United States 

Department of 

Agriculture 

Forest Service 

Pacific Southwest 

Research Station 

Research Paper 

PSW-RP-208 

Resistance of Ponderosa 

Pine to Western Dwarf 

Mistletoe in Central Oregon 

Robert F. Scharpf Lewis F. Roth 
Scharpf, Robert F.; Roth, Lewis F. 1992. Resistance of ponderosa pine to western dwarf 

mistletoe in central Oregon. Res. Paper PSW-RP-208. Albany, CA: Pacific Southwest 

Research Station, Forest Service, U.S. Department of Agriculture; 9 p. 

Ponderosa pines with little or no dwarf mistletoe in infested stands on the Deschutes, Ochoco, 

and Rogue River National Forests in Oregon were tested for resistance to dwarf mistletoe 

(Arceuthobium campylopodum). Small trees produced by grafting scions from the resistant and 

susceptible candidates onto seedling rootstock were planted in 1967-69 beneath a heavily 

infested stand of ponderosa pine in central Oregon. Between 12 and 15 grafted trees from each 

of 30 resistant parent selections and about the same number from each of eight susceptible parents 

were included in a replicated, randomized planting. Also included as controls were about 125 

ungrafted seedlings. In 1989, fewer than half of the resistant grafts from selections from the 

Deschutes and Ochoco National Forests were infected, and the average level of infection per tree 

was very low. Mortality among the resistant selections was also low over the test period. About 

85 percent of all Rogue River grafts, the susceptible grafts, and the seedlings were infected during 

the test period. Few Rogue River grafts died, but 52 percent of the susceptible grafts and seedlings 

died in comparison to 24 percent of the resistant trees. Size of the test trees, foliar habit, andcrown 

characteristics were not found to be correlated with resistance. High levels of resistance to dwarf 

mistletoe are present in some native ponderosa pines in central Oregon. 

Retrieval Terms: resistance, Pinus ponderosa, Arceuthobium campylopodum, parasitic plants, 

conifer diseases 

The Authors: 

ROBERT F. SCHARPF is supervisory research plant pathologist in charge of the Station's 

Forest Disease Research Unit at the Station headquarters in Berkeley, Calif. LEWIS F. ROTH 

is professor emeritus of forest pathology at Oregon State University, Corvallis. 

Cover: The tallest ponderosa pine in the center is only lightly infected with dwarf mistletoe, even 

though adjacent pines were heavily infected. Grafts made from this tall pine and exposed to 

heavily infected overstory also showed low levels of infection after 20 years in stands in northern 

California. 

Publisher: 

Pacific Southwest Research Station 

Albany, California 

(Mailing address: P.O. Box 245, Berkeley, California 94701-0245 

Telephone: 51 0-559-6300) 

March 1992 

In Brief. . . 

Scharpf, Robert F.; Roth, Lewis F. 1992. Resistance of 

ponderosa pine to western dwarf mistletoe in central 

Oregon. Res. Paper PSW-RP-208. Albany, CA: Pacific 

Southwest Research Station, Forest Service, U.S. Department of Agriculture; 9 p. 

Retrieval Terms: resistance, Pinus ponderosa, Arceuthobium 

campylopodum, parasitic plants, conifer diseases 

Several ponderosa pines (Pinus ponderosa) on the Deschutes, 

Ochoco, and Rogue River National Forests in Oregon were 

thought to be resistant to western dwarf mistletoe, Arceuthobium 

campylopodum. The pines on the Deschutes and Ochoco National Forests were considered to be resistant to infection, whereas 

the pines from the Rogue River National Forest were believed to 

escape infection because dwarf mistletoe seeds slide off their 

long, drooping needles. Scions taken from candidate trees on the 

three forests were grafted to seedlings for testing for resistance. 

Seedlings grafted with scions from susceptible trees were used 

as controls. Ungrafted seedlings were also included in the tests. 

All test trees were planted from 1967 to 1969 on the Pringle 

Butte Experimental Forest in central Oregon in a stand of ponderosa pines heavily infested with dwarf mistletoe. About 12- 15 

grafts from each of 30 resistant parent selections, and the same 

number of grafts from eight susceptible selections were outplanted 

in the infested stand in a replicated, randomized plot design. 

About 125 ungrafted seedlings grown from seed from the 

Deschutes National Forest seed bank were also planted as susceptible controIs. Infected overstory was maintained to provide 

abundant inoculum, with only a moderate level of suppression. 

The objectives of the study were 1) to determine the resistance of clonally propagated ponderosa pines from apparently 

resistant and susceptible trees, 2) to determine the survival of 

resistant and susceptible trees with different levels of infection, 

and 3) to determine whether resistance was correlated with tree 

size, foliar habit, or crown characteristics. It was also possible to 

compare the resistant and susceptible trees produced from scions 

to those grown from seeds. 

Tree survival, growth, crown size, and dwarf mistletoe 

infections were recorded in 1989. Of all the resistant grafts from 

selections on the Deschutes National Forest, only 29 percent 

became infected with dwarf mistletoe. Grafts from four of the 

resistant selections were disease free. In addition, the mean 

number of infections per tree was one or fewer in grafts from 20 

of the 25 selections. Fifty percent or fewer of the grafts from 

three of the five selections from the Ochoco National Forest 

were infected, and of these selections fewer than one infection 

per tree on the average was recorded. 

Grafts produced from the Rogue River source and the susceptible selections were much more frequently infected than 

trees produced from resistant selections. On the average, 85 

percent of all the susceptible grafts were infected, and the average number of infections per tree per source ranged from 1.9 to 

9.0. Similarly, 87 percent of the susceptible seedlings were 

infected, and the average number of infections per tree was 5.7. 

Seventy-six percent of the resistant and Rogue River grafts 

survived since outplanting, but only 48 percent of the susceptible 

grafts and seedlings were still alive in 1989. Seventy-eight 

percent of the dead resistant trees died from causes other than 

dwarf mistletoe, whereas dwarf mistletoe was involved in the 

death of about 73 percent of the susceptible trees. 

Dwarf mistletoe populations built up very slowly in resistant trees but more rapidly in susceptible ones from 1968 to 

1980. In 1981 an exceptionally large population increase occurred. In 1982 and 1983 the rate of increase returned to the 

previously established norm. Thereafter, the trend is not known 

because there was insufficient time between 1983 and 1986 for 

incipient infections to become visible. 

Tree growth and form varied markedly among the selections. Crafts from some selections grew straight and tall, some 

had few branches, whereas others branched profusely. However, neither tree size nor growth habit appeared to be correlated 

with disease intensity in most cases. In addition, the "drooping 

needle" pines from the Rogue River National Forest failed to 

develop the drooping habit when grown in central Oregon, and 

showed no resistance to dwarf mistletoe. On the other hand, 

most susceptible grafts showed lower levels of infection than the 

ungrafted seedling stock, demonstrating the presence of juvenile 

susceptibility among young seedlings compared to grafts from 

older susceptible trees. 

High levels of resistance were found in grafts produced 

from resistant selections of ponderosa pines on the Deschutes 

and Ochoco National Forests in Oregon. Dwarf mistletoe resistant ponderosa pines should be considered an important component of the native forest biodiversity, and should be identified, 

preserved, and used in future free improvement and pest management programs. 

USDA Forest Service Res. Paper PSW-RP-208. 1992. 

have been found by chance during the course of examining 

young stands for other purposes." 

Successful grafts were made with scions from resistant 

and susceptible selections grafted onto potted seedlings in 

the greenhouse. From twelve to fifteen grafts were made 

from each parent. Both resistant and susceptible grafts were 

made from the previous year's growth of parent trees that 

were of about the same age. Several years were required to 

develop a population of grafts. 

Another treatment was used in determining the "normal" 

level of susceptibility of ponderosa pines to dwarf mistletoe on 

the Deschutes and Ochoco National Forests. Ungrafted seedlings considered to have a normal background level of resistance 

were used. These were obtained from the USDA Forest Service 

nursery in Bend, Oregon, from seed of parent trees in which 

dwarf mistletoe was not considered in seed collection. These 

were of two groups: one group that was used to check the effect 

of root binding on resistance was held in containers large enough 

for the seedlings to develop root characteristics like those of the 

potted grafts. The other group was planted as bare root seedlings 

without the influence of root binding. 

The planting site was on the southwest side of Pringle Butte, 

Pringle Palls Experimental Forest in Central Oregon, and consisted 

of a heavily infected ponderosa pine stand that had been cleared of 

brush in 1965. Overstory trees were thinned enough to accommodate planting and exert only moderate suppression, but dense enough 

to provide sufilcient inoculum from dwarf mistletoe in the resjdual 

pines. Some overstory trees died from mistletoe late in the study, and 

a few were periodically removed to maintain a proper balance 

between overstory with dwarf mistletoe and growing space for the 

planted seedlings. Seventy-eight live overstory trees were on the plot 

in 1989, the basal area was about 50 square feet per acre (11.4 sq. 

meters per hectare), and the average dwarf mistletoe rating in the 

overstory trees was heavy (5.5) (Hawksworth 1977). 

A replicated, random plot design was used which consisted 

of three plots, each of which contained 12 square subplots large 

enough to contain 16 trees each. Trees in each subplot were 

planted at 4-feet (1.2411) intervals in four rows spaced 4 feet (1.2 

m) apart. At least 8 feet (2.4 m) separated subplots from one 

another. A third of all the resistant grafts, susceptible grafts, and 

seedlings were planted at random in each of the three large plots. 

On some subplots, fewer than 16 trees were planted. In all, about 

180 trees were planted in each plot for a total of 542 trees. 

The plots were visited periodically from 1967 to 1989, and 

brush and ocher unwanted vegetation removed as needed to 

reduce competition on the young trees. In September 1989, all 

trees were examined for survival, growth, and infection. Height 

growth and crown size (the cumulative length of all living and 

dead branches) were measured to the nearest inch (2.54 cm). 

Each dwarf mistletoe infection, dead or alive, was recorded, and 

the year in which each infection occurred was estimated by 

counting the number of annual growth segments on a stem 

beginning with the current segment and counting back to the one 

nearest the midpoint of the dwarf mistletoe infection. This method 

assumes that most infection occurs on current: year's growth. 

Branch infections and main stem infections were recorded separately. For dead trees, the cause and year of death were determined from growth segments, from external signs and symptoms, and from the stage of tree deterioration during the examination in 1989. Some trees disappeared for unknown reasons 

during the course of the study. 

Data were analyzed statistically to determine: 1) differences 

in levels of infection and proportion of resistant and susceptible 

trees infected, 2) the relationships between tree survival and 

infection level; and 3) relationships between level of dwarf 

mistletoe infection and tree height and total branch length. 

Results 

Number of Trees Infected and 

Intensify of Infection 

Resistant Tree Selections 

Only 29 percent of all the grafts from the 25 resistant 

selections from the Deschutes National Forest became infected with dwarf mistletoe over the 20- to 22-year test 

period (fable 2). Grafts from four of the selections were 

uninfected. Of another 14 selections, half or fewer of the 

grafts became infected. Only in seven selections did more 

than half of the grafts become infected. 

Mean number of infections per tree was low for all resistant 

selections. In only one of the 14 selections of which half or fewer 

of the grafts were infected was the mean number of infections 

per tree greater than one. On the other hand, for the selections 

with more than half the grafts infected, the number of infections 

ranged from a mean of 0.8 to 3.7 per tree. Therefore, a measure 

of resistance appears to be expressed both in the percentage of 

trees infected and in the number of infections per tree, and it is 

possible that some selections are immune. 

On the Ochoco National Forest, a trend similar to that on the 

Deschutes National Forest followed in the percentage of grafts 

infected and the mean number of infections per tree (table 3). No 

sources were uninfected, but three showed 50 percent or fewer 

of the grafts uninfected, and on these trees the mean infection 

level was fewer than one per tree. The other two sources had 

more than 75 percent of the trees infected, and means of 2.2 and 

3.3 infections per tree. 

Therefore, for nearly all sources selected for resistance on 

both the Deschutes and Ochoco National Forests, the percentage 

of grafts infected, and the mean infection level per tree were very 

low during the 20-22 years of the test. A Tukey test for mean 

separation showed that only five selections (R12, R20, and R47 

from the Deschutes National Forest, and R25 and R26 from the 

Ochoco National Forest) had trees with significantly higher 

mean numbers of infections per tree than the others, but even 

these levels of infection were regarded as low considering the 

duration of exposure to heavy overstory infection. 

USDA Forest Service Res. Paper PSW-RP-208. 1992. 

These results appear more dramatic, as discussed later in this 

paper, in that many of these seedlings were infected early in life 

and died over the duration of the test. 

Tree Survival 

Paired t-tests showed that the percentage of resistant trees 

that died from dwarf mistletoe on each of the three plots was not 

significantly different (table 5). Similarly, no significant differences were found in the percentage of susceptible trees killed by 

dwarf mistletoe among the three plots. Therefore, for further 

analysis of mortality, the data from resistant trees were pooled, 

and the data for all susceptible trees pooled. 

A paired t-test showed there was a statistically significant difference in the percentage of tree mortality from 1967 

to 1989 between the resistant and susceptible trees (table 5). 

Overall, 76 percent of the resistant trees were still alive after 

20-22 years, whereas only 48 percent of the susceptible trees 

were alive. Of the resistant trees that died, only 22 percent 

apparently died from dwarf mistletoe infection. The remaining 78 percent died mostly from cutting by rodents, poor 

survival after planting, or from unknown causes. 

Of the susceptible trees that died, we believe that 73 percent 

were killed by dwarf mistletoe mostly from one or a few infections 

occurring on the main stem, especially those that occmd early in 

the life of the tree. The remining 27 percent of the death among the 

susceptible trees was caused by other factors as mentioned above. 

There were no sigruficant differences in the proportion of trees bt 

died from other causes between the resistant and susceptible trees. 

The percentage of resistant and susceptible trees that died 

each year since the time of planting was determined for the years 

1977-1988 Cfig. 1). The trees that died before 1977 were either 

missing or too badly decomposed for us to determine in 1989 

Table 5--Survival of resistant andsusceptibleponderosdpines from I967to 

1989, and the percentage of death caused by dwarf mistletoe 

no. Pet PC* 

Plot 

Resistant trees' 

total 

Total 

trees 

Susceptible trees 

total 210 52 73 

Total dead 

trees 

'Based on all trees selected as resistant excluding those withdrmpy needles fmm 

the Rogue River National Forest. They wm not considered genetically resistant. 

Percentage of 

deaths caused by 

dwarf mistletoe 

their year of death. However, field observations over these early 

years suggest that most trees died mainly from factors mentioned earlier rather than from dwarf mistletoe. We estimated 

that about 13 percent of all the resistant and about 14 percent of 

all the susceptible trees died between 1967-69 and 1976. 

Mortality among the resistant trees was low and relatively constant from 1977 to 1988 (fig. I). Only in 1984 was 

the percentage of dead trees greater than 2 percent. On the 

other hand, mortaIity of the susceptible trees increased dramatically from 1981 to 1984, decreased somewhat, but remained relatively high, thereafter. We attribute this increased 

mortality in the susceptible trees, and somewhat increased 

mortality in the resistant ones to the buildup of dwarf mistletoe beginning in 198 1. 

We also noted that among the susceptible trees that died, 

64 percent of the seedlings planted as controls died from 

dwarf mistletoe in comparison to 30 percent of the grafts 

produced from older susceptible trees. These results support 

earlier studies by Roth (1974b) that showed that seedlings of 

ponderosa pine exhibit greater susceptibility to infection and 

damage from dwarf mistletoe than trees produced by grafting 

scions onto seedling rootstock. 

In summary, the resistant trees died mostly from causes 

other than dwarf mistletoe, whereas dwarf mistletoe appeared to 

be the primary cause of death of most of the susceptible trees. 

Population Dynamics 

of Dwarf Mistletoe 

The number of dwarf mistletoe infections and year in 

which each infection became established were determined 

for all infected resistant and susceptible trees (fig. 2). The 

numbers of infections occurring each year on the resistant 

selections were low and quite consistent from 1968 through 

1980. In none of these years did more than 19 infections 

become established, and only in 1981 were more than 26 

infections recorded. For the susceptible trees, the number of 

infections per year rose to 20 or more by 1972 and continued 

to increase steadily each year until 1980 when 50 infections 

were noted. A marked increase in the number of infections 

occurred in 1981 among both the resistant and susceptible 

trees. More than 50 infections were recorded on resistant 

trees, and nearly 150 recorded on susceptible trees in that 

year. Thereafter, infection levels dropped to about what they 

were in 1980, and continued to decrease to nearly none in 

1986. We attribute the apparent decrease in infection from 

about 1984 to 1989 to our inability to detect recent latent 

infections. Symptoms or the presence of dwarf mistletoe 

plants may not appear for several years after infection, particularly on slow growing trees. 

The differences between the levels of infection on resistant and susceptible trees are even greater when one considers that about twice as many resistant as susceptible trees 

were used in the study. In addition, about half of the susceptible trees died between 1968 and 1989, reducing opportuniUSDA Forest Service Res. Paper PSW-RP-208. 1992. 

ties for infection, whereas only about one quarter of the 

resistant trees died during this period. 

Tree Growth and Infection 

Before evaluating the effect of dwarf mistletoe on tree 

growth, it was necessary to understand the variability and characteristics of growth of both the infected and uninfected trees. 

Tree height and growth varied widely within and among the 

selections of trees. For example, some grafts grew straight, tall, 

and had relatively few short branches ($6. 3). Others grew 

equally as tall and branched profusely (fig. 4). Some trees hardly 

grew at all, and others lost apical dominance and developed into 

"coniferous brush" (fig. 5). In some cases, the growth characteristics of the tree were affected by dwarf mistletoe infection as in 

the loss of apical dominance, multiple tops, or brooming. Therefore, a wide range of tree growth characteristics were expressed 

among both the infected and the uninfected trees. 

The average tree heights from 1967-1989 for the resistant 

and susceptible selections were grouped as follows: 

Mean tree height No. tree selections 

inches (cm) resistant susceptible 

20-40 (51-103) 8 6 

41-60 (104-153) 18 7 

61-77 (154-196) 4 1 

These results show that, on the average, trees from all 

selections were more than a half meter tall after 22 years, but 

none averaged 2 meters in height. A few trees were about 3 

meters tall. These tree sizes represent poor growth rates for 

ponderosa pine with or without the influence of dwarf mistletoe, 

and were caused in large part by competition from the overstory. 

The area on F'ringle Butte on which these trees are growing is a 

poor site (low site IV) for growth of ponderosa pine. 

One assumption we made during the course of the study 

was that the number of mistletoe infections on a tree would be 

directly related to tree size (tree height) or to crown size (total 

length of living and dead branches). The larger the tree, the 

greater its probability over time to trap dwarf mistletoe seeds and 

become infected. However, results of the study showed no relationship between tree height and number of infections for nearly 

USDA Forest Service Res. Paper PSW-RP-208. 1992. 

Figure 6-A forked tree developed from a bud of a susceptible rootstock 

(left) and from a scion from a resistant parent selection (right). Note 

several dwarf mistletoe plants growing on the susceptible fork and the 

absence of infection on the resistant fork. The two forks were artificially 

held apart for photographing. 

Discussion and Conclusions 

One of the problems in determining resistance of a given 

selection of trees is in finding out what variables influence 

infection, and whether these variables operate the same or differently on both the resistant and susceptible selections. In this 

study we attempted to eliminate some of the complicating factors of genetic variability and experimental testing through the 

use of seedlings, clonally propagated material, and replicated, 

random plot &sign on a heavily diseased natural site. Other 

variables we believe could have been related to resistance to 

dwarf mistletoe in this study included foliar habit, tree size, and 

crown characteristics. 

In contrast to reports of resistance by Roth (1966), grafts 

propagated from parent trees with drooping needles from the 

Rogue River National Forest were found not to be resistant to 

dwarf mistletoe in our tests. A possible reason for the lack of 

resistance in these sources may be similar to that reported by 

Roth (1974a) for the lack of resistance in the drooping needle 

pines from the Deschutes National Forest. The needles of the 

coastal, Rogue River sources did not droop when the grafts 

were grown in central Oregon. As a consequence, these 

selections were as susceptible to infection as the local sources. 

Tree size and crown characteristics also were not related to 

number of trees infected or to infection intensity for most of the 

resistant selections, and for more than half the susceptible selections. These results agree with those of Scharpf and Vogler 

(1986) for young Jeffrey pines in California. Possibly there was 

not enough of arange in tree size to show a significant difference 

in intensity of infection based on height or total branch length. 

USDA Forest Service Res. Paper PSW-RP-208. 1992. 

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* U.S. GPO 1992-687-008 
Research Paper 

PS W-R P-208