Cronartium ribicola J. C. Fisch.
Basidiomycotina, Uredinales, Cronartiaceae
Hosts: In B.C., Cronartium ribicola affects native five-needle or soft pines, and has been reported on whitebark, sugar, western white, limber, eastern white, and Swiss stone pine (in order of relative susceptibility). Exotic soft pine species planted as ornamentals can also be infected by C. ribicola.
The telial hosts include currants and gooseberries (Ribes spp.).
Distribution: Five-needle pines are affected throughout their range in B.C., largely in the southern half of the province.
Identification: Infected trees can be identified by the presence of dead branches (red flagging) in the lower portion of the crown. Infections are characterized by diamond-shaped, orange-coloured cankers, evident on young stem and branch tissue with thin, smooth bark (Figs. 34a, 34b). Cankers on older stems have roughened, dead bark, often with resinosis (Fig. 34c). During the spring, white aecial blisters form on the canker, producing orange-coloured aeciospores (Fig. 34d). In the summer, spermatia develop around the margins of the canker in sticky, orangish droplets that dry out and leave small brown scars on the canker surface. Each year, aecia form in the tissue that produced spermatia the previous year.
Infection of Ribes spp. (the telial host) occurs shortly after aeciospores are released from pines in the spring. Uredinia appear as yellow-orange pustules on the lower side of leaves (Fig. 34e) in which orange-coloured urediniospores (Fig. 34f) are produced throughout the summer. Opposite the uredinia, on the upper leaf surface, chlorotic-necrotic spots are formed (Fig. 34g). In mid to late summer telial columns form in place of uredinia and appear as brownish-coloured hair-like structures on the lower side of the leaf (Fig. 34h). Heavily infected Ribes leaves can appear chlorotic and necrotic, and are sometimes shed prematurely.
Life Cycle: White pine blister rust alternates between five-needle pines and Ribes spp. (currants/gooseberries). Infection takes place through needles in the fall; the fungus grows into and down the branch toward the stem. The fungus grows in the phloem and bark with no visible symptoms for at least three years before spores are produced. In the spring of the 3rd or 4th years, spermatia are formed, followed by the production of aeciospores in white blisters that break through the bark (Fig. 34a). Aeciospores are capable of infecting only Ribes spp; approximately 10 days after infection, urediniospore development starts on the leaves and continues throughout the summer (Fig. 34e). Urediniospores are able to reinfect Ribes spp., thus intensifying the disease on this host. In the fall, teliospores and basidiospores are produced on Ribes spp. that carry the disease back to pine thereby completing the life cycle.
Microscopic Characteristics: Spermatia and aecia on pine caulicolous, spermatia orange, ellipsoid or ovoid. Aecial filaments on inner surface of peridium lacking or few. Aeciospores orange, short-ellipsoid, 18-20 x 22-31 µm, verrucose with a conspicuous smooth spot and warts up to 2 µm high. Urediospores on Ribes spp. hypophyllous, ellipsoid or obovoid, 14-22 x 19-35 µm, sparsely echinulate, orange. Teliospores oblong or cylindric, 8-12 x 30-60 µm, colourless, smooth, forming cinnamon-brown telial columns.
Damage: In many regions of B.C., the volume of white pine has been depleted to the point where it is no longer considered a viable commercial species. The disease is particularly serious in young trees; very few escape infection and many are killed within a few years. Cankers on young trees generally occur within 2.5 m of the ground, due to the presence of susceptible small branches and favourable environmental conditions. Similarly, open-grown trees with persistent branches are more likely to be infected than trees with self-pruned branches growing in dense stands. In older trees, the rust is often confined to isolated branches or the upper crown so that only part of the tree is killed.
Remarks: White pine blister rust is an introduced pathogen, and most natural populations of white pines are highly susceptible. It is believed to have been introduced to British Columbia in 1910 but was not discovered until 1921. It is the only stem rust on white pines and can therefore be easily distinguished from the other similar Cronartium rusts on the basis of this host preference. Basal stem cankers producing resin might also be confused with symptoms of Armillaria root rot. The root rot can be distinguished by the presence of white mycelial fans beneath the bark.
Hunt, R. S. 1983. White pine blister rust in British Columbia. Can. For. Serv., Forest Pest Leaf. No. 26. Victoria, B.C.
Ziller, W. G. 1974. The tree rusts of western Canada. Can. For. Serv., Publ. No. 1329. Victoria, B.C.
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Figure 34a: Aecial pustules of Cronartium ribicola on western white pine just before sporulation.