In the early days, pathogens were not seen as suitable biological
control agents. Early workers were primarily entomologists and searched
for insects only (Wilson
1964). Pathogens attacking prickly pears were
studied but were not deliberately introduced, though two appeared in
Australia, probably introduced on or with the insects. In the 1960s, the
USDA was studying a rust (Uromyces rumicis) for the control of Rumex
crispus,
but it was not introduced. The first deliberate introduction of a pathogen
was of the rust fungus Puccinia chondrillinae into Australia from Italy in
1971, to control skeleton weed (Chondrilla juncea). This fungus was
extremely successful in controlling the most common narrow-leaf form of the weed but the remaining two
forms were unaffected. New strains which will attack these forms are being
sought in south-eastern Europe, the centre of origin of Chondrilla.
Despite this success, doubts continued regarding the safety of
importing pathogens, and the next introduction was not until 1991 when the
rust Puccinia abrupta var. partheniicola was introduced into Australia for
the control of parthenium weed (Parthenium hysterophorus).
It is now accepted that pathogens are as narrowly host-specific as
insects, and equally safe to use as biological control agents. Several
have now been imported into the USA as well as Australia, New Zealand and
South Africa.
