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exclusively on native microorganisms. The bias toward microbes rather than
insects reflects the greater ease and lower cost of raising the large populations
needed for inundative releases, and the ability of certain microbial control
agents to mimic the ease of application and efficacy of chemical herbicides.
Greater reliance on native rather than exotic microbial taxa reflects greater
ease of collection, fewer regulatory requirements, and better preadaptation of
native species to local climatic conditions (Quimby & Walker, 1982).Although
there are important exceptions, shoot-attacking pathogens are more com-
monly used than those attacking roots because of greater ease and uniformity
of inoculum application, narrower host range, and more effective dispersal of
reproductive propagules in air than in soil (Hasan & Ayres, 1990).
In theory the use of inundative control agents is expected to have fewer
negative environmental impacts than the use of inoculative controls,since the
former are less well adapted for persistence and dispersal without human
intervention. That is not to say that mass releases of microorganisms to sup-
press weeds are without potential problems. TeBeest (1996) noted that
“almost without exception, these pathogens can and do infect cultivated and
horticulturally important plant species in controlled experiments.” Toxic
effects on livestock and people are another undesired but possible impact of
using microbial control agents. Phomopsis emicis ,a fungal pathogen considered
for use in Australia as a biocontrol agent against the weed Emex australis ,was
found to produce large quantities of a compound poisonous to grazing
animals (Auld & Morin, 1995). Thus, the host range and ecological impact of
organisms used for inundative control programs must be very thoroughly
studied, and prudence used in their deployment (Weidemann, 1991). Great
care must be taken to separate target and non-target species in both space and
time. No major accidents have yet been reported, but that may reflect limited
use more than a set of safety factors inherent to the organisms used. In antici-
pation of greater use of microorganisms for controlling weeds and other
pests, Canadian and American public regulatory agencies are now mandated
to consider how microbial products affect the environment, human health,
and food and feed quality (Makowski, 1997).
Mycoherbicides
More than 100 taxa of microorganisms have now been identified as
potential candidates for the control of at least 100 weed species (Charudattan,
1991; Kennedy & Kremer, 1996). The most intensively studied of these are
fungi (Watson, 1989). Typically, a fungus is collected and isolated from a dis-
eased weed, tested for its pathogenicity via inoculation of healthy weeds, cul-
tured and maintained on artificial media, and identified by taxonomic
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