Summary:
It has not been confirmed that the true Lactarius deterrimus is in the Pacific Northwest or even in North America. What is covered here is what Hesler(4) call Lactarius deliciosus var. deterrimus. Features include 1) orange milk that stains surfaces purplish vinaceous then dull dark green, 2) a cap that is viscid, bald, and pale orange buff or with a salmon tint, 3) adnate to decurrent, close, orange buff gills, 4) a stem that is moist to lubricous and colored about the same as the cap, and 5) microscopic characters including broadly elliptic spores with amyloid ornamentation forming a partial or broken reticulum with additional isolated warts and ridges. Lactarius deterrimus is a separate species from L. deliciosus according to Courtecuisse(1). Bessette(2) says L. deterrimus is the correct name for the North American species and L. deliciosus is European. Hesler(4) reduced L. deterrimus to subspecies of L. deliciosus, (and described L. deliciosus varieties areolatus, deterrimus, olivaceosordidus and piceus for North America but not variety deliciosus - although Methven(2) gives var. deliciosus as common in CA). Leuthy(1) treats L. deterrimus as a variety of L. deliciosus that has orange latex which stains tissue purple (vinaceous) before dull green, not becoming areolate like var. areolatus. Nuytinck(2) differentiates L. deterrimus (and L. fennoscandius) from L. deliciosus (and L. quieticolor), all in Europe, on the basis that the former has a stem that is not obviously scrobiculate, is associated with spruce, and spore ornamentation consists of isolated warts and short ridges, whereas the latter has stem mostly scrobiculate, is associated with pine, and has spores ornamented with an incomplete reticulum. However, despite the fact that Nuytinck(2) synonymizes Lactarius deliciosus var. deterrimus (Groeger) Hesler & A.H. Sm. with L. deterrimus Groeger, they are not convinced that the American taxon is conspecific with the European one. What follows is from Hesler(4) who use American material from MI and treat it as a variety of L. deliciosus.

Cap:
(3)8-13cm across, convex to convex-depressed, more or less flat-convex when old; pale orange-buff or with a salmon tint, duller when old, slowly staining greenish; viscid, bald, subzonate [somewhat zonate] to not zonate, at times with a sheen

Flesh:
fragile; pallid, becoming more or less "pinkish cinnamon" when old, soon staining purplish vinaceous above the gills, MILK moderately abundant; 'bright orange at first, staining cut surface purplish vinaceous then dull green'

Gills:
adnate to decurrent, close, many tiers of subgills, gills narrow to moderately broad, not showing an appreciable number forking; "orange buff" singly, "capucine orange" in mass (brighter), staining reddish vinaceous then dull dark green where injured

Stem:
2.5-7cm x 1.5-2.5cm, equal or narrowing downward, moderately fragile, becoming hollow; colored about as cap, staining reddish vinaceous where cut; "moist to scarcely lubricous but not viscid", not scrobiculate but may be slightly pitted to uneven

Veil:
[none]

Taste:
in young caps more or less peppery, in old ones mild or practically so

Microscopic spores:
spores from gills 7.5-9 x 6-7 microns, broadly elliptic, plage lacking diffusely distributed amyloid material, [amyloid] ornamentation forming a partial or broken reticulum with additional isolated warts and ridges, prominences 0.5-0.8 microns high; basidia 4-spored, 45-50 x 9-11 microns; pleurocystidia: macrocystidia absent or a few similar to cheilocystidia near edge, 35-50(65) x 3-6 microns, aciculate, pseudocystidia scattered, filamentous, cheilocystidia scattered, 30-40 x 3-6 microns, aciculate; cap cuticle "an ixocutis moderately well developed, lacking incrustations and apparent-amyloid debris"; stem cuticle "thin, not well developed, hyphae longitudinally arranged", no slime noted in KOH or Melzer''s reagent

Spore deposit:
pale buff

Notes:
Lactarius deterrimus Groeger was reported from WA by Jumpponen(1). There are collections from BC labeled as this species at Pacific Forestry Centre and the University of British Columbia. Hesler(4) material is from MI. There are collections so labeled at the University of Washington from Washington, Oregon, Alaska, and Vermont, but the sense is unclear - at one point Lactarius deterrimus was a name in use for any Pacific Northwest Lactarius deliciosus group member.

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