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Douglas-fir needle cast

Written by B.E. Callan Revision 2024

Red-brown spots caused by <em>Rhabdocline</em> species on infected Douglas-fir needles.
Red-brown spots caused by Rhabdocline species on infected Douglas-fir needles. Leo Unger

General information and importance

Douglas-fir needle cast is a foliar disease caused by five closely related species of Rhabdocline. The disease is host-specific to Douglas-fir where it occurs naturally in western North America. It also causes defoliation of Douglas-fir Christmas tree plantations where they are established elsewhere in North America. Yellow blotches on the current year’s needles are the first indication of infection, followed by reddish-orange patches of needle discolouration and needle cast. Severe recurring infections in young understory trees can result in defoliation of all but current year needles.

Distribution and hosts

In North America, all five Rhabdocline species are endemic in western Canada (British Columbia and western Alberta) and the western United States. They follow the natural distribution of both varieties of their hosts, Douglas-fir (Pseudotsuga menziesii var. menziesii) and Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca).

In eastern North America (Ontario, New Brunswick, and Nova Scotia), Rhabdocline pseudotsugae and R. oblonga are the most encountered introduced species in Douglas-fir Christmas tree plantations. Rhabdocline pseudotsugae is also the most common species in Europe, where it was introduced at the start of the 20th century along with its host. It has now become established in Douglas-fir plantations in fourteen European countries.

Tree parts affected

Douglas-fir needle cast causes browning and premature casting of foliage.

Symptoms and signs

Douglas-fir needle cast is caused by five closely related species of fungi, all of which cause similar symptoms and signs. They differ in microscopic features, location on needles, and to a certain extent, distribution ranges. Trees with heavy levels of infection have yellowing foliage and a thin crown. Small understory trees may only retain the current year’s needles.

Early symptoms of infection are pale yellow foliar blotches. They are 1 to 2 millimetres in diameter and darken to reddish-brown as the fruiting bodies within them mature. Severe defoliation results in trees with chlorotic foliage and open crowns. Only current year needles remain on the tree.

Rhabdocline pseudotsugae apothecia are orange to reddish-brown and hypophyllous (fruiting on the underside) of 1-year-old needles. They do not extend in width beyond the needle midrib and are slightly raised above the needle surface when mature and moist. They are roughly circular, 0.5 to 10 millimetres × 0.3 to 0.6 millimetres in size, and develop under the needle epidermis, splitting it open and lifting a flap to expose the hymenial layer. This layer consists of a tightly packed palisade of asci and paraphyses (sterile hyphae). Asci are 8-spored, club-shaped, and measure 120 to 160 micrometres × 16 to 22 micrometres. They narrow slightly at the flattened apex, which remains unstained with iodine. The ascus tip opens via a split to release the spores. Paraphyses are septate, up to 2.5 micrometres wide, slightly swollen at the tip, and are taller than the asci. They extend beyond the asci to form a thin cover layer (epithecium). The ascospores are initially hyaline (colourless), single-celled, oblong, and slightly constricted in the middle. They eventually develop a septum, with one cell turning dark brown. Ascospores measure 13 to 19 micrometres × 5 to 8 micrometres and are encased in a thick gelatinous sheath.

Rhabdocline epiphylla is very similar to R. pseudotsugae, except that most apothecia are epiphyllous (formed on the upper surface of the needle), and its paraphyses are wider, up to 10 micrometres across.

Rhabdocline weirii produces mostly hypophyllous apothecia, which look like R. pseudotsugae. Apothecia contain asci that are 8-spored, club-shaped, and have an apical pore that stains blue in iodine. They measure 100 to 130 micrometres × 15 to 20 micrometres. Paraphyses are septate, up to 2.5 micrometres thick, and slightly swollen at the tip. Ascospores are at first hyaline and single-celled, then become two-celled, with one cell turning brown after it is released from the ascus. They are oblong and slightly constricted in the middle. They measure 13 to 19 micrometres × 4 to 8 micrometres and have a thick gelatinous sheath. Rhabdocline weirii uniquely produces conidia in acervuli, which are linear pustules that measure up to 2 millimetres long and are produced on both upper and lower needle surfaces. The acervuli rupture the epidermis and cuticle to release masses of hyaline, single-celled conidia that are cylindrical to dumbbell-shaped, measuring 14 to 17 micormetres × 4 to 6 micrometres. Each has a slender mucoid apical appendage, measuring 3.5 to 5 micrometres × 1.5 to 2 micrometres.

Rhabdocline obovata is similar in appearance to R. weirii but produces apothecia on older (2-year-old and older) needles and does not produce a conidial state. Its apothecia bear asci with apical pores that stain in iodine. It differs in ascospore morphology, producing ascospores that are obovate (oval with one end slightly wider than the other), measuring 16 to 22 micrometres × 6 to 9 micrometres.

Rhabdocline oblonga is also similar to R. weirii, with an ascus pore bluing in iodine, but it lacks a conidial state and produces rectangular apothecia that grow across the full width of the needle (no other Rhabdocline species does this). Unlike other species, ascospores will turn brown while still in the ascus. 

Table 1: Key morphological characteristics of pathogenic Rhabdocline species on Douglas-fir needles (adapted from Stone and Gernandt 2005)
  Morphological characteristics
Species Apothecia mainly epiphyllous Apothecia hypophyllous Ascus with pore bluing in iodine Apothecia rectangular Other features
R. pseudotsugae Absent Present Absent Absent Not applicable
R. epiphylla Present Absent Absent Absent Paraphyses with swollen tips
R. weirii Absent Present Present Absent Conidial stage present
R. oblonga Absent Present (across full width of needle) Present Present Ascospore cell turns brown while in ascus

R. obovata

 Absent Present (on either side of needle midrib) Present Absent Ascospores obovate, on 2-year-old and older needles

In addition to the pathogenic Rhabdocline species associated with Douglas-fir needle cast, an endophytic species, R. parkeri, asymptomatically colonizes living Douglas-fir foliage. It fruits abundantly on needle galls caused by Contarinia, a dipteran fly, and on dead needles in the leaf litter, but not on living needles attached to trees that are free of galls. Its tiny apothecia, less than 0.5 millimetres in diameter, produce much smaller ascospores, measuring 10 to 13 micrometres × 4.5 to 5 micrometres.

Rhabdocline laricis is a needle cast pathogen of larch that reproduces only by conidia. It does not produce apothecia.

Disease cycle

Years with heavy infection levels are the result of favourable weather conditions during the infection period, namely warm, wet weather. Asci rupture to forcibly eject ascospores from apothecia on diseased 1-year-old or older needles still attached to branches. The infection period may last for several weeks. The ascospores are further spread by wind. They germinate if they land on tender, newly emerged, elongating needles under moist conditions. The germ tube penetrates the cuticle, and the fungus colonizes the needle. Only newly flushed spring growth is susceptible to infection. As the fungus colonizes the needle, a yellowish, then orange to reddish-orange blotch is formed. In late spring and summer, the apothecia develop under the epidermis in these blotches. As they grow and swell, the epidermis above them splits and is pushed back to expose the hymenium. Once the apothecium has finished releasing ascospores, the apothecium turns dark brown to black, and the needle is cast.

Damage

Persistent, severe infections can almost completely defoliate trees, with only a small tuft of current year needles remaining on the branches. This leads to loss of tree vigour and reduced growth. Understory trees and Christmas tree plantations are particularly susceptible to exposure to inoculum from nearby diseased overstory trees. Pure forests of Douglas-fir are more susceptible to severe outbreaks than mixed forests. Rocky Mountain Douglas-fir tends to develop more severe symptoms than coastal trees, especially when planted offsite in coastal areas. In natural forests in British Columbia, severe damage only occurs in the drier southern areas of the province. Heavy needle cast can cause substantial economic losses in Christmas tree plantations due to the discoloured foliage. For this reason, exported seedlots from central British Columbia are not recommended for planting in regions such as central Europe, where conditions favour Rhabdocline infections.

Prevention and management

Pest management strategies for a particular pest vary depending on several factors. These include:

Decisions about pest management strategies require information about each of these factors for informed decision-making. These various factors should then be weighed carefully in terms of costs and benefits before action is taken against any particular pest.

There is considerable variation in the susceptibility to Douglas-fir needle cast among individual trees. During forest management thinning programs, trees showing chronic high levels of disease should be selected for removal. Seeds should be gathered from trees that show some resistance in years when disease inoculum is high. Christmas tree plantations outside the natural range of Douglas-fir should be initiated using disease-free nursery stock. In nurseries, chemical control can be achieved using lime-sulphur at bud burst and repeated at 7 to 10-day intervals as the needles expand.

Pesticides registered for use against Douglas-fir needle cast under specific situations may change from year to year for various reasons. Therefore, please search Health Canada’s Pesticide Product Information Database for currently registered pesticides and product information for use against Douglas-fir needle cast. The application of any registered product should be based on population size and applied only when necessary and against the indicated life stage. It is also recommended to consult a local tree care professional. Chemical pesticides may be toxic to humans, animals, birds, fish, and other beneficial insects. Apply registered products only as necessary and according to all directions and precautions noted on the manufacturer’s label. In some jurisdictions and situations, only a licensed professional can apply pesticides. Consulting relevant local authorities to determine local regulations that are in place is recommended.

Photos

A Douglas-fir needle with fruiting bodies (apothecia) of one of five closely related <em>Rhabdocline</em> species that cause Douglas-fir needle cast.

A Douglas-fir needle with fruiting bodies (apothecia) of one of five closely related Rhabdocline species that cause Douglas-fir needle cast.

Alex Craigmyle

Thin crowns of Douglas-fir affected by Douglas-fir needle cast.

Thin crowns of Douglas-fir affected by Douglas-fir needle cast.

Leo Unger

Defoliation caused by Douglas-fir needle cast.

Defoliation caused by Douglas-fir needle cast.

Leo Unger

Red-brown spots caused by <em>Rhabdocline</em> species on infected Douglas-fir needles.

Red-brown spots caused by Rhabdocline species on infected Douglas-fir needles.

Leo Unger

Selected references

Allen, E.A.; Morrison, D.J.; Wallis, G.W. 1996. Common tree diseases of British Columbia. Natural Resources Canada, Canadian Forest Service, Pacific Forestry Centre. Victoria, British Columbia. 178 p.

Brandt, R.W. 1960. The Rhabdocline needle cast of Douglas-fir. State University College of Forestry, Syracuse University. Syracuse, New York. Technical Publication 84.

Chastagner, G.A. 2001. Susceptibility of intermountain Douglas-fir to Rhabdocline needle cast when grown in the Pacific Northwest. Online. Plant Health Progress 2(1). https://doi.org/10.1094/PHP-2001-1029-01-RS

Collis, D.G. 1973. Rhabdocline needle cast of Douglas-fir in British Columbia. Environment Canada, Canadian Forestry Service, Pacific Forest Research Centre. Victoria, British Columbia. FIDS Pest Leaflet 32. 4 p.

Morgenstern, K; Döring, M.; Krabel, D. 2014. Rhabdocline needle cast - most recent findings of the occurrence of Rhabdocline pseudotsugae in Douglas-fir seeds. Botany 92(6): 465–469. https://doi.org/10.1139/cjb-2013-0238

Myren, D.T. 1984. Needle cast of Douglas-fir caused by Rhabdocline pseudotsugae subsp. pseudotsugae and Phaeocryptopus gaeumannii in Ontario. Plant Disease 68: 1099.

Parker, A.K. 1962. The germination and growth of Rhabdocline pseudotsugae Syd. on artificial media. Canadian Department of Forestry Bi-monthly Progress Report 18(5): 3–4.

Parker, A.K. 1970. Effect of relative humidity and temperature on needle cast disease of Douglas-fir. Phytopathology 60: 1270–1273. https://doi.org/10.1094/Phyto-60-1270 

Samek, M.; Novotný, P.; Modlinger, R.; Fulín, M.; Beran, F.; Roy, A.; Pešková, V. 2019. Impact of Rhabdocline pseudotsugae and Phaeocryptopus gaeumannii on the selection of suitable provenances of Douglas-fir in central Europe. Forests 10(3): 204. https://doi.org/10.3390/f10030204

Scharpf, R.F. (technical coordinator). 1993. Diseases of Pacific coast conifers (revised). United States Department of Agriculture, Forest Service. Washington, D.C. Agriculture Handbook 521. 199 p.

Sherwood-Pike, M.; Stone, J.K.; Carroll, G.C. 1986. Rhabdocline parkeri, a ubiquitous foliar endophyte of Douglas-fir. Canadian Journal of Botany 64(9): 1849–1855. https://doi.org/10.1139/b86-245

Sinclair, W.A.; Lyon, H.H. 2005. Diseases of trees and shrubs. Second edition. Comstock Publishing Associates, Cornell University Press. Ithaca, New York. 660 p.

Stone, J.K.; Gernandt, D.S. 2005. A reassessment of Hemiphacidium, Rhabdocline, and Sarcotrochila (Hemiphacidiaceae). Mycotaxon 91: 121–126.

Wilhelmi, N.; Bennett, P.I.; Shaw, D.C.; Stone, J.K. 2021. Rhabdocline needle cast of Douglas-fir. United States Department of Agriculture, Forest Service. Washington, D.C. Forest Insect and Disease Leaflet 190. 19 p.

Cite this fact sheet

Callan, B.E. 2024. Douglas-fir needle cast. In J.P. Brandt, B.I. Daigle, J.-L. St-Germain, A.C. Skinner, B.C. Callan, and V.G. Nealis, editors. Trees, insects, mites, and diseases of Canada’s forests. Natural Resources Canada, Canadian Forest Service, Headquarters. Ottawa, Ontario.