Managing weeds with insects and pathogens - Ecological Management of Agricultural Weeds - page 395

Table 8.4. Effects of different spray applications on mortality of Sesbania exaltata
and soybean yield in field trials

Sesbania exaltata mortality (%) Soybean yield (kg ha 1 )

Spray treatment a 1989 1990 1990

Conidia in water 10 40 2160

Conidia in invert emulsion b 95 97 2593

Invert emulsion only 8 15 1408

Acifluorfen c 96 98 2618

Untreated 3 2 1309

Notes:
a Conidia concentrations of Colletotrichum truncatum were 1 10 7 mL 1 ; carrier volume for all
spray treatments was 187 Lha 1 .
b The invert emulsion was prepared from paraffinic wax and oil,monoglyceride emulsifier,
lanolin,and water.
c Acifluorfen was applied at 1.1 kg a.i.ha 1 .
Source: Adapted from Boyette et al .(1993).

Improvements in the efficacy of fungi as weed biocontrol agents can result
from the use of two or more species in combination. Infection of Senecio vul-
garis with the fungus Botrytis cinerea caused 10% mortality when B. cinerea was
applied alone, but 100% mortality when it was applied after the weed species
had been infected with Puccinia lagenophorae (Hallett, Paul & Ayres, 1990) .
Improvements in formulation technologies also can markedly increase the
efficacy of fungi used for inundative weed control. An illustrative example is
provided by Boyette et al . (1993), who examined the effects of carrier formula-
tion on infectiousness of Colletotrichum truncatum used to suppress Sesbania
exaltata ,a leguminous weed found in soybean,rice,and cotton in the southern
USA. The fungus is highly virulent, host-specific, and can be mass-produced
readily. Invert emulsions (water suspended in oil, rather than a standard
emulsion of oil suspended in water) are thought to retard evaporation of
water spray droplets, thereby reducing or eliminating the need for a 6- to 24-
hour dew period for infection by fungi used as biocontrol agents. In field
experiments, application of C.truncatum conidia in water (at 187 L ha 1 ) killed
an average of 25% of the S. exaltata population and increased soybean yield
65%, compared with an untreated control (Table 8.4). In contrast, application
of C. truncatum conidia in an invert emulsion with the same carrier volume
killed an average of 96% of the S. exaltata population and increased soybean
yield 98%. The level of weed suppression and crop yield increase obtained
from the invert emulsion formulation was similar to that obtained from the
herbicide acifluorfen (Table 8.4).