Oxford Dict. of English has:
adjective, denoting wood containing blackish irregular lines as a result of fungal decay, sometimes used to produce a decorative surface. Origin: 1970s, from dialect 'spalt', to split, splinter + -ED
Wood which is in the first stages of fungal decay, ie rot. Very often it is made manifest by irregular dark, or black, lines which run through the material. It is most common in beech but is found in many other hard woods. The fungae require damp conditions in which to grow; when the wood is dried (below about 20% MC) the fungae die and the process ceases. http://www.turningtools.co.uk/glossary/glossa...
If you want to add this to Leo (rarity is certainly no reason not to!) you'll need to find a German source. Duden is not helpful: all I can find from your suggestion is under "stocken": "4. Stockflecke bekommen <hat>: die alten Bücher haben gestockt." and "Stock|fleck, der: durch Schimmelpilze auf Textilien, Papier, Holz entstehender heller, bräunlicher od. grauschwarzer, muffig riechender Fleck: Auf der Rückseite dieser von -en verunstalteten Fotografie (Ransmayr, Welt 136). " © 2000 Dudenverlag
Stockfleck is translated as "patch of mildew" and stockig as "mildewy" in Muret-Sanders, which does not sound like the same thing as spalted wood to me:
mildew noun 1 any of various parasitic fungi that produce a fine white powdery coating on the surface of infected plants, or white or grey patches on the surface of paper, leather or other materials made from plant or animal material and subsequently kept in damp conditions. 2 the white powdery coating itself, or the white or grey patches produced by these fungi. verb (mildewed, mildewing) tr & intr to affect or become affected by mildew. mildewed adj. mildewy adj.
ETYMOLOGY: Anglo-Saxon mildeaw. http://www.chambersharrap.co.uk/chambers/chre...