The taxonomic value of the ornamentation of spores in ‘The Xerocomus-group’ of Boletus

The taxa of the Xerocomus-group are considered to belong to the genus Boletus. The spore surface of 17 taxa of this group and the related Phylloporus rhodoxanthus have been studied with SEM. The taxa of the B. chrysenteron-complex can be distinguished from the B. subtomentosus-complex and the other taxa examined by their striate spores. Boletus parasiticus can be distinguished by its conspicuously pitted spore surface. The spores of the taxa in the B. subtomentosus-complex, B. bubalinus sp. nov. and B. armeniacus show a fibrillose or a floccose to smooth spore surface. Entirely smooth spores have been found in B. badius, B. moravicus, B. pulverulentus, and B. ichnusanus comb. nov. The spores of P. rhodoxanthus show a fibrillose, a floccose or a pitted-and-floccose ornamentation. One new species is described, Boletus bubalinus Oolbekkink & Duin, and one new combination is proposed, Boletus ichnusanus (Alessio & al.) Oolbekkink. Boletus fragilipes is considered a nomen dubium. It is suggested that B. rubellus’ is merely a colour variant of B. chrysenteron. To elucidate the author’s present points of view, a chapter is added with a discussion on the taxonomic status of Xerocomus, a provisional key and descriptions of, and notes on taxa of the Xerocomus-group.