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Rhabdocline pseudotsugae Syd.
Ascomycotina, Rhytismatales, Hypodermataceae
Hosts: Douglas-fir needle blight occurs on both coastal and interior forms of Douglas-fir but is less severe on the coastal form.
Distribution: This fungus is widely distributed throughout the range of Douglas-fir in B.C.
Identification: Severely attacked trees usually have chlorotic foliage and very thin and open crowns (Figs. 55a, 55b). Infection takes place in the new foliage in the spring. Chlorotic, yellow spots 1-2 mm in diameter appear on both surfaces of 1st year needles in the fall, coalescing and darkening to red-brown during the winter (Fig. 55c). Some needles are shed during the winter. On needles that are retained, fruiting bodies (apothecia) form in the late spring. Apothecia are small, orange-brown, raised pustules, generally occurring on the lower side but occasionally on the upper side of needles (Fig. 55d). Spores are released in May or June, after which infected needles are shed. Identification is easiest in the late spring when needle spots and apothecia are visible.
Microscopic Characteristics: Apothecia chiefly hypophyllous, orange to red-brown, on one or both sides of needle midrib; erumpent by median splitting of overlying epidermis, or by circumsissile or lateral splitting when apothecia small; hypothecium poorly developed, no epithecial tissue, excipulum of marginal paraphyses only; situated in necrotic spots that are roughly circular, band-like or extending up to the entire length of needle, discrete or confluent in the necrotic spots, 0.5-10 long x 0.3-0.6 wide mm. Asci clavate, broadest below apex which is flattened, 8-spored, opening by a psuedooperculum or a bilabiate split of the apex, does not stain with iodine (J-), 120-160 x 16-22 µm. Ascospores at first hyaline and 1-celled, becoming 2-celled with one cell turning dark brown, oblong with obtuse ends, slightly constricted at the middle, 13-19 x 5-8 µm with a thick gelatinous sheath. Paraphyses septate, up to 2.5 µm thick, sometimes swollen at the tips, extending beyond the asci to form an epithecium.
Damage: Repeated severe infection almost completely defoliates trees, leaving only the current years needles. The impact of the disease therefore, is greatest on small trees because of their smaller total number of needles; large trees usually undergo only light defoliation and sustain little damage. Christmas tree plantations can be severely damaged.
Remarks: Douglas-fir needle blight is caused by a number of species and subspecies of the genus Rhabdocline. The most common in B.C. is R. pseudotsugae described here, and R. weirii (Parker & Reid). The symptoms caused by these organisms are similar, however, so precise identification is seldom important. Epidemic infection years occur in cycles and depend on climatic conditions during the infection period. Occasionally individual trees show marked resistance to the disease.
Collis, D. G. 1971. Rhabdocline needle cast of Douglas-fir in British Columbia. Can. For. Serv., Forest Pest Leaf. No. 32. Victoria, B.C.
Parker, A. K. and J. Reid. 1969. The genus Rhabdocline Syd. Can. J. Bot. 47: 1533-1545.
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Figure 55a: Thin crowns on Douglas-fir infected by Rhabdocline pseudotsugae.
Figure 55b: Defoliation caused by Rhabdocline pseudotsugae.
Figure 55c: Red-brown spots on Rhabdocline-infected Douglas-fir needles.
Figure 55d: Rhabdocline pseudotsugae fruiting bodies.