Eighteen isolates sampled from sugar beet roots associated with an unknown etiology were characterized based on observations of morphological characters, hyphal growth at temperatures ranging from 4 to 28 C, production of phenol oxidases, and sequence analysis of internal transcribed spacer (ITS) and large subunit (LSU) regions of the ribosomal DNA (rDNA). The isolates did not produce asexual or sexual spores, had binucleate hyphal cells with clamp connections, grew from 4 to 22 C with and estimated optimal growth at 14.5 C, and formed a dark brown pigment on potato dextrose or malt extract agar amended with 0.5% tannic acid. Color changes observed when solutions of gum guiac, guiacol, and syringaldzine were applied directly to mycelium grown on these media indicated that all isolates produced phenol oxidases. Sequences of ITS and LSU regions on the rDNA gene from 15 isolates were 99.2 to 100% identical and analysis of sequence data with Maximum Likelihood and Maximum Parsimony suggest that the isolates from sugar beet roots are phylogenetically related to Athelia bombacina, Granulobasidium vellereum, and Cyphella digitalis. High statistical support for both loci under different criteria confirmed that Athelia bombacina was consistently the closest known relative to the sugar beet isolates. Additional taxonomic investigations are needed before species can be clarified and designated for these isolates
