Dwarf Mistletoe

Arceuthobium spp.

From: Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54

Hosts. – Five species of dwarf mistletoe infect II native conifers in Idaho and Montana. Major hosts in this area are Douglas-fir, western larch, lodgepole pine, and ponderosa pine.

Distribution. – Generally the range of host species except Douglas-fir and ponderosa pine dwarf mistletoes. Douglas-fir dwarf mistletoe occurs only west of the Continental Divide. Ponderosa pine dwarf mistletoe is found near Coeur d'Alene, Idaho.

	can be large. Tree form is often affected as well. Bark beetles sometimes attack trees weakened by dwarf mistletoe infections.
Figure 76 - Douglas-Fir Dwarf Mistletoe causes witches brooms to form on infected branches.	Figure 77 - Larch Dwarf Mistletoe often causes nearly all branches in the crown to form witches brooms.
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Figure 78 - Lodgepole Pine Dwarf Mistletoe female shoots on an infected branch.	Figure 79 - Lodgepole Pine Dwarf Mistletoe male shoots.
Identification. – Dwarf mistletoe plants form shoots on branches or stems of host trees (figs. 78, 79, 80). The shoots may be simple or branched and from 1 to 4 inches in length. They are scattered along young twigs of the host or in groups of varying numbers on branches or stems. The jointed shoots have opposite pairs of scale-like leaves. Color varies from yellow to purple to brown or olive green. When shoots are shed, small basal cups often remain embedded in the bark. Witches brooms, cankers, and swellings on stems and branches are also indicators of dwarf mistletoe infections. See Table 3 for help in identifying species of dwarf mistletoe.	Image Unavailable
	Figure 80 - Douglas-Fir Dwarf Mistletoe shoots are about the same length as the tree's needles.

Similar damages. – Witches brooms, cankers, and swellings can be caused by a number of other agents. Stimulation brooms often are produced after stands have been thinned. Occasional witches brooms and swellings are caused by frost damage to growing cells in both cambium and buds. Elytroderma needle cast causes witches brooms on ponderosa pines. Cankers are caused by a number of common canker-causing fungi. Sunscald and mechanical injuries also cause damages which can be confused with cankers. Presence of dwarf mistletoe plants in damaged stands is the best assurance of dwarf mistletoe infection.

Link to Table 3 – Comparison of Dwarf Mistletoes Occurring in Idaho and Montana.

Link to Images in Forestry Images

References.

Anonymous. 1982. For. Insect & disease identification and management. USDA For. Serv.,Northern Region; Idaho Dept. of Lands, Insect and Disease Control; Montana Dept. of State Lands, Division of Forestry. 192 p.

Bega, R. V. 1978. Diseases of Pacific Coast conifers. USDA For. Serv. Ag. Hndbk. No. 521, 206 p.

Boyce, J. S. 1961. Forest Pathology. McGraw-Hill Book Co., New York, NY. 572 p.

Hawksworth, F. G. and O. J. Dooling. 1984. Lodgepole pine dwarf mistletoe. USDA For. Serv., For. Ins. & Dis. Leaflet 18, 11 p. 

Hepting, G. E. 1971. Diseases of forest and shade trees of the United States. USDA For. Serv. Ag. Hndbk. No. 386, 658 p.

Kimmey, J. W. and J. L. Mielke. 1959. Western dwarf mistletoe on ponderosa pine. USDA For. Serv., For. Pest Leaflet 40, 7 p.

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