Tsubaki mushrooms contain a lot of starch, and like potatoes they will oxidize and turn black after being cut.

If you don't use it right after cutting it, be sure to soak it in water to prevent oxidation. You can also wash the skin of the Izumi mushrooms, peel the skin, and then cut into slices or pieces, blanch them, cool them down and put them in the freezer of the refrigerator, and then take them out to defrost them when you want to eat them.