Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. United States Department of Agriculture Forest Service Pacific Northwest Research Station Research Note PNW-RN-506 August 1991 Abstract introduction Control of Western Dwarf Mistletoe With the Plant-Growth Regulator Ethephon Catherine A. Parks and James T. Hoffman Ethephon (Ethrel®), an ethylene-releasing plant-growth regulator, was applied with a hydraulic sprayer to ponderosa pine (Pinus ponderosa Dougl. ex Laws.) infected with dwarf mistletoe (Arceuthobium campylopodum Engelm. f. tsugense (Rosend.) Gill) in the Emmett Ranger District, Boise National Forest. Abscission rates of 60 to 100 percent of dwarf mistletoe shoots were consistently achieved with applications in both July and August of 2,400 p/m of ethephon plus surfactant: Ethephon did not kill the endophytic system of the parasite in the host tissue; new: joots began develap- ing on some of the infections within 2 months after treatmentdOne year later, mostiof the female and all the male plants flowered. Results indicates.acshorter period of shoot abscission for this dwarf mistletoe. Normal pollen proddétion is taterrupted for: if the year of treatment only, and seed production and dispersatwill occuin the He second year after treatment. A weet it” +, = ae iM r « ces a Keywords: Dwarf mistletoe control, ethephon. Q Coo... fies sees e 8 i ass Dwarf mistletoes are the most widespread forest diseases ingthe Pacific Northwest; infecting 41 percent of the commercially forested area in eastgin Washington and * Oregon (Bolsinger 1978). In the forests of the Intermountain West, more than 3.3 million acres are infected by the plant parasites (Johnson and Hawksworth 1985). The effects of dwarf mistletoe parasitism on trees—growth loss, branch and stem deformity, loss of vigor, and premature death—can seriously affect forest-manage- ment objectives. Control of dwarf mistletoes is centered on silvicultural methods of destroying infected branches or trees to prevent infection of uninfected stands. On September 1, 1987, the Environmental Protection Agency approved use of a growth-regulating chemical called ethephon for reducing the spread of dwarf mistletoes. Ethephon causes mistletoe shoot abscission but does not seem to affect the root system of the parasite embedded within host tissue. By abscising the seed-bearing shoots, however, seed dispersal and spread of the parasite are delayed until new seeds can develop. This delay could allow time for regeneration to become established under infected host trees before they are removed. Treatment is recom- mended for high-value trees only (recreation areas, administrative sites, or home- sites) and has limited use in commercial stands (Johnson and Hawksworth 1989). CATHERINE A. PARKS is plant pathologist, Pacific Northwest Research Station, Forestry Sciences Laboratory, 1401 Gekeler Lane, La Grande, Oregon 97850; and JAMES T. HOFFMAN is plant pathologist, Intermountain Region, Forest Pest Management, Boise Field Office, 1750 Front Street, Boise, Idaho 83702. Methods Results and Discussion Research with ethephon has been conducted on the following Arceuthobium species (Hawksworth and Wiens 1972): ™ | odgepole pine dwarf mistletoe (A. americanum Nutt. ex Engelm.) in Colorado ~and California ™ Black spruce dwarf mistletoe (A. pusillum Peck) in Minnesota ™ Southwestern dwarf mistletoe (A. vaginatum subsp. cryptopodum (Engelm.) Hawksw. & Wiens) in Colorado and New Mexico ™ Douglas-fir dwarf mistletoe (A. douglasii Engelm.) in Oregon ™ Western larch dwarf mistletoe (A. /aricis (Piper) St. John) in Oregon ™ Western dwarf mistletoe (A. campylopodum Engelm. f. typicum Gill.) on Jeffrery pine (Pinus jeffreyi Grev. & Balf.) in California. Shoot abscission ranges from 70 to 100 percent for most tests using ground-based equipment. Our study objective was to test the efficacy of using a hydraulic sprayer with ethephon at two application times to control western dwarf mistletoe (A. campylopodum) on ponderosa pine (Pinus ponderosa Doug. ex Laws.). A 22-year-old ponderosa pine plantation with moderate-to-heavy dwarf mistletoe infection was located within the Emmett Ranger District of the Boise National Forest. Fifty-two infected trees were selected and tagged at the base. From the lower crown of each tree, one to four dwarf mistletoe branch infections were selected and identified by a numbered metal tag. To delineate the size of each infection, the branch was tied off with yarn at the outer margins of the infection. Dwarf mistletoe shoots in each infection were counted before treatment and 4 weeks after treatment. A photographic record was made of selected individual infections and trees. All trees were randomly assigned one of two application dates (July or August). On each application date, 13 trees were sprayed with 2,400 p/m of Ethrel® (4-percent active ingredient ethephon) plus 0.1-percent Wilbur Ellis R-11 (surfactant).’ The other 13 trees served as controls and were treated with water and surfactant. High-volume sprays of either water or ethephon were applied to all Study trees by using a hydraulic sprayer. The crown of each tree was sprayed until runoff during morning applications with temperatures ranging from 65 to 80 °F. Abscission rates were evaluated 4 weeks after each application date by counting the remaining shoots of each tagged infection. Abscission rates ranged from 60 to 100 percent (average 95 percent) for both application dates. Effectiveness of treatment did not differ with application date (table 1). For the few infections where less than 90-percent abscission occurred, inadequate spray coverage may have been responsible because infections were always near the interior of the tree or on a more protected exposure on the branch. One tree from each application date exhib- ited a phytotoxicity symptom: the older foliage turned reddish brown. Others have reported this phenomena as well (Frankle and Adams 1989, Nicholls and others 1986); no cause has been identified. ' The use of trade and company names is for the benefit of the reader: such use does not constitute an official endorsement or approval of any service or product by the U.S. Department of Agriculture to the exclusion of others that may be suitable. | Acknowledgments | | Table 1—Number and percentage of abscission of dwarf mistletoe shoots on pon- derosa pine before and 4 weeks after spray application US ee ne Shoots RE eee eee ec ee ee eee cana aEETa-SIn ERRATA Date 4weeks Abscission Shoots regrown sprayed Treatment Before after rate at 1 year - - - -Percent - - - - July 13, 1989 2,400 p/m ethephon 1,800 109 94 87 Control 1,463 1,361 7 August 7, 1989 2,400 p/m ethephon 1,458 55 96 82 Control 1,626 1,696 0 Ph men ete el eee Two months after they were sprayed, many of the infections had begun to regrow (55 percent for the July application and 76 percent for the August). The regrowth was only small “nubs”; few mistletoe plants with more than one internode were observed. After 1 year, all the treated male plants were flowering, thereby suggesting that pollen production was reduced in the year of application only. Also, all but two of the female infections were in flower after 1 year. Twelve percent of the July-treated female infections and 4 percent of those treated in August produced a few seeds (1 -20) the next year. Although differences were not directly calculated, this represented a significant reduction from the seed produc- tion of the control infections, which often had hundreds of seeds. Those female infections producing seed in 1 year did not have complete abscission after treatment, which suggests that seeds formed on residual plants. These results differ from others (Johnson and Hawksworth 1989, Nichols and others 1986) who report an expected pollen and seed elimination of 3 to 5 years with ethephon applica- tions on other dwarf mistletoes. All the previously reported efficacy trials were made on spring-flowering dwarf mistletoe species. The accelerated regrowth and flowering of ethephon-treated A. campylopodum may be relevant to its fall flowering habit. Our results suggest that applications of ethephon from midsummer to late summer will successfully reduce the spread of western dwarf mistletoe by reducing seed production by 2 years. A ground-based hydraulic sprayer allows for fairly good coverage on midsized trees. The field assistance of K. Chocholak, B. Fuller, and B. Gardner is appreciated. We thank Rhone-Poulenc Inc. for supplying the ethephon and funding for other needed supplies. The cooperation of the Emmett Ranger District, Boise National Forest, also is appreciated. +c U.S. GOVERNMENT PRINTING OFFICE: 1991 - 591-001/40043 Literature Cited U.S. Department of Agriculture Bolsinger, Charles L. 1978. The extent to dwarf mistletoe in six principal softwoods in California, Oregon, and Washington, as determined from forest survey records. In: Proceedings, symposium on dwarf mistletoe control through forest manage- ment; 1978 April 11-13; Berkeley, CA. Gen. Tech. Rep. PSW-31. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Forest and Range Experiment Station: 45-54. Frankle, Susan; Adams, David. 1989. Reduction of dwarf mistletoe with the plant growth regulator ethephon. Forest Pest Management Rep. 89-1. San Francisco, CA: U.S. Department of Agriculture, Pacific Southwest Region. 5 p. Hawksworth, Frank G.; Wiens, Delbert. 1972. Biology and classification of dwarf mistletoe (Arceuthobium). Agric. Handb. 401. Washington, DC: U.S. Department of Agriculture. 234 p. Johnson, David W.; Hawksworth, Frank G. 1985. Candidates for control through cultural management. In: Loomis, Robert C.; Tucker, Susan; Hofacker, Thomas H. Insect and disease conditions in the United States, 1979-83: What else is growing in our forests? Gen. Tech. Rep. WO-46. Washington, DC: U.S. Department of Agriculture, Forest Service, State and Private Forestry, Forest Pest Management: 48-55. Johnson, David W.; Hawksworth, Frank G. 1989. Mistletoe contro! with ethephon: conclusions. In: Proceedings, 36th western international forest disease work conference; 1988 September 19-23; Park City, UT. Vancouver, BC: Department of Forest Sciences, University of British Columbia: 44-45. Nicholls, Thomas H.; Egeland, Leanne; Hawksworth, Frank G.; Johnson, David W. 1986. Control of dwarf mistletoe with a ethephon. In: Proceedings, 34th western international forest disease work conference; 1986 September 8-11; Juneau, AK. Portland, OR: U.S. Department of Agriculture, Forest Service, Forest Pest Management: 78-85. Pacific Northwest Research Station 319 SW. Pine Street P.O. Box 3890 Portland, Oregon 97208 Official Business Penalty for Private Use, $300