Royal bolete

The fruit bodies of Butyriboletus regius have caps that are initially convex before flattening out in maturity, reaching a diameter of 7–20 cm (2.8–7.9 in) wide. The cap surface is pink to red, occasionally with hints of yellow or brown, more so around the margin. Initially velvety to slightly tomentose (hairy) when young, these minute hairs tend to slough off with age, and the cap develops wrinkles and pits. The cap flesh is yellow, and slowly and erratically bruises blue in North American specimens. The pores on the underside of the cap are angular, and measure about 1–2 per millimetre. The color of the pore surface is bright yellow to start, but eventually darkens somewhat, and will stain blue with damage. The depth of the tubes comprising the pores extends to 0.8–2.5 cm (0.3–1.0 in). The stem measures 5–13 cm (2.0–5.1 in) long by 2.5–5 cm (1.0–2.0 in) thick, and typically has a thick, bulbous base. It is solid (i.e., not hollow), and a bright yellow color, often with reddish tones, particularly near the base of the stem. The stem surface can be covered with fine yellow reticulations either throughout its length, or just on the upper portion. Butyriboletus regius produces an olive-brown spore print. Its smooth, hyaline (translucent) spores are roughly elliptical to somewhat fusoid (wider in the middle and tapering toward the ends) to more or less cylindrical, and have dimensions of 12–17 by 4–5 μm. Chemical tests can be used to help identify Butyriboletus regius in the field. The cap cuticle will stain a pale purple color if FeSO4 is applied; this same test will turn the flesh grayish.