts of carbohy-
Seeds may travel 10-13 m, but
drates, the major function of aerial
most land within 3-5 m of the dis-
shoots 1s reproduction.
Male and female flowers are seminating shoot. A sticky seed coat-
small and produced on separate ing called viscin enables seeds to
plants. Southwestern dwarf mistletoe stick to most objects they strike.
flowers in the spring and western Foliage is the most common receiv-
dwarf mistletoe in the fall (Table 2). ing surface. Viscin acts as a lubricant
Insects and wind pollinate female when moistened by rain. Seeds slide
flowers. down and either fall off needles or
become lodged on bark near the base
Fruits complete their develop-
of needles. Seeds are fastened in
ment 12 tol5 months after pollina-
place when the viscin dries.
tion. The mature fruits contain one
seed averaging about ().2 inches (3 mm)
Seeds are often destroyed by visible. When shoots fall off of
insects and fungi or dislodged by rain infected branches they leave behind
or snow, so only a small proportion small cup-like structures embedded in
of the seeds dispersed actually sur- the bark. These are called basal cups.
vive and cause new infections.
As time passes and the endophyt-
Seeds of southwestern dwarf ic system of the parasite becomes
mistletoe germinate immediately after more extensive, the branching habit
dispersal, while those of western of the host may become distorted and
dwarf mistletoe over-winter and ger- witches’ brooms are formed.
minate in early spring. Witches' brooms are variously shaped
masses of abnormal branch and twig
A root-like structure called a
growth. Broomed branches usually
radicle emerges from the seed and
outlive their uninfected neighbors by
grows along the bark surface until an
many years. They divert food from
obstruction, usually a needle base, 1s
uninfected parts of the tree and thus
encountered. The radicle then forms
are instrumental in reducing vigor
a mound of tissue called a holdfast.
and causing premature death of
The holdfast develops an infection
severely infected trees.
peg, which penetrates into host tissue.
The mistletoe's endophytic system Long-lived witches' brooms also
then develops in the outer bark and result in excessively large knots; it is
wood of the infected part of the host. not uncommon for the base of
broomed branches on otherwise clear
Infection occurs most readily in
trunks to become over 10 inches (25
twigs less than five years old because
cm) in diameter. Resinous cankers
their bark is more easily penetrated
may result from mistletoe infection of
than older twigs.
the main stem, especially on smaller
Aerial shoots typically appear 2 trees.
to 3 years after initial infection.
Severely infested pine stands typ-
Infections that have not yet produced
ically have many trees with stunted
aerial shoots are called latent infec-
growth, witches' brooms, dying or
tions. The typical length of time
dead tops and branches, and dead
needed for female plants to complete
trees. Dieback occurs as nutrients
their life cycle from initial establish-
and water needed by growing tree
ment to dissemination of the first
tops are diverted to infections that are
seed crop is 4 to 5 years for both of
usually concentrated in the lower or
these dwarf mistletoes.
mid crowns.
Symptoms and Signs of Foliage near infections becomes
sparse and off-color and gradually the
Infection
upper branches die. Eventually
height growth slows, and such trees
The first symptom of dwarf mistletoe
may subsequently be attacked and
infection is the appearance of slight
killed by bark beetles. These stands
swellings at infection sites. Swellings
eventually contain numerous dying
become visible | to 2 years after
and dead trees, usually bearing rem-
infection occurs. Aerial portions of
nants of witches’ brooms.
male and female plants appear after a
year or two after the swellings are
Spread and Intensification crown third have infections). The
three ratings are then added to obtain
Several interrelated factors influence a tree rating ranging from 0 (healthy
tree-to-tree spread of ponderosa pine trees) to 6 (severely infected trees).
dwarf mistletoes. These include tree The tree ratings of all live trees in a
size, tree spacing, stand structure, stand or plot (including uninfected
species composition of stands, and ones) are then summed and the total
infection position. divided by the number of live trees to
obtain an average rating for the stand.
In single-storied stands, spread is
Average dwarf mistletoe ratings for a
estimated to be 2-3 feet (0.6-1.0 m)
stand can then be used to help deter-
per year. Spread in multi-storied
mine what management options can
stands is more rapid because under-
be considered for the infested area.
story trees are exposed to dwarf
mistletoe seeds from infected over- Dwarf mistletoe ratings can also
story trees. Presence of non-host tree be used to help decide which trees
species can slow the spread of pon- should be removed or retained if a
derosa pine dwarf mistletoe. Spread decision is made to conduct harvest-
rates in very dense stands are less ing or thinning treatments in the
than in more open stands because infested area.
dwarf mistletoe seed production is
As a rough rule-of-thumb, inten-
usually reduced due to limited light
sification of pine dwarf mistletoe
and lower host vigor, and many seeds
averages about one DMR class per
are trapped before they travel far.
decade for individual trees, but varies
Dwarf mistletoe seeds from plants
with tree size, stand position, and
high in tree crowns tend to travel far-
overstory infection. Infection intensi-
ther than those in lower portions of
fies most rapidly in sapling or pole-
crowns.
size trees under severely infected
Nearly all spread is local and larger trees.
results from explosive discharge of
Impacts
seeds. Wind exerts a minor influence
on distance and direction of seed
travel. Birds and other animals are Infection of ponderosa pine by dwarf
responsible for some long-distance mistletoe causes increased mortality,
spread when seeds stick to them and reduced growth rates and loss of
later are rubbed off onto susceptible vigor, lowered timber quality,
TEES reduced cone and seed production,
and increased susceptibility to other
The 6-class dwarf mistletoe rat-
damaging agents. These damaging
ing (DMR) system is useful for quan-
effects result from the dwarf mistle-
tifying intensity of infection in trees
toe plants taking food and water from
and stands. For this system, the live
the host, thus reducing the amount
crown of the tree is visually divided
available for the tree's normal growth,
into thirds and each third rated as: 0
protective, and reproductive processes.
= no visible infection, | = light infec-
tion (less than half of the branches in The effect of dwarf mistletoe on
the crown third have dwarf mistletoe growth increases with severity of
infections), or 2 = heavy infection infection and is especially acute in
(more than half of the branches in the severely infected trees (DMR = 5 or
6). Tree volume growth can be
reduced by as much as 50% in Management of ponderosa pine dwarf
severely infected trees. Severely mistletoes must recognize their value
infected trees (DMR = 6) usually live as functional components of forest
less than 10 years in the Southwest. ecosystems. In areas where timber
production or developed recreation is
Quality of lumber in infected
the primary goal, direct control of
trees can be reduced because dwarf
dwarf mistletoe may be warranted. In
mistletoe sometimes infects the boles
other areas, where wildlife or esthetic
of trees. Knots often are associated
values are more important, maintain-
with branches supporting witches'
ing or even increasing dwarf mistle-
brooms. Severely infected trees typi-
toe populations may be appropriate.
cally produce few cones, and those
that are produced are smaller than
Cultural Controls
normal and contain fewer seeds.
Dwarf mistletoes can adversely The only practical control of pon-
affect other forest values. Witches' derosa pine dwarf mistletoes over
brooms and dead branches can large forested areas is through cultur-
increase the hazard potential in recre- al treatments. No chemical or biolog-
ation sites because they may cause ical controls are available for treating
trees and branches to break and fall. stands.
Dead and dying trees detract from
Profitable production of timber in
visual quality. Potential for wildfires
many local areas often depends on
is increased because of resin soaked
management of dwarf mistletoe.
live branches, dead branches,
Because the parasite can cause large
increased tree mortality, and the accu-
reductions in yield, management
mulation of dead, resinous branches
should be considered in all timber-
around the bases of infected trees.
producing stands where dwarf mistle-
However, dwarf mistletoe infec- toes occur.
tion can also have beneficial effects.
Successful timber volume pro-
Flowers, shoots, and fruits are food
duction is almost impossible in multi-
for insects, birds, and mammals.
storied, infested stands. Silvicultural
Mortality caused by dwarf mistletoe,
treatments designed to achieve sin-
either directly or by predisposing
gle-storied stands offer the best
trees to other agents, provides snags
prospects of preventing unacceptable
as habitat for cavity-nesting birds
losses to dwarf mistletoes.
and, eventually, coarse-woody debris
on the forest floor. Dwarf mistletoe The most effective method for
brooms create additional structure eliminating dwarf mistletoes from
and various species of wildlife use timber-producing forests is complete
the witches’ brooms as nesting sites. harvest of infested stands by clearcut-
ting. After usable trees are harvested,
Management all remaining infected trees should be
killed.
In forest ecosystems, dwarf mistle-
To minimize invasion of young
toes have value as individual, biologi-
pine stands by dwarf mistletoe from
cal species and act as disturbance
infected border trees, the ratio of
agents, influencing both the structure
perimeter to area of clearcuts should
and function of forest communities.
be as low as possible. That is, cut decline rapidly about 10 years after
openings should be roughly circular they are exposed to full sunlight by
and not long, narrow strips. thinning.
Advantage should be taken of Because of their rapid decline,
any potential barriers to dwarf mistle- moderately infected trees should not
toe spread, such as roads, ridgetops, be left when stands are being sani-
natural openings, and changes in tim- tized unless they can be expected to
ber types when laying out the bound- reach merchantable size within 15
ary of a clearcut. years.
When clearcutting of dwarf Thinning priorities should be
mistletoe-infested stands is not appro- based on both dwarf mistletoe severi-
priate, shelterwood and seed tree har- ty and live crown ratios. First priori-
vests can be good alternative even- ty for removal should be all trees
age management methods. Trees with a live crown ratio < 40% or
selected to provide shelter or seeds trees with live crown ratios < 60%
should be uninfected or only lightly with a DMR of 5 or 6. Last trees to
infected (DMR < 3). be removed should be those with a
live crown ratio > 80% and a DMR <3.
Moderately and severely infected
trees, in addition to being a source of In stands where management of
dwarf mistletoe seeds, produce poorer hiding and thermal cover for wildlife
crops of tree seeds. Infected shelter- is a primary consideration, infected
wood or seed trees should be trees with large witches' broom
removed as soon as susceptible repro- should be treated by girdling rather
duction becomes established. As a than felling.
general rule for most dwarf mistle-
Reexamination of stands 5 years
toes it is desirable to remove the
after sanitation is desirable to deter-
infected overstory before the young
mine if additional sanitation 1s need-
stand is 3 feet (1 m) tall or 10 years
ed. Moderately and severely infested
old, whichever occurs first.
stands should not be sanitized
In mixed-species stands that con- because many trees with latent infec-
tain pines infected by dwarf mistle- tions will be missed, and once these
toe, silvicultural treatments should trees are released from competition,
favor other tree species. Non-hosts the number of dwarf mistletoe plants
left between infected and non-infect- will increase rapidly within their
ed pines prevent or slow spread and crowns. Such stands should either be
intensification of the parasite. clearcut and regenerated or left
unthinned to avoid their rapid
Thinning or sanitation by
decline.
removal of infected trees can be an
effective treatment in lightly infested
Latent dwarf mistletoe infections in
stands. Lightly infested is defined as
trees in densely stocked stands and in
those stands in which there are
shaded reproduction are very difficult
acceptable numbers of desirable
to detect. It should be assumed that
dwarf mistletoe-free or lightly infect-
understory trees more than 3 feet (1
ed (DMR <3) trees. Trees with one-
m) tall that have been overtopped by
half or more of their crowns infected
infected trees for at least 10 years are
by dwarf mistletoe (DMR >3) may
probably infected.
Recreation Management Assistance
In recreation areas, sanitation treat- Resource managers can get more
ments that favor non-susceptible information about the identification
hosts or remove infected overstory and management of dwarf mistletoes
trees are appropriate. Pruning, in of ponderosa pine by contacting a
conjunction with removal of severely County Cooperative Extension agent,
infected trees, can prolong the life of their local state forestry office, or
individual trees. their regional USDA Forest Service,
Forest Health Protection office.
For high value trees, removal of
live witches' brooms that exert a large
References
drain on infected trees can improve
their vigor and prolong their life.
Hawksworth, F.G. 1961.
Trees may have to be repruned occa-
Dwarfmistletoe of ponderosa pine in
sionally to remove developing witch-
the Southwest. USDA Forest Service
es' brooms. Candidates for pruning
Technical Bulletin No. 1246, 112 p.
are trees with infections only in the
lower half of their crown.
Hawksworth, F.G. 1977. The 6-class
In addition to prolonging tree dwarf mistletoe rating system.
life, pruning can reduce the danger of USDA Forest Service Research Note
trees or branches breaking and pre- RM-48, 7 p.
vent damage and subsequent liability
from falling branches. Trees with Hawksworth, F.G., and D. Wiens.
bole infections may contain decayed 1996. Dwarf mistletoes: biology,
wood and therefore may have a high- pathology, and systematics. USDA
er potential to fail. Forest Service Agriculture Handbook
709, 410 p.
Pesticides used improperly can be injurious to
humans, animals, and plants. Follow direc-
tions and read all precautions on the labels.
Consult your local forest pathologist, county
agricultural agent, or State extension agent
about restrictions and registered uses of par-
ticular pesticides.
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