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In California, both Armillaria mellea and A. ostoyae have a thick, felty annulus and they are the only Armillaria species here to have that character. They differ by the color of both the cap and the annulus.
A. mellea has a white to yellow edge on the annulus, the cap is honey yellow and the disc is darker than the margin giving it a two-tone appearance. The cap is smooth and the hairs are indistinct.
A. ostoyae has a brown edge on the annulus, the cap is brown with dark hairs.
What rule do you use? I generally use A. mellea for the ones that grow on hardwoods and A. ostoyae for the ones on conifers. I do know that Tom Volk id’ed a collection from southern California growing on hardwood as a slightly unusual form of A. mellea. I also sent him a sample from Santa Cruz that came out as A. mellea. Apparently the California material mated with the true A. mellea, but it was not a typical mating. Macroscopically I remember Tom noting that the California material has a scalier cap than classic A. mellea. Based on his key it looks like we should be doing more microscopy and looking for clamps at the base of the basidia. Personally I have a hard enough time find a good basidia, much less figuring out whether there’s a clamp at the base. Have you had much success with that? Any recommendations?