This section is from the book "Our Edible Toadstools and Mushrooms and How to Distinguish Them", by W. Hamilton Gibson. Also available from Amazon: Our Edible Toadstools And Mushrooms And How To Distinguish Them.
The general contour of the present species - Boletus subtomentosus (Plate 22, fig. 1) - resembles the foregoing, but it is easily distinguished by the color of its cap and tube surface, the pileus being usually olive, olive-brown, or red of various shades; the color, however, does not extend to the flesh beneath the peeled cuticle, as in Boletus chrysenteron, Fig. 2. The surface is soft and dry - subtomentous - to the touch. Cracks in the cap become yellow, on which account this species is called the "yellow-cracked Boletus," in contradistinction to the red-cracked Boletus chrysen-teron. Its most important distinction, however, is of a chemical nature.
The stem is stout, unequal, firm, yellowish, and more or less ribbed, occasionally tinted, minutely dotted, or faintly striped with the color of the cap. The taste of the flesh is sweet and agreeable. Palmer compares it to the flavor of walnuts. The tube surface is yellow or yellowish green, and the tubes and yellowish flesh of cap and stem turn a rich peacock-blue immediately on fracture, becoming deeper moment by moment until the entire exposed portion becomes leaden - especially noticeable in mature specimens. The pore surface shows a similar blue stain whenever bruised. The tubes are angular-sided instead of round, and much larger than in the Boletus edulis; spores ochre colored.
This blue stain was formerly, and is even now, deemed sufficient with many mycophagists to place this mushroom on the black-list, but is believed by Mr. Palmer and Mr. Mcllvaine to be unwarranted as a stigma, assuming that fresh specimens are employed. The Boletus subtomen-tosus is also among the eleven edible Boleti in the list of Dr. Curtis, given on a previous page, and the present author has habitually eaten the species with enjoyment and without unpleasant results. Fresh young specimens with the least change of color would perhaps be the wiser choice for the novice.
The blue stain. An unwarranted stigma
Boletus subtomentosus Boletus chrysenteron
1. Boletus subtomentosus
Pileus: Diameter three to six inches. Color, varying in different individuals, yellowish brown, olive, or subdued tan color; epidermis soft and dry, with a fine pubescence. Cracks in surface become yellow.
Flesh: Creamy white in mature specimens, changing to blue, and at length leaden on fracture.
Tube surface: Yellow or yellowish green, becoming bluish when bruised; opening of tubes large and angled.
Stem: Stout; yellowish; minutely roughened with scurfy dots, or faintly striped with brown.
Spores: Brownish ochre.
Taste: Sweet and agreeable.
Season: Summer and autumn.
Another species having this peculiar property of "turning blue" even in a more marked degree, and named, in consequence, the Boletus cyanescens, though always heretofore considered poisonous, is now pronounced by certain prominent mycoph-agists to be not only harmless but esculent. It is still advisable, however, to caution moderation in its use as food, if only on the ground of idiosyncrasy. The spores of this species are white, which, with the more minute tube openings, form a sufficient discrimination from subtomentosus. The spores should be obtained by a deposit on black or dark-colored paper. The flesh is white also. Other blue-stain species, such as Boletus alveolatus (Plate 24), are still considered with suspicion, presumably groundless.