I've heard from wood turners that covering the bowl blanks (basically large cylinders) in a pile of leaves for a year will encourage the mold to grow and spread. You might be able to see the black line (mapping) on a cross cut, but it is more obvious when it is cut in a lengthwise direction (because that the direction the fungus migrates). Just my 2 cents.
I did have a gorgeous bowl roughed out of plain variety hackberry, and I wrapped it in the shavings and placed in a plastic shopping bag. I had every intention to get it finished the next weekend - but it was 2 months before I got to it. The pure white nothing interesting going on wood had turned into a gray mold infested blank. I had to wear a face mask and breathing mask to get rid of the mold, but the bowl turned out very pretty.