When a fungal spore germinates, the living strand that comes out is called
hyphae (high-fay). When the hyphae increase in mass, it is then referred to as
mycelium, and it can take on different forms, or colors. The mycelium can be cottony, filamentous or if the hyphal filaments branch, and those branches continue to branch, the mycelium is described as rhizomorphic (having the form of rhizomes) and are occasionally referred to as 'rhizomorphs'.
This is probably much more than you wanted to know about early fungal structure, but I'm a geek and this stuff has always fascinated me.
Anyway, many, many species of fungi start their life cycle from spore in much the same way; germination, hyphal development and growth, and they spend most all of their life in this form- mycelium. The mycelium produces powerful enzmes, usually cellulases, but often lignases and some both.
To identify the fungus by mycelium alone is next to impossible, unless you plate it out on nutrient agar, attain a pure culture, propagate it on sterilized substrate and fruit it out. This is not at all difficult to do if you have the lab facilities and knowledge, but it can take weeks or months to get fruitbody production. There are hundreds of species whose mycelium is white and rhizomorphic. The color of
Armillaria mycelium is often more cream-to-honey colored, but can be brown, dark rust or black, so you can increase the probability of identification by that. I've never seen it to be a true white. Whenever I find
Armillaria (not very often) I do a bark peel and note the form and color of the mycelium, as well as the smell (farinaceous and slightly sharp and acrid).
Armillaria does like oak, but so do a lot of other species of fungus.
The true ID can only be done by 1) DNA profile or 2) Fruitbody structure, gill color (white) and spore color (also white). You need a microscope to see the spores, or create a sporeprint on both white and colored paper with the cap of a mushroom whose stem has been cut off. Armillaria produces abundant white spores.
Armillaria (
Armillaria mellea) tends to grow in clusters, but not always is brownish-tan-honey color, holds it's veil on the stem after the cap opens and can grow out of the tree itself, or out of the soil where a root is near the surface but very often at the stump-soil line. The stem is usually stringy and rather tough to break (does not snap off easily or cleanly) This is an edible mushroom and is really very good pan-fried in butter (not margarine) and a little salt. Keep the heat high and keep em moving. The caps are better than the stems as the outside of the stem, as mentioned before, is kinda tough and stringy.
This most certainly is more than you wanted to know.