Rhabdocline Needle Cast
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Contents |
Hosts
Douglas-fir.
Distribution
Rhabdocline needle cast occurs throughout the range of the host in Idaho and Montana. Swiss needle cast also may be widely distributed. It is common in northwestern Montana and northern Idaho.
Damage
Infected needles are killed and shed. Growth loss probably results when trees are severely infected. Economic damage to Christmas trees often occurs.
Identification
Rhabdocline causes red-brown transverse banding of needles (fig. 107). Newly developing needles are infected but symptoms are not seen until fall or the following spring. By spring one year following infection, fruiting bodies develop on the undersides of leaves in the discolored bands. Spores are released when flaps of leaf epidermis fold back on either side of the midrib exposing the light brown cushion of fungus. Needles are cast to the ground shortly after spore release.
Swiss needle cast also infects newly developing needles but varies greatly from tree to tree and among locations in length of time before needles are cast after infection. Needles generally become chlorotic overall although they occasionally turn red-brown. Most diagnostic of Swiss needle cast are the black fruiting bodies (fig. 108) on the undersides of infected needles. The tiny black dots (pseudothecia) emerge through the stomata in rows on either side of the midrib. Infected needles usually have a few fruiting bodies one year after infection with more visible each year until the needle is shed. The fruiting bodies can be seen at any time of the year making them especially useful for diagnosis of the disease.
Similar damages
Conditions such as root disease which cause overall decline in trees, particularly saplings, often appear superficially similar to needle cast. Douglas-fir needle midge, Cooley spruce gall adelgid, and occasionally aphids cause damage to Douglas-fir foliage.
Comparison of Needle Problems of Douglas-Fir
| Disease | Symptoms | Location | Fruiting Bodies Description | Fruiting Season | Needles Shed |
|---|---|---|---|---|---|
| Rhabdocline Needle Cast | Red-brown banding | Underside; either side of midrib | Tan with flaps of epidermis | Fall and spring | 12-15 months following infection |
| Swiss Needle Cast | General chlorosis or red-brown tips | Underside, emerging from stomata in line on either side of midrib | Round, black, tiny | Year round | 1-4 years following infection |
| Insect | Appearance | Season present | |||
| Douglas-Fir Needle Midge | Yellow to red spots with swelling | Within galls inside needles | Tiny, yellow, legless maggots | Spring to late summer | Late summer |
| Cooley Spruce Gall Adelgid | Yellow spots, kinked needles | Upper surface new growth | White tufts of wax cover adult adelgid; tiny black crawlers in spring | Spring to fall |
Gallery
 Photo by USDA Forest Service Archive, USDA Forest Service, Bugwood.org |
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.
Funk, A. 1985. Foliar fungi of western trees. Canadian Forestry Service, Pacific Forest Research Centre BC-X-265, 159 p.
Hepting, G. E. 1971. Diseases of forest and shade trees of the United States. USDA For. Serv. Ag. Hndbk. No. 386, 658 p.
Field Guide to Diseases and Insect Pests of Idaho and Montana Forests, USDA Forest Service Northern Region, Publication Number R1-89-54
