European Mistletoe | Viscum album L | Pest Rating Proposals and Final Ratings

Figure 1:
Viscum album (2001 CDFA)

California Pest Rating for

Viscum album L: European mistletoe

Santalales: Viscaceae

Pest Rating: A | Proposed Seed Rating: R

PEST RATING PROFILE

Initiating Event:

This plant has been rated as “B” on the CDFA Plant Pest Rating list for some years.

History & Status:

Background: European mistletoe (Viscum album) is a hemiparasite of broad-leaved trees and shrubs that can be found on its hosts stems. It depends on the host for water, mineral nutrients, and some carbohydrates. Depending on the health of the host plant and severity of infestation it weakens its host, leaving it susceptible to damage from insects, increasing host mortality rates. European mistletoe is spread by seed dispersal from birds eating its berries and expelling the viscin covered seeds.

Found natively in Eurasia and North Africa this plant was introduced to California in the early 1900s by noted plant breeder Luther Burbank at his experimental farm outside Sebastopol, where it still occurs.

European mistletoe (Fig. 1) is sometimes confused with the native California mistletoe (Phoradendron serotinum) (Fig. 2) that is common in the area where European mistletoe is adventive. Differences in leaf form are the easiest way to distinguish them; European mistletoe has narrow propeller-shaped leaves, while California mistletoe has widely ovate leaves. European mistletoe has dichotomously branched stems that diverge at >40%, while California mistletoe has branches that generally diverge at <45%. European mistletoe has only a few fruits per cluster (generally <5), while California mistletoe has many fruits per cluster (generally >5).

Figure 2:
Phoradendron serotinum ©2011 Jorg & Mimi Fleige

Worldwide Distribution: European mistletoe has a native range from North Africa to southern England, southern Scandinavia, and western Russia. It can infect numerous species of broad-leaved trees and conifers in temperate to tropical regions.

Official Control: Viscum album is a prohibited plant in New Zealand.

California Distribution: European mistletoe was introduced to the Sebastopol area of Sonoma County as an ornamental from Eurasia around 1900, but it was not until 1966 that the taxon was recorded as present in California (Howell 1966). Surveys performed in 1971 covered 16 square miles in Sebastopol and Graton, and found 310 infected trees in 21 species (Scharpf and McCartney 1975). Surveys performed in 1984 covered 63 square miles in Sebastopol, Graton, Forestville, Santa Rosa, and Cotati, and found 554 infected trees in 22 species (Hawksworth and Scharpf 1986). Surveys performed in 1984 covered 71 square miles across Occidental, Forestville, Fulton, Santa Rosa, and Cotati, and found 664 infected trees in 23 species (Hawksworth et al 1991). Current specimen records show that it is present in Sebastopol and Santa Rosa. There is one recorded occurrence in Sacramento, but this record is most likely spurious.

California Interceptions: None

The risk European mistletoe would pose to California is evaluated below.

Consequences of Introduction:

1) Climate/Host Interaction: California has a climate suitable to European The area it has established in is surrounded by scattered oaks and conifers forests. while it has yet to be recorded expanding to these trees, there is potential as it has established itself on oaks and conifers forests in its native range. Therefore, it scores as Medium (2) in this category.

-Low (1) Not likely to establish in California; or likely to establish in very limited areas.

-Medium (2) may be able to establish in a larger but limited part of California.

-High (3) likely to establish a widespread distribution in California.

2) Known Pest Host Range: Risk is Medium (2) as European mistletoe can infect numerous species of broad-leaved trees and conifers in temperate to tropical regions, which are found throughout California. In California, it has been detected on native bigleaf maple (Acer macrophyllum), red alder (Alnus rubra) California buckeye (Aesculus californica), Fremont Cottonwood (Populus fremontii), and Pacific willow (Salix lasiandra). It has also been found on introduced species of birch (Betula), persimmon (Diospyros spp.), locust (Robinia spp.), hickory (Carya spp.), ash (Fraxinus spp.), hawthorn (Crataegus spp.) apple (Malus spp.), plum (Prunus spp.), pyracantha (Pyracantha spp.), mountain-ash (Sorbus spp.), Maple (Acer spp.), elm (Ulmus spp.), and poplar (Populus spp.) Therefore, it scores as Medium (2) in this category.

-Low (1) has a very limited host range.

-Medium (2) has a moderate host range.

-High (3) has a wide host range.

3) Pest Dispersal Potential: Viscum album is spread primarily by birds, which eat and carry the fruit to other trees. Since introduction to Sonoma County, spread has increased from point of origin to more than 71 square miles at last survey in 1991. The current distribution and pattern of infestation is not known. Therefore, it scores as Low (1) in this category.

-Low (1) does not have high reproductive or dispersal potential.

-Medium (2) has either high reproductive or dispersal potential.

-High (3) has both high reproduction and dispersal potential.

4) Economic Impact: Within its native range European mistletoe infects apple and other commercial fruit trees; however, it’s damage is limited as these hosts are pruned regularly, preventing further damage and slowing its spread. Therefore, it scores as High (3) in this category.

A, B, C, D

A. The pest could lower crop

B. The pest could lower crop value (includes increasing crop production costs).

C. The pest could trigger the loss of markets (includes quarantines).

D. The pest could negatively change normal cultural

E. The pest can vector, or is vectored, by another pestiferous

F. The organism is injurious or poisonous to agriculturally important

G. The organism can interfere with the delivery or supply of water for agricultural uses.

-Low (1) causes 0 or 1 of these impacts.

-Medium (2) causes 2 of these impacts.

-High (3) causes 3 or more of these impacts.

5) Environmental Impact: European mistletoe has potential to find hosts in a variety of California trees and shrubs. Many of these hosts are found within riparian corridors, such as willows, and impacts to this environment could affect multiple special status species that depend on a riparian habitat. In Sonoma County riparian trees provide nesting habitat for a variety of birds, including Swanson hawk, and roots provide shelter for California freshwater shrimp. European mistletoe also impacts urban street trees and fruit orchards in Europe, leading to increased pruning to prevent damage to the trees. Therefore, it scores High (3) in this category.

A. The pest could have a significant environmental impact such as lowering biodiversity, disrupting natural communities, or changing ecosystem processes.

B. The pest could directly affect threatened or endangered

C. The pest could impact threatened or endangered species by disrupting critical habitats.

D. The pest could trigger additional official or private treatment

E. The pest significantly impacts cultural practices, home/urban gardening or ornamental plantings.

Score:

Low (1) causes none of the above to occur.

Medium (2) causes one of the above to occur.

High (3) causes two or more of the above to occur.

Consequences of Introduction to California for European mistletoe:

Total score: 11

Low = 5-8 points

Medium = 9-12 points

High = 13-15 points

6) Post Entry Distribution and Survey Information: Evaluate the known distribution in California. Only official records identified by a taxonomic expert and supported by voucher specimens deposited in natural history collections should be considered. Pest incursions that have been eradicated, are under eradication, or have been delimited with no further detections should not be included. -1

-Not established (0) Pest never detected in California, or known only from incursions.

-Low (-1) Pest has a localized distribution in California, or is established in one suitable climate/host area: Sebastopol area of Sonoma County.

-Medium (-2) Pest is widespread in California but not fully established in the endangered area, or pest established in two contiguous suitable climate/host areas.

–High (-3) Pest has fully established in the endangered area, or pest is reported in more than two contiguous or non-contiguous suitable climate/host areas.

Final Score:

The final score is the consequences of introduction score minus the post entry distribution and survey information score: 10

Uncertainty:

Medium. This plant has had the opportunity to spread further in California, but it has not succeeded so far. This plant has hosts and dispersal techniques that are adaptable to California, but it has a history in the state with little impacts. It is known from younger street trees less than 15 years in place, so the reasons for its restriction are not known; it is possible that it is still in its lag phase and may increase its rate of spread once it becomes more prevalent.

Conclusion and Rating Justification:

European mistletoe currently is known from only Sonoma County in CA; it has the potential to move beyond its established area in California. As the current list of infected hosts shows, it could use riparian corridors to move throughout the state. If this plant does spread it might have significant impacts to native trees and commercial orchards. Despite its current slow rate of spread, an A rating is justified.

References

Calflora: Information on California plants for education, research and conservation. Berkeley, California: The Calflora Database: http://www.calflora.org/ (Accessed: March 20, 2018).

California Department of Food and Agriculture, Encycloweedia: Data Sheets: https://www.cdfa.ca.gov/plant/IPC/encycloweedia/weedinfo/viscum.htm (Accessed: March 20, 2018).

Consortium of California Herbaria: ucjeps.berkeley.edu/consortium/ (Accessed: March 20, 2018).

Hawksworth, F.G., Scharpf, R.F., & Marosy, M. 1991. European mistletoe continues to spread in Sonoma County. California Agriculture. 45: 39-40.

Howell, J. T. 1966. Viscum album in California. Leaflets of Western Botany 10(13):244.

Scharpf, R., and F. Hawksworth. 1976. Luther Burbank introduced European mistletoe into California. Plant Disease Reporter 60(9):740-742.

Photo Sources:

https://calphotos.berkeley.edu/cgi/img_query?enlarge=0175+3301+2364+0094

Author:

Rachel Avila, Environmental Scientist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6813; plant.health[@]cdfa.ca.gov

Responsible Party:

Dean G. Kelch, Primary Botanist; California Department of Food and Agriculture; 1220 N Street, Sacramento, CA 95814; Tel. (916) 403-6650; plant.health[@]cdfa.ca.gov.

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