ABSTRACT
The false ragweed (Parthenium hysterophorus) , a native of Central America, is a problem weed of Australian rangeland, particularly in Queensland. Following field surveys in Mexico, the rust fungus Puccinia abrupta var. partheniicola was selected as a potential biological control agent (Parker et al. 1994). One isolate was chosen for further investigation. Infection with the rust hastened leaf senescence, significantly decreased the life-span and dry weight of false ragweed plants, and led to a tenfold reduction in flower production. Subsequent studies showed that the rust was sufficiently hostspecific to be considered for introduction. In Bangalore, India, Cassia uniflora, a leguminous undershrub of some economic value, has replaced, over a five-year period, more than 90 per cent of the false ragweed on a 4,800 m2 site (Joshi 1991). Leachates from Cassia uniflora are allelopathic, inhibiting false-ragweed seed germination and hampering the establishment of a summer falseragweed generation. The colonies of Cassia uniflora are robust enough to prevent a false-ragweed generation from forming below them in winter.
