Emulsion-formulation of microbial herbicides.
Development of microbial herbicides is constrained by unreliability in the field where
conditions are often sub-optimal for infection. Crucially, sufficient moisture, often dew, is required
to establish infection. Two model systems, Colletotrichum dematium (Pers. ex Fr. ) Grove on
Chenopodium album L. and Mycocentrospora acerina (Hartig) Deighton on Viola arvensis Murr.,
have been investigated and formulation requirements for each system identified, principally to
reduce the dew period necessary for infection.
Effects of adjuvants (surfactants, stickers and humectants) on spore germination and
appressorium formation were investigated in vitro. Few were toxic and then, principally, at high
concentration. The surfactants Tween 40,60 and 80 were compatible with both pathogens.
Similarly, the stickers acacia, ghatti, guar, karaya, locust bean and xanthan gums and low viscosity
alginic acid were all non-toxic as was the humectant glycerol. Each pathogen reacted differently
to the adjuvants and any potential microbial herbicide will need individual matching of adjuvants
to give an effective formulation.
A working formulation (rapeseed oil-in-water (1: 10 v/v) emulsion using 0.1 % v/v Tween
40 as the emulsifier) was found to reduce the dew period requirement of M. acerina from 36 to
18 hours. The formulation protected spores from desiccation for 24 hours after application, or for
16 hours following a sub-optimum dew period occurring immediately after application.
Scanning electron microscopy showed that the applied spores, and the developing
mycelium, were immersed in the oil deposit. Transmission electron microscopy of sections through
formulation deposits on the leaf revealed that some inversion of the emulsion, to form a water-inoil
deposit, had occurred, suggesting a mechanism of protection against desiccation. The oil phase
infiltrated the cortical intercellular spaces only when the leaf was infected. This intercellular oil
contained more water than that on the leaf surface.
Emulsion-formulation applied to run-off with an 'air brush', consistently gave significantly
better weed control under sub-optimal dew conditions than a formulation of surfactant only. When
applied with a conventional hydraulic nozzle at 400 1 ha" the emulsion was only occasionally
superior to the surfactant alone. Such interactions require further in-depth investigation.
The importance of correct inoculum placement for maximum effectiveness, independent
of formulation type, was highlighted. Unless all meristems are killed, survivors quickly grow,
despite the death of neighbouring leaves and petioles, and the weed suffers merely a growth check.
Formulation as emulsion improved diseasee stablishmenta nd diseasee xpressioni n the
target weedo nly in somec ircumstancesF. urther researchin to spraya pplicationm ethodsa ndt heir
interactions with formulation, host and environment is clearly necessary