Type: Caulicolous Fungi; Description: Basidiomata perennial, effused-reflexed, pileate. Pilei projecting 10–30 mm from the substrate, separate or clustered, 50–100 mm in diameter, 1–2 mm thick. Pileus surface irregular, finely tomentose, rusty brown to dark brown, margin lighter. Pore surface grey to brown-violet with a lilac tinge, later ochraceous brown, smooth to tuberculate, margin somewhat thickened and finely tomentose. Hyphal system dimitic. Cystidia abundant, encrusted, 40–60 × 6–9 μm, yellowish brown. Basidio-spores cylindrical to narrowly ellipsoid, 5–7 × 2–3 μm, hyaline, smooth. Arthrospores produced in special intersegmental sacs of the adult female wood wasp Sirex noctilio.

Distribution: Northland, Auckland, Coromandel, Waikato, Bay of Plenty, Taranaki, Taupo, Rangitikei, Wanganui, Wellington, Gisborne, Hawkes Bay, Wairarapa, Nelson, Buller, Westland, Marlborough, North Canterbury, Mid Canterbury, South Canterbury, Central Otago, Dunedin, Southland.; 1st Record: Clark (1933: as ‘the Sirex fungus’).

Significance: The fungus, which is carried by the adult female wood wasp Sirex noctilio Fabricius in special intersegmental sacs, causes a sudden wilt disease of exotic conifers in New Zealand. Arthrospores and a toxic mucus are deposited along with an egg by the ovipositing insect in tunnels made in tree stems. The mucus causes wilting and yellowing of foliage. It facilitates spread of the fungus, which rapidly kills cambium cells and invades the wood, disrupting water movement. The combined effect of mucus and fungus can kill a tree weakened by drought, suppression, or subjected to massive oviposition. Although present in New Zealand at least since 1900 (Miller & Clark 1935), the first major outbreak did not occur until 1946. By 1951, 25–30% of Pinus radiata trees growing on about 100,000 ha had been killed. Shortage of labour in New Zealand during the Second World War meant that most tree stands were untended and heavily overstocked. Most of the trees killed had been suppressed and mortality was beneficial because it provided a much-needed light thinning. At present, Sirex noctilio populations are controlled by the introduced larval parasitoids Rhyssa persuasoria persuasoria (Linnaeus), Megarhyssa nortoni nortoni (Cresson) and Ibalia leucospoides leucospoides (Hochenwarth) and the nematode Deladenus siricidicola Bedding which parasitises ovaries and renders the female wasps sterile. Currently, the presence of the fungus is unimportant in well-tended stands. Occasional local outbreaks after long periods of drought have been short-lived and the damage caused has been minor.; Host(s): Resistant i.e., attacked, but suffering little damage: Abies sp., Larix decidua, L. kaempferi, Picea abies, P. sitchensis, Pseudotsuga menziesii. Moderately resistant: Pinus palustris, P. pinaster. Susceptible: Pinus contorta, P. echinata, P. elliottii, P. muricata, P. nigra subsp. nigra, P. nigra subsp. laricio, P. patula, P. ponderosa, P. radiata, P. strobus, P. sylvestris, P. taeda.