Xanthium spinsom L. Bathurst burr

Bathurst burr, Xanthium sp inosum (Asteraceae), is a
widespread summer annual weed in rangeland, pastures
and crops in eastern Australia. Possibilities for both the
inu ndative (bioherbicide) and classical biocontrol of
Bathurst burr have been explored. In the 1980s-90s, the
anthracnose fungus Colletotrich um orbiculare that naturally
occurs on Bathurst burr in Australia was extensively
investigated to develop a bioherbicide for use in infested
crops and pastures. Despite initial optimism, commercial
development of the potential bioherbicide did not go
ahead. Prospects for classical biocontrol of Bathurst burr
were first explored in the 1930s then again in the 1990s
with surveys in the USA and South America, the putative
centre of origin. A range of arthropod natural enemies
was identified; only one, a tetranychid webbing mite, was
investigated further and found not to be sufficiently specific
for biocontrol. Three of the fungal pathogens most
freq uently recovered during the 1990s surveys, the
anthracnose fungus, the leaf spot fungus Cercospora xanthicola
and the powdery mildew Golovinomyces
cichoracearum, were studied in more detail. Isolates of
the anthracnose fungus collected in semi-arid and arid
areas of Argentina were found not to be superior, in terms
of dew requirement for infection, to the Australian isolate
of the fungus used in the bioherbicide resea rch. Bathurst
burr and three of the eight sunflower cultivars tested with
Argentinian isolates of the powdery mildew became
heavily infected . Isolates of the leaf spot fungus caused
severe necrosis on Bathurst burr leaves, but did not infect
stems. They were also shown in preliminary tests to cause
small necrotic lesions on lucerne and five sun flower cultivars.
These fungi were not investigated further as possible
biocontrol agents due to lack of specificity.
Key words: anthracnose fu ngus Colletotrich um orbiculare,
powdery mildew fungus Golovinomyces cichoracearum,
leaf spot fungus Cercospora xanthicola, bioherbicide.