SPORES: spore print yellowish brown (between Cinnamon and Clay Color); spores pallid melleous, elliptic-subfusiform to oblong-elliptical, 7.8-9.1-(9.8) X 3.0-3.6-(3.9) µm, smooth. HYMENIUM: basidia hyaline, clavate, 21-30 X 5-7.5 µm., 4-spored; cystidia crowded in groups at or near tube mouths, no solitary cystidia seen, thin-walled, subclavate to clavate, 35-70 X 5.5-8.5 µm., vinaceous in KOH. HYMENOPHORAL TRAMA: bilateral of the Boletus subtype; clamp connections absent. CONTEXT OF PILEUS: white to pallid yellow, usually with large brownish areas, unchanging on exposure to air. TASTE: mild. SMELL: not distinctive. CHEMICAL REACTIONS: KOH on pileus—dark brown with vinaceous tints; on context of pileus—immediate, red flush turning mauve; NH4OH on pileus—dark reddish brown; on 'context— faint red to mauve.

Suillus subacerbus is probably of North American origin. In New Zealand it has been found only in association with Pinus radiata.

Suillus subacerbus appears to belong to the S. granulatus - S. pungens -S. acerbus complex of species as described by Smith and Thiers (1964) in their extensive study of North American Suilli. Of these species, S. subacerbus is most closely related to S. pungens Thiers & A. H. Smith and S. acerbus A. H. Smith & Thiers, both of which are recently described species naturally associated with Pinus radiata in California. However, it differs from both these species in a number of characters. In pileus colour, Suillus subacerbus most closely resembles S. pungens, but differs in the absence of a marginal roll of white tissue when young, shorter spores, vinaceous colour produced by KOH on cuticle and cystidia, and absence of a harsh taste and strong pungent smell. It is perhaps more closely related to S. acerbus, from which it may be separated by the combination of colours in the pileus, smaller spores, constant vinaceous colour produced by KOH on cuticle and cystidia, and absence of an unpleasant taste. Although these may appear to be but minor differences, the distinctions between many species of Suillus are not great, and S. subacerbus cannot satisfactorily be accommodated within any of the species so far described.

S. subacerbus may be distinguished from other Suilli occurring in New Zealand by the combination of colours in the pileus, presence of glandulae, absence of an annulus, and vinaceous colour reaction to KOH.