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Sclerotia of Leucocoprinus cretaceus cultured in a jar of brown rice and wood, though mostly growing on the rice as the wood may be too dry.

This was an old jar of sterilised substrate that had been sat unused for a while and had dried up somewhat. An agar wedge from a recently cleaned up culture was dropped in primarily to double check for any contamination. Sclerotia were not noticed on the agar when it was collect but did appear on the agar that remained after. Sclerotia growth started on the rice immediately after being inoculated and resulted in substantial growth.

Immature sclerotia developing on agar can be seen here:

https://www.inaturalist.org/observations/175642154

Day by day growth log of the mushrooms is here:

https://www.inaturalist.org/observations/181123489

The jar was opened to collect sclerotia on myceliated rice in order to inoculate several more jars with various substrates and water levels to test growing conditions. A small amount was also collected in the process for preliminary microscopy.

To the naked eye they appear initially as small, fluffy white specks but as they develop further the white mycelium seems to peel away to reveal a yellow orange colour coming through. Then brown or almost black as they mature. In another jar left sitting for some months no trace of mycelium remains and the sclerotia appear like tiny black seeds.

Dissection of these tiny, hard structures is awkward and they readily stick to instruments. They can be easily picked up just by touching the tip of a needle or scalpel to them are aren't so easily removed from the surface.

Placing them under the microscope at 40X with top illumination on a slide with a drop of 10% KOH seemed to make it easier. It's not clear if it helped soften them but it did at least kept them from floating around and sticking to the instruments. Beneath the outer brown surface, yellow-orange is visible inside. The outer layer appears like a thin brown/black envelope surrounding this, though was not easily photographed.

From measuring around 20 sclerotium the following size range was determined: 245 - 735 µm. The smallest of which had developed the brown/black colour so appeared fully formed rather than immature. The average size being around 440-490 µm. However I would expect these figures to change with a larger sample.

If they are isolated and have enough space to grow they form into almost perfect spheres or are ovoid or elliptical with relatively symmetrical proportions. However in this densely clustered growth in which maybe 50 sclerotia were on a single grain of rice they deform where they touch each other. Some are left with almost geometric looking shapes and flat edges. Subsequently measuring in situ is complicated by sometimes being unable to tell where one ends and another begins and in isolation by the non uniform shape.

The growth rate is fast with this jar having been inoculated less than a month prior. Sclerotia began growing immediately and appeared with the mycelium as it spread down the jar, rather than after the substrate had already colonised. Specimens could be observed to develop fully and then start transitioning to brown/black within a matter of days.

On the rice in this jar or card in another they still stood out but in a natural environment they would be virtually indistinguishable amongst soil and easily mistaken for insect frass on wood.

By comparison, a size of 500-820 µm is given for sclerotia of L. birnbaumii:

https://www.jstage.jst.go.jp/article/jjom/52/1/52_jjom.H22-05/_article/-char/en

I will be checking the size in a culture I have soon but L. cretaceus sclerotia do appear smaller and more discreet to the naked eye, though no less abundant in number.