The agaric Crinipellis perniciosa (Tricholomataceae) is a hemibiotrophic pathogen which causes witches' broom disease of cacao and has recently decimated the Brazilian cacao industry. In addition to the pathogenic cacao (C-) biotype, other biotypes are found in association with unrelated plant taxa, notably bignoniaceous lianas (L-biotype), solanaceous hosts (S-biotype), and the shrub Heteropferys acutifolia (H-biotype). The C-and S-biotypes are non-outcrossing and form broom symptoms on hosts, whereas the L-biotype is outcrossing and asymptomatic. Phylogenetic analysis of several regions of the rRNA locus revealed near identity between C- and S-biotype isolates from diverse locations, with the L- and H-biotypes forming separate groupings. Preliminary analysis of sequence data from Moniliophthora roreri, causal agent of frosty pod disease, indicates that this morphologically distinct pathogen may be closely related to C. pernicinsa. Similarities in host infection between C. perniciosa and M. roreri have previously been noted but it is difficult to reconcile the gross morphological differences. Pairings between C. perniciosa and M. roreri gave rise to a clamped dikaryotic mycelium suggestive of a hybridisation event.