Cytospora chrysosperma (Pers.:Fr.) Fr.
(= Cytospora pulcherrima Dearn. & Hansbrough)
Deuteromycotina, Coelomycetes, Sphaeropsidales, Sphaeropsidaceae
(teleomorph = Valsa sordida Nitschke)
Ascomycotina, Diaporthales, Valsaceae
Hosts: In B.C., Cytospora chrysosperma has been reported on maple, cottonwood, trembling aspen, lombardy poplar, and willow. Elsewhere in North America it has also been found on white and water birch, apple, white poplar, chokecherry, oak, elderberry, and mountain-ash. It is likely that all willow and poplar species are susceptible to this fungus.
Distribution: This fungus is widely distributed throughout the range of its hosts in B.C. It is common throughout the northern hemisphere and Australia, and is likely to accompany imported host plant material.
Identification: The disease occurs on stems, branches, and twigs, forming elongate cankers, regular or irregular in outline, generally with defined borders (Figs. 40a, 40b). Cankers appear as discoloured sunken zones, with a slightly raised perimeter formed by annual callus growth. The inner bark of diseased tissue turns brown to black and may have a distinct foul odour. The sapwood associated with cankers is light to reddish brown. Several years after infection, dead bark lifts away from the stem and falls off. Fruiting bodies (pycnidial stromata) of the asexual stage of the fungus (Cytospora) form within the cankers, just beneath the cuticle of the dead bark, appearing as short grey-black cones, 0.5-1 mm in diameter. Sticky masses of conidia ooze out of the pycnidia forming long, orange-red, coiled "spore tendrils" (Fig. 40c). Perithecia of the sexual stage (Valsa) are more rare. They appear as spherical black structures beneath the bark in clusters of 6-12, each 0.5 mm in diameter. Blackstem disease, a disorder of cottonwood seedlings and cuttings is thought to be caused in part by C. chrysosperma. Symptoms of blackstem are orange, brown, or black, water-soaked, necrotic stem lesions with well defined margins, and pimple-like fruiting bodies in infected bark.
Microscopic Characteristics: Stromata conic to truncate-conic, 0.5-2 mm diameter, breaking through the bark to expose a prominent dark grey disc (ectostroma). Perithecia embedded in the bark, 6-12 in a cluster, globose to compressed, 300-500 µm diameter, dark brown; ostiolar necks collectively erumpent through ectostroma, 500-700 µm long. Asci clavate, with apical ring, 8-spored, 30-45 x 5-7 µm; ascospores hyaline, allantoid, unicellular, both ends rounded, 7-12 x 1.5-2.5 µm. Pycnidial stroma immersed in the bark, 0.5-1.5 mm diameter, multilocular, ostiole erumpent, wall indistinct. Conidiophores usually branched, hyaline, phialidic, 10-40 mm long. Conidia hyaline, allantoid, unicellular, 3-5 x 1-1.5 µm, often emerging in long orange tendrils.
Damage: This disease is rarely a problem of economic importance in natural stands, but can cause serious damage in forest nurseries, young plantations, and in horticultural settings.
Remarks:Cytospora chrysosperma has been shown to inhabit healthy bark of aspen and beech, causing disease only in trees or branches of low vigour or when the hosts are stressed by drought, injury, sunscald, fire, or other pathological disorders. The presence of this disease generally indicates that the trees are under stress. Other species of Valsa, including Valsa abietis Fr. (anamorph: C. abietis Sacc.) and V. pini (Alb. & Schw. ex Fr.) Fr. (anamorph: C. pini Desmaz.) are associated with cankers and dieback of conifers.
References:
Bloomberg, W J. 1962. Cytospora canker on poplar. Can. J. Bot. 40:1271-1292.
Chapela, I. H. 1989. Fungi in healthy stems and branches of American beech and aspen: a comparative study. New Phytologist 113: 64-75.
Spielman, L. J. 1985. A monograph of Valsa on hardwoods in North America. Can. J. Bot. 63:1355-1378.
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Figure 40a: Cankers caused by Cytospora chrysosperma on trembling aspen.
Figure 40b: Cankers caused by Cytospora chrysosperma on trembling aspen.
Figure 40c: Spore tendrils of C. chrysosperma on trembling aspen.