Summary:
Lactarius necator is characterized by a dark green to olive brown, fibrillose to fibrillose-scaly cap, a viscid stem, white to pale yellow, unchanging milk, staining the gills dark brown to olive brown where cut, and a magenta staining reaction when KOH is applied to cap surface, occurring in association with birch. Hesler(4) describe a collection of Lactarius necator from England and say that in the concept they present (1979), they are not aware that the species has been found in North America. The online Species Fungorum, accessed April 1, 2016 and September 27, 2020, says that both the Lactarius necator of many authors and the Lactarius plumbeus of some authors should be called Lactarius turpis (Weinm.) Fr., and list Lactarius necator (Bull.) Pers. and Lactarius plumbeus (Bull.) Gray separately. Methven concluded that the name L. necator should be used over L. turpis, "Burlingham (1919) and Hesler and Smith (1979) correctly concluded that L. necator and L. turpis were synonymous. Since L. necator is the older, correctly applied epithet, it has priority and should be used over L. turpis." (Methven(2)). Trudell(4) on the other hand uses the name Lactarius plumbeus (Bull.: Fr.) S.F. Gray (=L. necator, L. turpis).

Cap:
6-9cm across, convex to flat, margin incurved to decurved [downcurved]; azonate, olive brown to brown or dark green; moist to subviscid, fibrillose to fibrillose-scaly, margin densely tomentose, (Methven), up to 15cm across, margin inrolled for a long time; dirty, olive-brown, putty-colored, center blackish or more greenish yellow; vaguely fibrillose zoned, margin slightly shaggy, (Courtecuisse), becomes broadly depressed when mature; "strong olivaceous tones usually mixed with yellow at first and brown or green when mature", "somewhat sticky surface that is velvety to hairy or somewhat scaly, and the edge often appears veined from above", (Trudell)

Flesh:
1-1.5cm thick at disc; pale yellow, unstaining on exposure, (Methven), dingy, browning slightly, (Courtecuisse), all parts of fruitbody darken when broken or bruised (Trudell), MILK white to pale yellow, unchanging, staining the gills dark brown to olive brown where cut, (Methven), drying greenish yellow on the gills (Courtecuisse), copious; white, (Trudell)

Gills:
"adnate to subdecurrent, close, broad", forking near stem; pale orange, not marginate, "staining dark brown to olive brown where cut", (Methven), subdecurrent, close; "white then tinged greenish, spotted russet or olive brown", (Courtecuisse), crowded, usually forked near stem; "whitish with pale orange to pale yellow tints when fresh", (Trudell)

Stem:
4-5cm x 3-4cm, equal or narrowing toward base, round in cross-section, stuffed to hollow; grayish yellow at top, brown near base; subviscid, bald, not scrobiculate, (Methven), up to 7cm long and 3cm wide, more or less concolorous, sometimes pitted, (Courtecuisse), cylindric to barrel-shaped; "strong olivaceous tones usually mixed with yellow at first and brown or green when mature"; slightly sticky, sometimes has spots, (Trudell)

Odor:
not distinctive (Methven)

Taste:
slowly peppery (Methven), mild at first, then very peppery, (Trudell)

Microscopic spores:
spores 7-9 x 4.5-6(6.5) microns, elliptic, amyloid ornamentation a broken to partial reticulum up to 0.5 microns high; basidia 35-55 x 4.5-9 microns; pleurocystidia: macrocystidia 50-65 x 6-9 microns, often with one or more subapical constrictions, cheilocystidia 35-50 x 4.5-6 microns, fusoid to fusoid-ventricose, often with one or more subapical constrictions; cap cuticle an ixocutis, producing a diffuse magenta pigment in KOH; stem cuticle a distinct ixocutis, producing a diffuse magenta pigment in KOH, (Methven)

Spore deposit:
pale yellow in mass (Methven)

Notes:
There are collections designated as this species from BC and NB at the University of British Columbia, from WA, AK, NB, Austria, and Sweden at the University of Washington, and from OR at Oregon State University. It occurs in CA (Methven(2)) and WA (Trudell(4) as L. plumbeus).

EDIBILITY

Synonyms and Alternate Names:
Lactarius plumbeus "(Bull.) Gray, "
Lactarius turpis (Weinm.) Fr.