Annals of Botany

Parasite prefers to do it on grass

Continuing the parasite theme, we now consider a hemi-parasite, Thesium chinensis an Asian member of the Santalaceae. Thesium is a widely distributed Old World genus, all members of which are parasitic or hemiparasitic. Several members of the genus have a nuisance value as agricultural weeds and thus it is important to gain knowledge of their ecology and host range. Suetsugu et al. (Kyoto, Japan, pp. 49–55) point out that for T. chinensis such knowledge is very fragmentary. Further, there is a world of difference in observing proximity in the field to potential host species and actually knowing that parasitism is occurring. There is also a difference between study of interactions with selected plants under controlled conditions and study of what happens in the wild. The authors first carried out an association analysis in the field. Of the 38 species recorded at the study site only two (Eragrostis curvula and Lespedeza juncea) were present at greater frequency than expected. When haustorial connections were examined by excavating around the plants, 22 of the 38 species were shown to be parasitized, indicating a broad host range such as is seen in many hemiparasites. However, there was clear evidence that some species were more effective hosts than others: estimation of haustorium numbers showed that for many host species numbers differed significantly from expected. On this basis, grass species (Poaceae) were very good hosts whilst members of the Rosaceae and Caryophyllaceae were poor hosts. Other families, such as the Fabaceae contained both good (L. juncea) and poor (Pueraria lobata) hosts. A slightly puzzling feature of the data is that haustoria on Fabaceae were larger than on other families. Nevertheless, the results indicate that T. chinensis has a clear preference for Poaceae whilst retaining the ability to utilize a range of other species when preferred hosts are not in reach.