A Proposed Mode of Action of Oil-Based Formulations of a Microbial Herbicide

Abstract

Formulation of some microbial herbicides in emulsions of vegetable oil can signiWcantly reduce their dependency on long dew periods to allow infection and control of the host weed. Examination of leaves of Viola arvensis, by a range of microscopy techniques, after spraying with an oilin-water emulsion formulation of the potential microbial herbicide Mycocentrospora acerina, suggests that the eVect arises as a result of inversion of water into both oil droplets on the leafsurface and oil that penetrates into the leaf tissue. This oil probably mixes with aqueous components from the leaf tissue and, in doing so, forms an invert emulsion. We suggest that this supplies the water required by the fungus to initiate and sustain growth so that infection can result.

Where are the bioherbicides?

“…Despite this, our understanding of formulations for bioherbicides remains poor because most studies have been limited in scope, apparently performed to find a workable method for testing a candidate bioherbicide, and then discontinued. Few studies have aimed to provide a detailed understanding of the effect of formulations on the biology of the bioherbicide or the target weed, with the exception of Greaves et al (2001). Consequently, generally held assumptions such as the assumption that bioherbicide formulations protect spores from desiccation on the leaf surface should be treated with caution.…”

“…Water/oil formulations significantly improve efficacy compared with simple aqueous sprays; however, these improvements are limited and other difficulties exist, including more cumbersome product preparation, crop phytotoxicity, and high viscosity (McWhorter et al 1988;Quimby et al 1988). Invert emulsions retard the evaporation of water from droplets (Connick et al 1991;Greaves et al 2001), enhance the germination of conidia in high-humidity environments (Egley et al 1993), and may have other effects, such as causing localized damage to the plant epidermis and cuticle. A better understanding of the mechanisms by which formulations enhance bioherbicide effect would be valuable.…”

Oil emulsions increase efficacy ofPhoma herbarumto control dandelion but are phytotoxic

“…In this case, eight of the 10 formulations tested increased the efficacy of the pathogen P. herbarum to cause disease on dandelion. This may have been due to the emulsion preserving the aqueous phase containing the fungal mycelium in the formulation, capturing moisture from the atmosphere or the plant surface (Lawrie et al 2000), penetrating the plant via stomata (Gauvrit & Cabanne 1993;Greaves et al 2001) or damaging the cuticle (Amsellem et al 1991;Klein et al 1995) which may assist infection, or by stimulating the growth of the fungal pathogen (Womack & Burge 1993;Abbas & Egley 1996). However, significant phytotoxicity was observed and this is the first such report associated with these previously described formulations, as prior studies have recorded little or no phytotoxic symptoms (Daigle et al 1990;Auld 1993;Womack et al 1996).…”

Advances in bioherbicide formulation

“…The mechanism by which simple emulsions may reduce dew dependence is not clear. Greaves et al . (2001) suggest that invert emulsions are formed on the leaf surface and also as oil penetrates leaf tissue and mixes with aqueous components of leaf tissue.…”