Plant species in the family Alismataceae are significant weeds of rice in Australia and South-East Asia. In Australia and Korea, parallel research programs have investigated the use of inundative biological control of these weeds using plant-pathogenic fungi. Recent studies have shown a close phylogenetic relationship between the organisms under investigation in the two countries. We surveyed Alismataceae weeds in southern South Korea for the presence of diseases that produced symptoms similar to those previously recorded in Australia and Korea. Fungal isolations were made from the diseased specimens. Phylogenetic studies of the relatedness of the isolates collected from Korea and Australia revealed that the isolates overlapped between those previously described in Korea as Plectosporium tabacinum and the newly named P. alismatis. Pathogenicity tests on Australian weeds (Alisma lanceolatum, Damasonium minus and Sagittaria graminae) were undertaken in the glasshouse. The host range testing showed that the isolates from Korea were less pathogenic on the Australian weeds tested than the Australian isolates. Therefore, although the isolates were phylogenetically related, the isolates from Korea did not show greater virulence or a wider or different host range than the Australian isolates.