Hypoxylon mammatum (Wahlenberg) J. H. Miller
(=Hypoxylon pruinatum (Klotzch) Cke.)
Ascomycotina, Sphaeriales, Xylariaceae
Hosts: Hypoxylon mammatum is found only on hardwoods, most commonly on poplar and willow. In B.C., it has been reported on aspen, willow, and Sitka alder. Elsewhere in North America it has also been found on other poplar spp., birch, apple, oak, and hophornbeam.
Distribution:Hypoxylon canker has been reported with low frequency from all parts of the province.
Identification: Hypoxylon cankers first appear as sunken, yellow-orange areas on the bark of stems, centered around dead branch stubs or injuries. The bark associated with older cankers becomes mottled, then necrotic with small blisters of dead bark appearing at the canker margins (Fig. 43a). Cankers enlarge rapidly attaining lengths > 1 m, and often girdle the stems. Hyphal pegs, or pillars, bearing conidia form beneath dead bark, pushing it away from the underlying cortical tissues. Two to three years after the initial infection, perithecia form on the surface of the canker. The perithecia are embedded in stromata and have a prominent nipple-like tip through which ascospores are released (43b, 43c). The perithecial stroma is fertile for only 1 year but may persist for several years. It is greyish at first, and becomes black with age.
Microscopic Characteristics: Ascostromata immersed or erumpent, discrete, whitish pruinose at first, becoming black, 2-5 mm in diameter, 1-2 mm thick, usually coalescing to form effused stromata up to 25 mm in length, smooth except for papillate ostioles, carbonaceous, sometimes tuberculate; perithecia globose, single or up to 30 in a stroma, 0.7-1 mm in diameter; asci cylindric, with J+ apical plugs, 140-200 x 12-16 µm with stipe 30-40 µm long; ascospores uniseriate, ellipsoid, dark brown, with a germination slit running along the axis of the spore, 20-30 x 9-12 µm. Paraphyses present but indistinct and gelatinizing. Conidial state: Grey, pillar-like hyphal pegs arising from a brownish subiculum below the periderm. Conidiophores branched, with 2-3 terminal conidiogenous branches, 75-150 µm high. Conidia developing on geniculationson conidiogenous branches, hyaline, ellipsoid, 6-9 x 2-4 µm.
Damage: Trees with main stem cankers usually die within 5 years or are structurally weakened and break in the wind. Most damage occurs on injured or stressed trees, and on trees growing in poorly stocked or open stands. In some regions of the eastern USA, annual losses are estimated to be 30% of the net growth of aspen.
Remarks: Although H. mammatum has been recorded throughout British Columbia, the incidence and loss to the disease are low. In contrast, the fungus is commonly found on aspen in Alberta. The factors contributing to infection by H. mammatum appear to be very complex and it is difficult to predict where and when disease will occur. There does seem to be variation in both susceptibility among poplar clones, and virulence of the fungus. Cankers caused by H. mammatum could be confused with other cankers or bark anomalies, but the presence of hyphal pegs and perithecia are unique diagnostic features.
References:
Anderson, R. L. and G. W. Anderson. 1969. Hypoxylon canker of aspen. USDA For. Serv., Pest Leaf. No. 6. Washington, D.C.
Rogers, J. D. 1979. The Xylariaceae: systematic, biological and evolutionary aspects. Mycologia 71: 1-43.
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Figure 43a: Dead bark of trembling aspen associated with Hypoxylon canker.
Figure 43b: Perithecia embedded in stroma.
Figure 43c: Perithecia embedded in stroma.