United Stales
Department of
Agriculture
Forest Service
Pacif ie Southwest
Forest and Range
Experiment Station
1960 Addison Street
Berkeley
California 94704
Research Note
PSW-386
July 1986
Robert E Scharpf Oetlev Vogler
estern dwarf mistletoe (Arceuthobiurn campylopodum Engelm.) is a
damaging parasite of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and Jeffrey pine (P. jejjfreyi Grev. & Balf.)
throughout much of southern California.
Large, old trees are often severely parasitized by this organism, but the extent to
which younger trees-psticularly seedlings-are infected and damaged is not
well understood.
A smdy conducted on Laguna Mountain, on the Cleveland National Forest,
California, was designed to determine the
Res. Note PSW-386. Berkeley. CA: Pacific
Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture; 1986. 2 R.
Many young, understory Jeffrey pines (Pinus
jefreyi Grev. & Balf.) were found to be infected
by western dwarf mistletoe (Arcerrthobium cantpylopodurn Engelm.) on Laguna Mountain,
Cleveland National Forest, in southern California. Under heavily infected overstory, about
three-fourths of the young pines (about 15 years
old on the average) were infected by dwarf mistletoe. Infections per tree averaged about two and
a half, two-thirds of these on the bole. Attempts
to regenerate Jeffrey pine under an infected
overstory in southern California will probably
lead to a severely infected and damaged understory. Planting nonhost species or resistant selections of Jeffrey pine would be practical in
managing dwarf mistletoe on these highly valuable forest recreational areas.
Retrieval Terms: Arceurhobiurn carrzp~lopodurn,
dwarf mistletoe, forest diseases, Jeffrey pine,
mistletoes-host resistance, Pinus jefreyi-injury by mistletoe, reproduction, understory vegetation
pine seedlings to infection by western
dwarf mistletoe. Test results indicate high
susceptibility to infection and damage
from dwarf mistletoe. Neither seedling age
nor height seemed related to infection.
bowledge of the degree of susceptibility within a stand will aid the manager
in determining ( I ) how rapidly young
seedlings become infected by dwad mistletoe in the overstory, (2) how many become idected over time, (3) when the
overstory needs to be removed or treated
in order to protect the young stand, and (4)
whether to convert to nonhost species or
resistant planting stock in order to achieve
a healthy understory.
C e r ~ i nconditions under which a seedling grows can determine the extent of infection. Understory ponderosa and Jeffrey
pine growing in partial sunlight were more
readily infected than those growing under
full or continuous sunlight.' On the other
hand, partial or full sunlight and favorabletemperatures provided favorable conditions for infection of Digger pine, (I? sabininna Dougl. by A . occidentnle
~n~elm.).
Inherent characteristics of pine also appear to be involved in its susceptibility to
inkction by dwarf mistletoe. In 1953,
~ 0 t reported
h ~
noticeable differences in
resistance to western dwarf mistletoe
among understory ponderosa pine in Oregon. In a subsequent study, ~ 0 t h planted
'
young ponderosa pine grafted with "resistant" and "susceptible" scions in the
field among heavily infected overstory.
A%er 12 years, inherent resistance was
found among certain selections of pine
grafted to the young trees. More recently,
~ 0 t hreported
~
that scions from young
ponderosa pine grafted to seedlings were
more susceptible to infection by western
dwarf mistletoe than scions from older
trees.
~awksworth~
found that infection of
ponderosa pine seedlings by A. vagirmtum
subsp. crypropodurn (Engelm. ) Hawks.
and Wiens in the Southwest is not common-though the trees are susceptiblebecause they present a very small target on
which infection can occur. In Hawksworth's shdy, only 19 percent of the trees
in the I-inch d.b.h. class were infected by
dwarf mistletoe.
Differences in resistance to western
dwarf mistletoe have also been reported
among several seed sources of Jeffrey pine
*.'
in ~aliforniia."."Therefore, it is difficult to
determiii~eor predict the level of infection
that will occur in young pondemsa or Jeffrey pine surrounded by overstory inkction without having some idea of the
general level of resistance arm a given seed
source or stand.
METHODS
One area on Laguna Mountain was selected to determine the incidence of dwarf
mistletoe in undea-story trees. The site selected was considered representative of
many stands on the mountain and contained several heavily infected overstory
Jeffrey pine and an abundance of understory seedlings. Moreover, Laguna Mornntain is representative of many high value,
high use recreational forests that are badly
infested by western dwarf mistletoe in
southern California. All seedlings I inch
(2.54 em) or less in diameter at ground
level were examined fix dwarf mistletoe
infectio~~.
Only seedlings g m i n g under
the drip line of the oventory tree crown
were taken. Seedling height was masured
to the nearest inch and the number of bole
(main stem) and branch infections recorded. The seedlings were then cut and
taken into the laboratory where tree age
and age of the oldest infection were determined." A total of 52 seedlings were
sampled.
WSULTS AND DISCUSSION
Results of the study indicate that young
Jeffrey pine at Eaguna Mountain are
highly susceptible to infection and damage
from western dwarf mistletoe. Of the 52
seedlings sampled, 40 (77 pct) were infected. Of the infected seedlings, 95 percent had one or more bole infections, and
30 percent had both bole and branch infections; and one seedling had branch infections only. Bole infections comprised
about two-thirds of all infections. Bole infections are considered particularly dam-
aging because they reduce vigor and stunt
seedling growth.
Levels of infection were low. Forty percent had only a single infection, most of
these in the bole. The average number of
infections per infected tree was 2.4 (range
1 to 8). The average age of the oldest infection on each seedling was 5.2 years
(range 2.to I8 years) and the average age
of the brmch or bole at the time of infection was 4.3 years (range iZ to 10 years).
Neither seedling age nor height seemed
related to infection. The average age of the
B 2 uninfected trees was B 5.8 yeas (range
6 to 21 years), whereas that for the 40 infected trees was 15.9 years (range 9 to 26
years). Uninfected trees averaged 25.9
inches (66 cm) in height (rmge 10-39
inches [25 to 99 cm]), a d infected trees
averaged 24.7 inches (63 cm) in height
(range 5 to 42 inches [ 13 to 107 cm]).
These results somewhat contrast those
of ~ 0 t in
h ~which uninfected ponderosa
pine seedlings were noticeably taller than
infeckd individuals after 12 yeas. 81though the results of this study-do not ina
effect
growth, the
dominant seedlings were ~ o ~ ~ i b l first
y
to become infected and stunted. The
smaller, unidected trees were then able to
continue growing and attain sizes
to the infected trees before
of these
idso became infected. In fact, seedlings
w s t reach a certain size before they provide sufficient target area for dwarf mistletoe seeds to hit. Thus, seeds of the
parasite have gzater probability of landing
on larger seedlings.
A dismrbing findhg of this study was
the high percenhge of fairly young seedlings which were infected by western
dwarf mistletoe, most cont~ningdamaging bole infections. On Laguna Mounhin
and possibly other areas of southern California where Jeffrey pine is ideckd, it
remay not be practical to attempt
generation of pine under an infected Overstory. unless the overstory is removed
when the seedlings are very small, many
seedlings likely will kcome infected and
stunted. Favoring or planting nonhost species or planting resistant selections of Jeffrey pines would be practical alternatives
to establishing a natural understory in
heavily infected, high value recreationd
areas.
ENlD NOTES A m WFEWNCES
' ~ a ~ e n eWillis
r,
W. The inJluence of light on establishment and growth of dfi~a*istletoe on ponderosa and JeffP-ey pines. Res. Note PSW-181.
Berkeley, CA: Pacific Southwest Forest and Range
Experiment Station, Forest Service, U.S. Department of Agriculture; 1961. 5 p.
2~charpf,Robert E Temperature afects penetration and injection of pines by dwarf mistletoe. Forest
Sci. 15(2):149-151; 1969.
3~charpf,Robert E Light afects penetration and
infection ofpirzes by dwarfmistletoe. Phytopathology
62:1271-1273; 1972.
4 ~ o t hLewis
,
F. Pine dwarfmistletoe on the Pringle
Falls Experimental Forest. Res. Note PNW-91. Corvallis, OR: Pacific Northwest Forest and Range Experiment Station, Forest Service, U. S . Department
)griculture; 1953. p.
Roth, Lewis E Resistance of ponderosa pine to
dwarfmistletoe. Phytopafhology 57:1003; 1967.
Lewis F. Juvenile susceptibility- o- f -ponderosapine to dwarfmistletoe. lohytopathology 64689692; 1974'Hawksworth, Frank G. Dwarfmistletoe of ponderosapine in the southwest. Tech. Bull. 1246.Wshineton.
DC: U. S. Deoartment of Agriculture.
Forest
"
"
Service; 1961. 112 p:
'Scharpf, Robert F. Host resistance to dwarfmistletoes. In: Nawksworth, Frank 6 . ; Scharpf, Robert
F., tech. coords. Biology of Dwarf Mistletoes: Proceedings of the Symposium, 1984 August 8; Fort
Collins, CO. Gen. Tech. Rep. RM-I 11. Fort Collins,
CO: Rocky Mountain Forest and Range Experiment
Station, Forest Service, U.S. Department of Agriculture; 1984: 70-76.
'~charpf,Robert F. ;Parmeter, J. R., Jr. Spread of
dwarfmistletoe in Jeffrey pine plantation: trees infected a@er22 years. Res. Note PSW-141. Berkeley,
CA: Pacific Southwest Forest and Range Experiment
Station, Forest Service, U.S. Department of Agriculture; 1967. 6 P.
I0~charpf,Robert E; Parmeter, J. R., J r Determining the age of dwa~@istletoeinfections on redjr.
Res, Note PSW-105 Berkeley, CA: Pacific Southwest Forest and Range Experiment Station, Forest
Service, U.S. Department of Agriculture; 1966.5 p.
*
The Authors:
Robert E Sclnarpf is project leader, forest disease research, with headquarters in Berkeley,
Calif. He earned a bachelor's degree in forestry (1953) at the University of Missouri, and a
master's degree in forestry (1957) and a doctorate in plant pithology (1963) at the University
of California, Berkeley. He joined the Forest Service in 1960 and has been with the Station's
staff since then. Detlev 'vw1er is plant pathologist, with headquarters in San Francisco. He
earned bachelor's (1965) and master's (1972) degrees from San Francisco State University,
and a master's degree (1979) in plant pathology from the Univ&sity of California, Berkeley.
Me joined the Forest Service in 1973.
-