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Remarks (internal):Habitat and distribution: Only known from montane rainforest in Java, probably associated with Pinus.
Collections examined: INDONESIA, Java, Bogor, north slope of Mt. Salak, Curug Nangka, 910 m a.s.l., under Pinus merkusii (native to Sumatra), planted in secondary broad-leaf montane rainforest, on soil, 02.01.2000, E. Horak 8336 (ZT), holotype – Java, Bogor, north slope of Mt. Salak, Curug Nangka, 910 m a.s.l., under Pinus merkusii (native to Sumatra), planted in secondary broad-leaf montane rainforest, on soil, 01.11.2000, leg. A. Retnovati, AWW 30, E. Horak 9983 (ZT).
Discussion: Lactarius horakii is a small species with reddish-brown colours in the pileus, a purple-grey stipe and red latex. It was collected in montane rainforest under Pinus merkusii. It has rather densely ornamented, small spores (7.3-7.7 — 5.6-5.8 µm on average), which are mostly broadly ellipsoid.
Our molecular analyses reveal a close relationship to L. hatsudake. The most striking morphological differences with L. hatsudake are the general colour of the basidiocarps (L. hatsudake often has orange tinges) and the size of the basidiocarps. Microscopically the spores of L. horakii are clearly smaller than those of L. hatsudake (8.3-9 — 6.2-6.6 µm on average in L. hatsudake) and L. horakii lacks cheilomacrocystidia.
Description type:Original description 
Description:Pileus up to 4.5-6.5 cm diam., plano-convex when young with an incurved margin, when expanding becoming depressed with straight margins to deeply infundibuliform, occasionally papillate in the centre; margin translucently-striate, entire to eroded; surface glabrous, sticky when moist, reddish-brown with a pale purplish tinge and with conspicuously darker ring-like zones when young, mature specimens light pink (6-8A2) to greyish-pink (8C4), aged specimens changing to dirty pale beige and losing zoned aspect, staining lemon-green (30A6/7) to lime-green (27/28B8). Lamellae shortly decurrent, crowded, often anastomosing at the base; colour pale pink (8/9A/B2), greening upon bruising; edge slightly paler. Stipe 4-4.5 — 0.9-1 cm, cylindric, equal or slowly tapering towards the base, apex often costate otherwise smooth; surface dry, purple-grey (8C4) to pale pink (6-8A2), base and especially upon handling turning to pale or deep green; whitish mycelial tomentum or rhizomorphic strands present at base, this basal tomentum turning blue-green. Context hollow in the stipe; whitish to pale beige, lower half of the stipe blue-green, rind of stipe and above the lamellae blood red-brown, becoming pale to deep green after exposure; smell not distinctive to pleasant; taste mild to bitter. Latex vinaceous red (10/11D/E8).
Spores 6.8-7.3-7.7-8.2 — (5.1) 5.2-5.6-5.8-6.2 µm, mainly broadly ellipsoid to sometimes ellipsoid (Q = 1.25-1.32-1.34-1.43); ornamentation up to 0.5 µm high, composed of medium broad ridges and some isolated warts forming a rather dense, incomplete to almost complete reticulum; plage very slightly amyloid distally. Basidia 35-45 — 7-9 µm, subclavate to subcylindric, hyaline or with a needle-like content, 4-spored; sterigmata up to 5.2 µm high. Pleuromacrocystidia abundant near the lamella edge, scarce elsewhere, 35-55 — 6-8 µm, emergent, fusiform, with an acute to moniliform apex, with needle- to cube-shaped crystals, thin-walled. Pseudocystidia abundant, 3-4 µm broad, cylindrical to tortuous, not to slightly emergent, with a refringent content. Lamella edge sterile without cheilomacrocystidia; cheiloleptocystidia 7.5-15 — 2.5-5 µm, subclavate to irregularly shaped, hyaline, thin-walled. Subhymenium irregular, appearing like angular cells, more or less arranged in rows. Hymenophoral trama irregularly filamentous, with abundant, ochre coloured lactifers. Pileipellis an ixocutis, 50-100 µm thick, composed of thin-walled hyphae +/- parallel to the surface; with abundant shrivelled and gelatinous hyphae in the surface layer, hyphae 1-4 µm diam. Stipitipellis a cutis, 30-50 µm thick, composed of hyaline hyphae with somewhat refractive walls, arranged parallel to the surface; no shrivelled or gelatinised hyphae.
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