Summary:
Features include 1) growth on living stems of herbaceous plants, sometimes on leaves, or on soil, or on wood and bark lying on the ground, 2) a fruitbody that is cobwebby to pellicular, whitish to cream, more or less separable, 3) spores that are elliptic, smooth, inamyloid, and colorless, 4) basidia that are 4-(6-)spored and short-cylindric, 5) a monomitic hyphal system, the hyphae without clamp connections, the basal hyphae light brown. Thanatephorus cucumeris is best known under the names of the anamorph (Rhizoctonia solani Kuhn or Moniliopsis solani (Kuhn) R.T. Moore). The online Species Fungorum, accessed August 25, 2020, lists the former as its current name, presumably because basidiomycetes and their anamorphs are now supposed to have a single name and the Rhizoctonia name has priority. MycoBank, accessed the same day, listed the current name as Thanatephorus cucumeris.

Microscopic:
SPORES 8-12 x 5-6 microns but varying in size, elliptic (adaxial side mostly convex or straight), smooth, inamyloid, acyanophilic, colorless, thin-walled, some producing secondary spores although not seen in all specimens; BASIDIA (12)15-20 x (8)10-12 microns, short-cylindric, normally 4-spored with rather stout 8-10 microns long sterigmata; HYPHAE monomitic, without clamp connections, basal ones colorless or some with a light brown color, "thin or usually with walls thickened or more rarely distinctly thick-walled, 10-12 microns wide, other hyphae thin-walled and forming a very thin subhymenial tissue", (Hjortstam), SPORES 8-14 x 4-6 microns, even, flattened on one side, colorless; BASIDIA 10-20 x 7.5-11 microns, not forming a compact hymenium, "with 4-6 sterigmata 6-10 microns long and more or less swollen towards the basidium"; in section "60-100 microns thick, composed of a few loosely interwoven hyphae running along the substratum and sending out short branches which bear the basidia", "hyphae in contact with substratum may be slightly brownish", colorless elsewhere, "not incrusted, not nodose-septate, up to 6-10 microns in diameter, with branches smaller", (Burt)

Notes:
Thanatephorus cucumeris has been found in BC, WA, OR, ID, AB, MB, NB, NS, NT, ON, PQ, SK, AK, AL, AR, AZ, CA, CO, CT, DC, DE, FL, GA, IA, IL, IN, KY, LA, MA, MD, ME, MI, MN, MO, MS, MT, NC, ND, NE, NJ, NY, OH, OK, PA, RI, SC, SD, TX, UT, VA, WI, WV, and WY, (Ginns). Distribution includes the West Indies, Europe, India, and Australia, (Burt), as well as Denmark (Hjortstam).