Considerations when using Ethephon for Suppressing Dwarf and Leafy Mistletoe Infestations in Ornamental Landscapes

Mistletoe Biology

Mistletoes are serious pests of conifer and many hardwood tree species (12,37). As parasites, they harm their hosts by shunting plant photo-synthate (dwarf mistletoes) or water and nutrients (both mistletoes) to their tissues at the expense of host nutritional and water needs. Dwarf mistletoes (Arceuthobium spp.) are leafless, yellow-orange, essentially obligate parasites on conifers (Pinaceae and Cupressaceae) (14). Leafy mistletoes are larger, green leaved hemi-parasitic plants found on many hardwood genera and occasionally on conifers (13). Two genera of leafy mistletoes are found in the United States: the native Phoradendron and the introduced European mistletoe, Viscum, found only in California (28, 37).

Health of lightly infected trees is usually not seriously affected by mistletoe presence. However, moderately to heavily infected trees may suffer from reduced tree vigor, increased susceptibility to insects and diseases, deformity—including branch dieback, and often, untimely death.

Both dwarf and leafy mistletoes can be long lived—potentially living until their host tissue dies. Dwarf and leafy mistletoes generally produce localized infections, but infection may become systemic in a few dwarf mistletoe host-parasite combinations (21). Both mistletoes are dioecious (have separate male and female plants), have endophytic systems within host tissues and epiphytic (external) stems for flower and seed production (14, 37). Dwarf mistletoes are highly specialized plants that have a narrowly defined host range, while leafy mistletoes have a broader host range.

Dwarf mistletoe infections develop slowly. A viscous-coated seed landing and sticking onto a needle of a host conifer may slide down the needle to a branch segment after rain, become “cemented” in place and overwinter there to germinate in the spring. A primary haustorium emerges from the seed to penetrate host tissue; usually tissue less than five years old. Haustorial (cortical) strands arise from the primary haustorium and grow longitudinally through host cortex and outer phloem. Sinkers grow into the branch phloem and xylem (2,20). In three to four years the mistletoe epiphytic system breaks through the bark with subsequent fruit development (14). Pollination occurs by both insect transmission and wind. When mature, dwarf mistletoe seeds are disseminated by an explosive discharge mechanism that can shoot the seed, actually a naked endosperm, up to 35 feet away.

Mechanical seed dispersal limits spread of dwarf mistletoe to within the host tree and closely adjacent trees; and due to the large numbers of seeds per mistletoe plant, very high host infection potential exists. Some plants may produce several thousand fruits per year. Birds and rodents may be incidently involved in dwarf mistletoe spread, accounting for new infections in previously uninfested areas.

In leafy mistletoes, the berries serve as food for birds, particularly in winter. Berries usually contain a single seed which is surrounded by a fleshy pulp. The berry pulp is digested by the bird and the seeds are evacuated onto branches, or the sticky seeds are inadvertently transported from their beaks and feet onto branches elsewhere. A wetted seed produces a viscous coating that cements the seed in place when dried, and in the spring the germinating radicle penetrates into living host tissue (37). Infection occurs primarily through young bark, however older wood with deeply furrowed bark may also become infected. Haustorial development follows a similar course as for dwarf mistletoe, however, epiphytic shoots are normally developed within one year. The plant may start producing berries in three to four years.

Since leafy mistletoe is disseminated by birds, there is a different host infection pattern from that of dwarf mistletoe. New infections are most often found in the larger trees of an area, in isolated trees, and in bird-favored susceptible roosting trees.

Mistletoe Control Methods

Pruning out infected branches, tree removal, and favoring nonsusceptible tree species are all useful techniques for managing mistletoe. Biological control with diseases and parasites of dwarf mistletoe has been tried without success (19).

Pruning out infections is not always desirable or possible. Conifers are difficult to prune for mistletoe control without significantly detracting from their expected form in the landscape. Also, latent and very you ng or small infections are easily overlooked during pruning. Small infected branches can often be removed without affecting tree appearance, but removing large branches may well affect host symmetry. Bole infections cannot be removed without seriously threatening tree health and integrity. In hardwoods, small infected branches can be removed without affecting tree appearance. Judicious removal of some larger branches may not detract from tree appearance, but wounds created in the process may provide entrance courts for decay organisms.

The difficulty and cost of pruning has inspired pest managers to search for chemical methods for mistletoe control. Through 1978, over 60 different chemicals have been tested for use in controlling dwarf mistletoes, but none has proven effective (19). 2,4-D foam (no longer marketed) and black plastic have had some practical use for topical treatment of pruned leafy mistletoe infections in hardwoods (5,23,38), and asphalt-based pruning paint has provided control of mistletoe regrowth when sprayed onto freshly cut leafy mistletoe branch stubs and surrounding host tissue (24, 25).

Recent research with ethephon has renewed hope for an effective chemical for mistletoe control. Ethephon releases ethylene during absorption by plant tissue, this enhances the natural ripening process leading to abscission of mature dwarf and leafy mistletoe shoots. Ethephon is marketed by Rhone-Poulenc Ag Company under the name Florel^®. Florel has a “Caution” warning on the label. It is a strong acid and is harmful if absorbed through the skin and eyes, inhaled, or swallowed. Health hazards associated with ethephon are mainly due to its irritant properties on mucosal surfaces. Mutagenicity studies “in vivo” and “in vitro” were negative. The active ingredient has been shown not to be teratogenic or carcinogenic in animal studies (35). It is damaging to swamps, wetlands, and marshes and is not to be applied through irrigation water. Plants sprayed with Florel often flower 7 to 10 days earlier, may experience temporary growth inhibition, premature needle drop, and foliage burn or yellowing (35). At high concentration it can damage acrylic plastics, metals and paints (such as car finishes).

The Florel Brand labeled as “FRUIT ELIMINATOR” states, as one of its uses, that is used “as a foliar spray on ornamental con ifers and ornamental deciduous trees for removal of dwarf mistletoe shoots and seeds” (29). The label goes on to advise “Applications made in conjunction with silvicultural dwarf mistletoe management will prevent spread of the mistletoe parasite to other parts of the tree and to other trees”, and to treat any dwarf and leafy mistletoe regrowth before seed dispersal occurs.

It is well documented that thorough application of ethephon to dwarf mistletoe shoots will cause shoot defoliation; many studies have reported reductions of between 60 and 100 percent due to application of ethephon (Table 1 references). However, contrary to the 1989 label statement concerning “dwarf mistletoe removal”(34), Florel does not remove dwarf mistletoe from the tree. What it does, is cause the abscission of adequately treated, mature shoots present at the time of treatment. Since the endophytic system of the dwarf mistletoe plant is usually not affected by ethephon treatment, new plant growth often commences following defoliation and retreatment may be required. The mistletoe plant continues to produce aerial stems at the plant margins and over the next several years will again produce fruits (15). Despite this significant limitation, with proper use ethephon can be a useful chemical for suppression of dwarf mistletoe fruiting. The following section provides background information and summarizes research work done with ethephon.

Dwarf Mistletoe Control

For dwarf mistletoe control, Florel’s label rate is 2,700 ppm, and use of a surfactant is recommended, as is spraying to “wet” (29). Florel should be used within four hours of mixing as chemical effectiveness decreases after that length of time (29). Experimentally, ethephon has been tested at rates ranging from 1,200 -5,000 ppm (4, 6, 7,16, 17, 27, 30, 31, 32, 39). Ethephon is not translocated within the tree, it must be placed onto the target to be effective. Therefore treatment success is dependent upon complete mistletoe plant coverage (39).

The volume of ethephon needed for dwarf mistletoe control varies with the application method, tree size, and number of dwarf mistletoe plants to be treated. Direct plant treatment requires the least material, whole-tree ground application requires more, while aerial application has not been successful with rates used experimentally. Frankel et al. (9) used 1000 mis. of 2700 ppm solution in a backpack sprayer to directly spray 30 individual mistletoe plants to runoff. Whole tree spraying required between 200 to 240 gallons per acre to be effective; aerial application using 10-40 gallons per acre was not effective (3, 11,36).

Many different spray devices have been used to apply ethephon to dwarf mistletoe in conifers. Hand, backpack, gravity-fed, hydraulic sprayers and aerial application have been tested. In groundbased treatments spray applications became less effective as coverage was required higher in the tree. In our studies, we attribute this to the difficulty in fully covering higher targets. Lower targets are easier to spray and may receive additional application as chemical drips down when infections above them are sprayed (9). Due to the difficulty of adequately treating all fruiting infections in conifers, some fruiting shoots on large mistletoe plants and small fruiting infections may be missed. Chemical intercepted by needle clusters has caused incomplete target mistletoe shoot removal (9).

Timing of ethephon application in relation to dwarf mistletoe seed discharge is not critical. Application should be made priorto seed dispersion but no differences were seen for June, July, and August treatments for control of Arceuthobium vaginatum on ponderosa pine in Colorado (15). Windy and rainy situations should be avoided during application for environmental safety and most effective use of chemical.

Following ethephon application, dwarf mistletoe resprouting occurred after different lengths of time depending upon application rate and hostspecies combination. Three to four weeks after fall treatment of western dwarf mistletoe on Jeffrey pine, emerging aerial shoots were observed on both treatment and control plants at Latour Demonstration State Forest in northern California (8). Lodgepole pine dwarf mistletoe treated in an adjacent stand did not resprout for over one year (9). Johnson (15) noted development of small immature ponderosa pine dwarf mistletoe shoots one year after ethephon application and production of mature shoots and fruits four years after treatment with 2200 and 2700 ppm ethephon. Parks and Hoffman (32) noted some resprouting in Douglas-fir which varied with ethephon application rate: 37% of the 1,260 ppm treatment infections resprouted, while no resprouting occurred in the 5,000 ppm treatment. Label rate of 2,700 ppm appears to be adequate for shoot defoliation on ponderosa pine provided mistletoe plants are thoroughly wetted (7), but does not seem to provide long term control (8). No resprouting occurred in tests at 5,000 ppm (32).

Some toxicity to host trees has been observed after Florel application. Needle yellowing and branch dieback was found on black spruce (27) and lodgepole pine (7).

Leafy Mistletoe Control

Ethephon has been tested for use as a direct, spray-applied application on intact mistletoe clusters for shoot removal and as a treatment applied to pruned mistletoe stubs forthe prevention of regrowth. Spray application timing is critical for successful shoot reduction of leafy mistletoe. Host dormant treatments were effective for shoot removal, however, ethephon application with temperatures below 40°F or after mistletoe growth had begun (March-April) were ineffective (6). Laboratory experiments determined that ethephon is rapidly absorbed by leafy mistletoe and indicate that ethephon treatment results would not be affected with at least a 1 hour interval between application and rainfall (10).

Guidelines for Ethephon Application

After reviewing research results and observing ethephon trials, we recommend the following guidelines for mistletoe control.

Before any dwarf or leafy mistletoe control work is begun, present and future mistletoe impact in the landscape should be evaluated. Mistletoe reinfestations from within and surrounding the planting must be considered for the risk they present. Present mistletoe infection levels should be determined for all susceptible trees; for dwarf mistletoe, the Hawksworth 6-Class Rating System is useful for this task (1).

Severely infested conifers and hardwoods must be judged for their value in the landscape when evaluated for mistletoe control or removal. Older, heavily infected trees are likely to decline over time and become “centers” for insects and disease. Younger, vigorous trees are generally more amenable to severe pruning to reduce or eliminate mistletoe infection than are older trees. Pruning should be done following standards published by the Western Chapter of the International Society of Arboriculture (ISA) (40), or similar, before any chemical treatment of mistletoe is begun.

The frequency of mistletoe treatment needed is dependent upon the level of mistletoe infection tolerable to both host and arborist, and property owner/client. Dwarf mistletoe host reinfestation depends upon the location and number of female plants in the tree and closely adjacent susceptible trees and on seed production, while leafy mistletoe reinfestation depends upon host presence, mistletoe fruit availability and bird tree preferences. These factors must be measured against host tree age, vitality, and value. General guidelines for retreatment are necessarily site specific and will need to be developed by a pest control advisor and/or an arborist for the desired end result.

If treatment with ethephon is deemed useful, application technique and equipment should be carefully chosen to meet the conditions present in the infested area. Proper weather conditions, complete mistletoe plant coverage, and use of a surfactant are critical for satisfactory results for dwarf mistletoe shoot removal (39). Due to the hazards associated with spray drift and incomplete target coverage, spraying infected trees from the ground must be done with caution and may not be appropriate for many urban sites. Ethephon at label rate is not efficacious for leafy mistletoe control.

Conclusions

Dwarf Mistletoe. Since the endophytic system of the dwarf mistletoe plant is usually not affected by ethephon treatment, resprouting will occur following mistletoe shoot removal. Because mistletoe control is temporary, ethephon use for ornamental conifers at label rates should be intended only to inhibit seed production, thereby lessening further build-up of dwarf mistletoe in infected trees and reducing spread to adjacent trees. Female plants must be retreated periodically to afford continued relief from mistletoe infection. The female plants will produce fruit again after several years, and both male and female plants will continue to demand ever-increasing carbohydrates, mineral nutrients and water from their hosts. Careful pruning may be used to remove some mistletoe plants without significantly affecting tree appearance.

Leafy Mistletoe. Due to the deleterious effects of leafy mistletoe on its host, this pest should be removed from high value landscape trees to avoid host stress and tree deformity. Mistletoe clusters should be removed through pruning wherever possible and appropriate in keeping with well-respected pruning standards (such as those of the ISA). Remaining mistletoe clusters—those on desirable branches or scaffolds—should be pruned flush to the host branch (without damaging the host). Experimentally, ethephon application (at 10% a.i.) to stubs of mistletoe clusters has provided effective control, as has an asphalt-based wound paint applied as an aerosol (Tree Seal®), however neither of these treatments are currently registered for use.