Pileo 60–100 mm lato, hemispherico, dein plano-convexo, glutinoso, margine involuto, flavo, flavobrunneo, interdum rufo-brunneo maculato, Lamellis emarginatis, pallide luteis, Stipite 50–100 mm longo, bulbo marginato 30–50 mm lato, Velo universale albido. Carne albida, cortina copiosa, rufo-brunnea, sapore miti. Sporis 10–12 × 5.5–7 μm, amygdaliformibus usque limoniformibus, grosse verrucosis, basidiis 30–40 × 7–10 μm, tetrasporigeris, fibulis praesentibus.
Type: “USA, California, Contra Costa County, Tilden Park, Berkeley, 2009/11/15 col. Dimitar Bojantchev DBB26451 UCB Herbarium: Holotype UC 1860808 Genbank nrITS HQ441244”
Etymology: from the Greek: xanthos = yellow, drys = oak, philios = loving
Stature pileocarpous bulbopodium, very variable in aspect ratio. Pileus 60–100 mm diam, hemispherical to convex to plano-convex to uplifted in age. Margin persistently involute. Colors rather uniform, predominantly in yellow shades, starting pale straw- to sulphur-yellow (2.5Y 8/6) becoming yellow- brown (10YR 8/6-8/8), darker near the center (10YR 6/6-8/8), frequently with reddish-brown discolorations. Surface glutinous when wet, glabrous to dull glossy when dry, at age developing cracks and areolations near the disk, remaining smoother near the margin. Lamellae L=80–120, crowded, 8–15 mm broad, pale sulphur-yellow (2.5Y 8/6-8/8) to off-white when young, turning various shades of yellow-brown to brown (7.5R 6/6-5/6) as the spores mature. Edges even to occasionally slightly wavy, frequently eroded with age. Attachment notched. Lamellulae abundant, with widely varying extensions, 15–75%, series of 3–5. Stipe 50–100 mm long, 15–30 mm wide, cylindrical to subclavate above the bulb. Mostly white, but occasionally with light bluish tinges in the upper part. Bulb 30–50 mm diam at the widest point, always well-developed, abruptly emarginated, tapering below, the subterrestrial part with a white cottony mycelial felt. Context mostly white, slowly bruising brownish, few of the basidiomata feature a strong bluish cast in the upper stipe context, more pronounced near the surface. A watery grayish-blue cast above the lamellae is present in almost all basidiomata, rather faint in dry conditions, but persisting deep into maturity. Universal Veil white, frequently leaving floccose patches on the pileus surface, occasionally forming volva-like extensions on the bulb margin in recently expanded basidiomata. Cortina white to pale yellow, turning rusty brown due to mature spore drop, copious, persistent, leaving an annular zone of dense fibrils on the stipe and frequently forming a hairy appendiculate zone on the pileal margin. The cortina deposits form a distinct brownish belt on the bulb edge, which can be fused into a gelatinized matrix on the periphery. Macrochemical Reactions KOH5% light reddish brown on the pileus surface, stronger on the brownish spots near the disk. On the context the reaction varies from little to none on very young material, to yellowish-brown on mature basidiomata, stronger in the lower stipe and near the surfaces. No reaction was observed on the basal mycelium. UV no reaction was detected with both fresh and dry material. Odor mild, leafy, and earthy. Taste mild, earthy.
Basidiospores (9.5–)10–12(–13) × (5–)5.5–7(–7.5) μm (mean 11.2 × 6.3 μm) Q = 1.66–1.85, Qav = 1.78 (N = 213, 7 collections, 14 basidiomata), amygdaliform to citriform, distinctly and coarsely verrucose, deep rusty brown in deposit, slightly dextrinoid. Basidia 30–40 × 7–10 μm, 4-spored, cylindro- clavate, clamped. Hymenial layer not reacting to alkaline or iodine solutions. Cystidia none observed. Pileipellis a cutis, simplex, no hypodermium detected, composed of parallel to interwoven hyphae in a dense gelatinous matrix 150–250 μm thick. The outer 10–15 layers of hyphae 2–4 μm diam, entangled, some erect, irregularly shaped, strangulated to twisted, commonly with non–parallel walls, mostly with refractive cytoplasmic pigment. Lower layer of cuticle hyphae 3–7 μm diam, mostly parallel, with thicker yellow walls and hyaline content. The yellow pigmentation is emphasized when mounted in KOH. No distinct reactions to Melzer’s reagent were observed. Clamp connections common in all parts. Trama composed of cylindrical cells 10–15(–20) μm diam, hyaline with pale yellow walls. Occasional oleiferous hyphae present.
Habitat and distribution – Solitary to gregarious under oaks. This species is common in California under live oak (Quercus agrifolia). In the Sierra Nevada foothills we have collected it under interior live oak (Q. wislizenii) and canyon live oak (Q. chrysolepis). In Mendocino Co. it was collected under tanoak (Notholithocarpus densiflorus). Based on a molecular data match there is a collection (Genbank DQ974721) under blue oak (Q. douglasii) in the Central Valley. There is also a collection (Genbank GQ159771) from Vancouver Island, British Columbia, under garry oak (Q. garryana).