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Control of Dwarf Mistletoes with a Plant
Growth Regulator
Thomas H. Nicholls, Leanne Egeland, Frank G. Hawksworth,
David W. Johnson, and M. Kathryn Robbins 1
I
Abstract--This paper summarizes test results of ethephon, an ethylene-releasing plant growth regulator, as a control for two dwarf mistletoes:
Arceuthobium americanum on Pinus contorta and A. vaginatum subsp.
cryptopodum on P. ponderosa in Colorado. Ethephon at 2,500 ppm with
a surfactant was tested using three application methods: a bottle sprayer,
a bacl<pack mlstblower, and a hydraulic sprayer. Dwarf mistletoe shoot
abscission rates of 74% to 100% were consistently achieved using these
grour;d application methods. Mistletoe seed dispersal the year after
ethephon application was much less In the treated plots than In the
nontreated control plots. An evaluation of aerial application methods Is In
progress.
toxic by-products when the compound breaks down. Ethylene
also exists naturally in conifers and is presumably responsible
for natural abscission of aging dwarf mistletoe shoots. Base.d
on these facts and the encouraging results achieved in the
eastern dwarf mistletoe study (Livingston et a1. 1985), we
decided to test ethephon on two other extremely damaging
dwarf mi.stletoe species in the western United States: .lL
amcr;canur,., on lodgepole pine and A vaginatum subsp.
cryptopodum on ponderosa pine.
Dwarf mistletoes, Arceuthobium spp., are parasitic seed
plants. They are regarded as one of the most damaging disease
age.nts of conifers in the United States. An estimated 418
million cubic feet of wood fiber is lost annually to these
pathogens either through growth reduction or tree mortality
(Drummond 1982). These obligate parasites also lower wood
quality and reduce cone and seed production of infected trees.
It has long been felt that an effective, safe, and economical
chemical control of dwarf mistletoes would significantly reduc~ the impact of a major forest disease, especially in highvalue stands, recreational areas, seed orchards, and ornamental plantings around homes, cabins, and business e.stablishments.
~luch successful research has been done on silvicultural
control of dwarf mistletoes. But until recently, no effective
chemical control had been found. Livingston and Brenner
(1983) te.ste.d ethephon on A. pusillum on black spruce, Picea
mariana, and consistently achieved 74 to 100 percent abscission of dwarf mistletoe shoots. They predict that abscission of
aerial dwarf mistletoe shoots can prevent the spread of this
disease in treated black spruce for at least 2 years.
Ethephon is quite safe and is routinely used on food crops
to promote the abscission of leaves and fruits (DeWilde 1971).
As ethephon is absorbed into plant tissues, ethylene is released, causing susceptible plant parts to abscise. There are no
Methods
Studies of ethephon control of A. amen'canum on lodgepole pine were conducted in Colorado on the Fraser Experimental Forest from 1983 to 1986 and at Cutthroat Bay in the
Arapaho National Recreation Area in 1985. We also set up a
preliminary study on A. vaginafum-infected ponderosa pine
near Mee.ker Park, Colorado, in the Roosevelt National Forest
in 198.? Ethylene-~eleasing agents containing ethephon,
FlorelCH) and Ethre.1, (!l) at 2,500 ppm in water with a. surfactant
were tested onA. americanum, using three ground application
methods: a bottle sprayer, a backpack mistblower, and a
hydraulic sprayer. A bottle sprayer was used to apply
e.thephon, at the same concentration, to A. vaginatum. Two
methods of evaluating the effectiveness of the treatments were
used: the 6··Class Dwarf !'Jistietoe Rating (DMR)
(Hawksworth 1977) and the Total Dwarf ~listletoe Area
Rating (square inches) (Nicholls et a1. 1987). Also, a(~rial
applications of 1,200 and 2,400 ppm ethephon with surfactants
were tested in 1986. Details of all 1983 to 1986 application and
1Nicholls and Robbins are Research Plant Pathologists, and Ege·
land is a Forestry Technician at the r"orth Central Forest Experimfmt
Station, USDA Forest Service, St. Paul, Minnesota 5!51()8. Hawksworth is
a Research Plant Pathologist at the Rocky Mountain Forest and f~ange
Experiment Station, USDA Forest Service, Fort Collins, Colorado 80$26.
Johnson is a Plant Pathologist in Region 2, Forest Pest Management,
USDA Forest Service, Lakewood, Colorado 80225.
154
Table 2.-~Percent 6-class Dwarf Mistletoe Rating (OMR) reduction of
Arceuthoblum amerlcanulD on lodgepole pine caused by 2,500
ppm ethephon compared to control treatments (N =102lndlvldual trees), Arapaho National Forest, Colorado, 1985.
evaluation methods are in the paper by Nicholls et a1. (1987)
or in progress reports on file at the North Central Forest
Experiment Station. This paper summariz.es those results.
Study area
Results :md Discussion
Cutthroat Bay
Overstory trees
Dwarf mistletoe shoot area abscission rates of 74 to 100
percent (table 1) and DMR abscission rates of 30 to 100
percent (table 2) were ac.hieved using ground application
methods. Most shoot abscission occurred within 2 to 5 weeks
after ethephon was applied (fig. 1). Mistletoe seed dispersal
the. year after ethephon applic\ltion was significantly less in
treate.d plots than in nontreated control plots. The bottle
sprayer worked we.!l for treating individual infections; the
backpack mistblower worked well for trees 10 to 15 ft. tall; and
the hydraulic sprayer was useful for trees up to 55 ft. tall. Some
foliage turned brown on lodgepole pine and ground juniper
(Jrmipenls communis subsp. alpina) when the hydraulic
sprayer was used. This was pFobably due to the increased
volume of ethe.phon solution that was applied with the hydraulic sprayer. Approximately 1 gallon of solution per tree was
applied with the hydraulic sprayer; 0.4 gallon per tree was
applied with the mistblower. However, the next year's buds
and foliage developed normally.
Ethephon does not destroy the parasite's endophytic system in the host tissue. Therefore, new shoots began developing
on some infections 1 to 2 years after treatment (fig. 2), and
some fihoots produced a few seeds 3 years after treatment.
However, it will take time for the parasite to replace the. large
masses of mature shoots and seed that abscised after treatment. Therefore, we expect seed production and dissemination to be greatly reduced during this period. Because A.
arnericanum and A vaginatum have approximately 6 to 7-year
life cydes, we feel that ethephon control will prevent or reduce
significant spread and infection of dwarf mistletoe for up to 4
years and, perhaps, longer.
Understory trees
Treatment
Fraser Experimental Forest
Ethephon/surfactant
Control
I Ethephon/surfactant
Control water
Ethephon/surfactant
Control water
Fool (1983)
Gage (1984)
Headquarters (1985)
Cutthroat Bay (1985)
A. vagjnatym
Meeker Park (1985)
Ethephon!surfact,int
Control
95
74
100
4
9
93
100
0
99
55
100
5
87
0
10
97
62
97
o
78
o
o
o
81
70
o
90
o
100
o
An effective, safe, and economical control of dwarf mistletoes would significantly reduce the impact of a major forest
disease, especially in high-value stands. It would also provide
forest managers with another control option to use in conjunction with silvicultural control methods.
Acknowledgements
A. americanum
89
30
Conclusion
% Total shoot area reduction
Ethephon/surfactant
Ethephon/surfactant/DM SO 1
Control water/surfactant
Ethephon!surfactant
Control water/surfactant
Ethephon/surfactant
Control water
Ethephon!surfactant
Control water
Ethephon!surfactant
Control
(femah~ and male
% DMR reduction
Aerial application of ethe.phon is currently being evalu~
a.ted. This method may be useful in large stands of high
recreational-value with understory regeneration. By controlling dwarf mistletoe in overstory trees, it may be possible to
protect the understory trees from infection until they reach a
size where dwarf mistletoe impact is less serious.
1\listletoe shoot abscission was not observed after the 1986
aerial application of ethephon. Although it is possible that the
evaluation methods were not sensitive enough to measure
effects, a more likely explanation is that not enough ethephon
was applied to the plots to be absorbed by the mistletoe plants.
The 10 gallons per acre applied in this study is much lower than
the volumes applied during the successful ground applications: approximately 200 to 240 gallons per acre in the
mistblower and hydraulic sprayer studies.
Female Male
Sage (1983)
Ethe phon/surfactant
Control
Ethephon/surfactant
Control
Overstory trees
(Area 1)
Overstory trees
(Area 2)
Understory trees
(Area 2)
Table 1.--Percent reduction of Arcel.lthoblum amerlcanum and A.
yaglnatum female and male shoot area caused by 2,500 ppm
ethephon COml)ared to control treatments (N = 652 Individual
Infections), Colorado, 1983-1985.
Study area
Treatment
This was a cooperative effort of the USDA Forest Service
(Forest Pest 1\fa.nagement, Region 2; North Central Forest
Experiment Station; Rocky l\fountain Forest and Range
Experiment Station; Arapaho and Roosevelt National Forests), the University of 1\finnesota, and Union Carbide. The
authors wish to thank BiH and Ulrike Livingston, Jim Kemper,
Tim Garvey, John Beddy, Robert Alexander, Laura Merrill,
V\Tayne Sheppard, 1\lark and Nora Nilles, 1\lary Doupe, Martin
Ouanc.i, J\.lary Lou Nic.holls, Prasad Rao, Thomas Schwartz,
Charles Troendle, Robert Blanchette, Dave French, 1\1ark
Brenner, Ann Acheson, John Sprac.kling, Robert Resendez,
Diane Legler, and Frans Schuitemaker for their help during
this study.
combined)
78
0
1DMSO = Dimethyl Sulfoxide
155
Figure 1. Ethephon control of Arceuthoblum amerlcanum on lodgepole pine: Presence of
aerial shoots before and 5 weeks after ethephon was applied with a hand-held bottle
sprayer', Fraser Experimental Forest, Colorado.
Literature Cited
Livi.ngston, W. H., and 1\1. L. Bre.nner.1983. Ethephon stimulates abscission of eastern dwarf mistletoe aerial shoots on
black spruce. Plant: Disease 67:909-910.
De\Vilde, R.C.1971. Practical applications of (2-chloroethyl)
phosphonic add in agricultural production. Hortscience
6:364-370.
Drummond, David. B. 1982. Timber loss estimates for the
coniferous forests of the United States due to dwarf
mistletoes. USDA Forest Service, Forest Pest 1\fanagement, 1\fethods Application Group, Fort Collins, Colorado, 24 p., Report 83-2,
Hawksworth, F.G. 1977. The 6-dass dwarf mistletoe rating
system. USDA Forest Service, General Technical Report
RM-48, 7 p.
Livingston, W. H., R. A. Blanchette, 1\1. L. Brenner, and K. J.
Zuzek. 1985. Effective use of e,thyle.ne-re.1e,asing a.gents to
prevent spread of eastern dwarf mistletoe. on black
spruce. Canadian Journal of Forest Research 15:872-876.
Nicholls, T. H., L. Egeland, F. G. Hawksworth, and D. W.
Johnson. 1987. Control of dwarf mistletoe with ethe.phon.
In: Proceedings of 34th Annual "'estern International
Forest Disease '''ork Conference. [June.au, Alaska, Se.p·
tember 9-12, 1986], p. 78-85.
Figur'e 2. Ethephon contn)1 of Al'ceythobjum amer'jcanum on Ic)dgepole pine: Presence of
a.erlal shoots before and 3 years a,fter ethephon was a.pplied with a hand-held bottle
sprayer, Fraser Experimental Forest, Colorado.
156