Abstract
SUMMARY Mass-spore-derived plasmodia from a heterothallic Honduran isolate of Didymium iridis were compatible in all 15 combinations tested, whereas plasmodia from the same isolate, but derived from crossing single-spore clones were compatible in only 2 of 21 combinations. It is suggested that an incompatibility mechanism exists in this and other heterothallic isolates. On the other hand, when 20 combinations of single-spore-derived plasmodia were tested from a homothallic isolate from Costa Rica, all of these showed compatibility.