Type: Caulicolous Fungi; Description: Ascomata perithecial, stromatic, solitary or aggregated in groups of 2–3, globose to subglobose, up to 0.4 mm in diameter, with necks up to 200 μm long; on cankers on stems and branches. Asci clavate, 90–170 c 9–12 μm. Ascospores oblong to ellipsoidal, 3-septate, 17–25 8–9 μm, brown. Conidiomata stromatic, pycnidial, scattered to gregarious, immersed, elongate to oval, dark brown to black, 0.2–0.6 mm long, opening by an irregular split; on cankers on stems and branches. Conidia fusiform, straight or slightly curved, 5-septate, 24–30 × 8–10 μm, smooth, apical and basal cells hyaline, intermediate cells pale brown to brown; apical cell with a single, tubular appendage, 5–8 μm long; basal cell appendage single, tubular, centric, 3–6 μm long.

Distribution: Northland, Auckland, Coromandel, Waikato, Bay of Plenty, Taranaki, Taupo, Rangitikei, Wanganui, Wellington, Gisborne, Hawkes Bay, Wairarapa, Nelson, Buller, Westland, Fiordland, Marlborough Sounds, Marlborough, North Canterbury, Mid Canterbury, South Canterbury, Mackenzie, Otago Lakes, Central Otago, Dunedin, Southland.; 1st Record: Fuller & Newhook (1954: as Monochaetia unicornis).

Notes: The teleomorph is present but uncommon in New Zealand, and isolations from cypress cankers almost invariably yield the anamorph. Boesewinkel (1983) considered that many of the collections identified as Seiridium unicorne in New Zealand could be distinguished as a separate species, S. cupressi, but Chou (1989) found that S. cupressi and S. unicorne were morphologically indistinguishable. Barnes et al. (2001) reported that while S. unicorne and S. cupressi could not be distinguished on morphological grounds, they could be separated using sequence data from either the β-tubulin or histone H3 gene. The two clades defined by Barnes et al. (2001) did not coincide exactly with Boesewinkel’s two morphological groupings, but they concluded that the two New Zealand isolates of ‘S. unicorne’ included in their study should be referred to S. cupressi. Recent reports from Europe, based on molecular characteristics (Graniti 1998; Moricca et al. 2000) support the existence of three species there, and indicate that a fourth species may be present in Australia (Graniti 1998). Until the New Zealand situation is clarified, I have included records of S. cupressi under S. unicorne.

Significance: One of the fungi causing cypress canker.Cypress canker is common throughout New Zealand (van der Werff 1988, Hood et al. 2001). The fungi cause cankering on branches and stems, resulting in crown dieback which leads to malformation, reduced wood quality and, occasionally stem breakage. Some mortality has been recorded, particularly in young stands (Hood et al. 2001). No economic evaluation of the losses is available. Conidia are dispersed by water splash. New cankers are initiated if conidia land on wounds or cracks in otherwise healthy tissue. Pruning increased spread of the disease (Self & Chou 1994, Hood et al. 2001). Van der Werff (1988) reported that incidence of cypress canker in Cupressus macrocarpa decreased as tree age increased and increased with a rise in average mean temperature. No correlation with rainfall was found. Hood et al. (2001) observed a gradual increase in severity of the disease from south to north of the country. A farm forester of much experience with cypresses has noted that C. macrocarpa seems to be much healthier on southern slopes and sheltered sites, indicating that site temperature and exposure are important factors influencing disease severity. He also noted that the disease also seemed to be worse on the windward site of a shelterbelt (D.Hocking, pers. comm.). There is considerable difference in susceptibility between the host species (Gilmour 1966a; Hood et al. (2001): highly susceptible—Chamaecyparis lawsoniana, Cupressus macrocarpa; susceptible—×Cupressocyparis leylandii, Cupressus lusitanica; resistant —Cryptomeria japonica, Cupressus arizonica, C. torulosa, Thuja plicata. Records of Seiridium spp. in the Forest Health database maintained by New Zealand Forest Research Institute show that Seiridium unicorne is more common (77%) than S. cardinale (23%). An exception occurs in the Canterbury region (North, Mid and South Canterbury), where records of S. cardinale were as frequent as those of S. unicorne. Most records of S. unicorne are from C. macrocarpa, C. lawsoniana, and ×Cupressocyparis leylandii. Nearly all records of S. cardinale came from C. macrocarpa. Only S. unicorne was recorded on C. lusitanica. Chemical control of cypress canker is not considered to be necessary. Resistance to this disease is one of the criteria used in the selection of superior individuals of C. macrocarpa for tree improvement field trials (Gea & Low 1997).; Host(s): Callitris rhomboidea, Calocedrus decurrens, Chamaecyparis lawsoniana, C. nootkatensis, C. pisifera, C. thyoides, Cryptomeria japonica, ×Cupressocyparis leylandii, ×C. ovensii, Cupressus arizonica, C. arizonica × torulosa, C. arizonica var. glabra, C. duclouxiana, C. guadalupensis, C. lusitanica, C. macrocarpa, C. sempervirens, C. torulosa, Sequoiadendron giganteum, Thuja occidentalis, T. plicata, Widdringtonia sp.