Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 393

specialists. Host range of the pathogen is then determined, optimum condi-
tions for infection and disease development are identified,and mechanisms of
action of the pathogen are investigated. Next, particularly virulent strains of
the pathogen are mass-produced, formulated to improve their infective
ability, evaluated for efficacy on a field scale, and registered with regulatory
agencies. Through this process, a number of fungi have been developed as
sprayable, potentially marketable products, commonly called “mycoherbi-
cides.”These include Alternaria cassiae ('CASST'), for use against Cassia obtusifo-
lia , C. occidentalis , and Crotalaria spectabilis; Colletotrichum gloeosporioides f.sp.
aeschynomene ('Collego'), for use against Aeschynomene virginica; Colletotrichum
gloeosporioides f.sp. cuscutae ('Lubao'), for use against Cuscuta australis , C. chinen-
sis , and C. maritima; Colletotrichum gloeosporioides f.sp. malvae ('BioMal'), for use
against Malva pusilla; Phytophthora palmivora ('DeVine'),for use against Morrenia
odorata; and Puccinia canaliculata ('Dr. Biosedge'), for use against Cyperus esculen-
tus (TeBeest, 1996; Boyetchko, 1997).
In nature, widespread infection of host plants by pathogens and subse-
quent disease development are relatively rare: inoculum levels may be too low,
host plants may resist infection, and environmental conditions may not be
conducive to infection, attack on host tissues, and propagule dispersal. The
mycoherbicide approach to weed control overcomes most barriers to infection
and disease development by applying large doses of inoculum (e.g., 10 10 -10 11
spores ha 1 ) to susceptible life stages of target weeds using carrier formula-
tions that promote attack (Watson, 1989; Auld & Morin, 1995). The timing of
applications may be chosen to make use of environmental conditions favor-
able to plant disease, or environmental conditions may be modified directly.
Because leaf wetness is an especially critical factor affecting infection and
disease development by many foliar pathogens (TeBeest, 1991), delivery of a
high concentration of inoculum in an optimally moist environment is desir-
able. Mabbayad & Watson (1995) found, for example, that increasing both the
conidia concentration of an Alternaria species and the volume of water carry-
ing the fungus improved suppression of the weed Sphenoclea zeylanica in rice
fields (Table 8.3). The fungus was effective against both seedling and adult
stages of the target weed.
By reducing weed density and suppressing weed biomass production,
application of mycoherbicides can diminish the competitive effects of weeds
on crops and increase crop yields. For example, in experiments conducted by
Kempenaar,Horsten & Scheepens (1996),application of Ascochyta caulina spore
suspensions improved the performance of maize and sugar beet grown with
Chenopodium album. The fungus created necrotic lesions on leaves and stems of
C. album and reduced C. album density up to 65%, but did not harm the two