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Plantwise Technical Factsheet

juniper dwarf mistletoe (Arceuthobium oxycedri)

Host plants / species affected
Juniperus communis (common juniper)
Juniperus excelsa (eastern savin)
Juniperus oxycedrus (prickly juniper)
Juniperus phoenicea (phoenician juniper)
Juniperus sabina (savin juniper)
List of symptoms/signs
Stems  -  distortion
Stems  -  witches broom
Description
Like other Arceuthobium spp., A. oxycedri is an obligate parasite with an endophytic 'root' system ramifying within the host branch. This endophyte expands within the cortex and becomes embedded in the xylem for some years before aerial shoots are produced, encircling the infected branch and growing along it. A. oxycedri shoots are 5 to 10 cm high, but can grow up to 20 cm high, with verticillate branching. Staminate (male) flowers 1.5 - 2.0 mm across, perianth mostly 3-merous (ca. 95%), occasionally 4-merous (ca. 5%), rarely 2-merous (Hawksworth and Wiens, 1996). The mature fruit is about 3.0 mm long and 1.5 - 2.0 mm wide.
Prevention and control
In its natural range, A. oxycedri is not an invasive pest, nor are its hosts of importance for forestry, and it is therefore not subject to any particular control measures. On the contrary, as the sole European representative of its genus, it has value for biological diversity in Europe. If, however, it was introduced into other continents and attacked Juniperus spp., or other Cupressaceae, valuable for forestry, the measures applied for North American species would no doubt be appropriate.

Herbicides have been investigated in Spain for the control of A. oxycedri, and 2,4-D and MCPA was found to be most effective (Rios-Unsua, 1994). The only chemical approved for use against dwarf mistletoes is the ethylene-releasing growth regulator, ethephon, which can cause abscission of the shoots and delay fresh seeding for 2-4 years, but there is eventual re-growth from the endophyte. It is difficult to achieve good coverage in larger trees from the ground, whereas applications from the air fail to penetrate the canopy adequately. It is not known whether this treatment has been tested on A.oxycedri.
Impact
Though A. oxycedri damages its Juniperus hosts, this is of no real economic significance, since they are not forest trees grown for wood, nor important amenity trees. Juniperus spp. are significant components of Mediterranean vegetation, but their status in this respect is not impaired by Arceuthobium infection. A. oxycedri has the potential for economic impact however, as in North America Cupressaceae, (e.g. Juniperus virginiana), are of major economic importance.
Summary of invasiveness
Arceuthobium spp. do not spread rapidly and cannot be considered highly invasive. They do, however, constitute a serious threat as a result of their ability to build up gradually over the life of a forest and cause severe damaging effects on a number of important forest species.

Their potential to establish in other areas is limited by the need for the living parasite to survive on the pathway and reproduce after entry. Nevertheless, the risk of economic impact is considerable if host species are available. The conifers at greatest risk would be species, known to be hosts, planted as exotics in other continents, but there is also a certain possibility of spread to related species, not known to be hosts.
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