Methods for selecting hypervirulent biocontrol agents of weeds: why and how

Abstract

A considerable number of plant pathogens have been studied for their possible use in weed control. Some have proven virulent enough to control weed species and to compete commercially with chemical herbicides. However, most pathogens of weeds are not useful in their wild form because they are not sufficiently host-specific and/or virulent. The authors believe that these barriers can be overcome. The present research has focused on the inhibitory effects of certain amino acids on the growth and development of specific plants. Pathogens that overproduce these selected amino acids can be easily selected from a pool of spontaneous mutants. Such mutants can have increased pathogenicity to their target weed and enhanced field performance as biocontrol agents. Enhancement of biocontrol efficacy in three separate pathogen-host systems, two with Fusarium and one with Pseudomonas, has already been reported. It is proposed to use the same technology to enhance the biocontrol efficacy of the two species of Fusarium that are host-specific pathogens of the broomrape group of parasitic weeds. The stepwise approach outlined can lead to obtaining enhanced biocontrol agents capable of producing inhibitory levels of selected amino acids in situ. It is proposed that these approaches, in combination with other methods of virulence enhancement, will lead to sustainable systems of biological control of parasitic weeds.

Investigation of Amino Acids As Herbicides for Control of Orobanche minor Parasitism in Red Clover

“…As a green alternative for weed control, the enzymes targeted in weeds by glyphosate and imidazolinone herbicides, can also be inhibited by the exogenous addition of certain amino acids (Sands and Pilgeram, 2009). The rationale is based in the mode of enzymatic regulation in amino acid biosynthetic pathways.…”

“…In addition, amino acids from unrelated pathways can rescue the plant growth by giving general growth stimulus or reactivating the targeted enzyme in the case of end product of competitive pathways. Active amino acids for plant growth inhibition and plant growth rescue are dependent on the plant species, the plant developmental stage and concentration of application (Henke et al, 1974) and therefore, the development of this strategy for weed management systems requires the determination of which amino acids and concentrations are efficacious against the targeted weed species and whether the inhibition is reversible by other amino acids potentially present in the soil (Sands and Pilgeram, 2009). Properly implemented this strategy assumes low toxicity for plants other than the targeted weed and low persistence in the soil through microbial metabolization.…”

“…In addition, because the targeted enzymes are absent in animals it is also assumed low toxicity in them. Although this approach has been reported effective for control of Canada thistle, red bromegrass, kudzu and cannabis, the direct application of amino acids as herbicides has not been put into field practice yet (Sands and Pilgeram, 2009). …”

“…Given the predicted low persistence of amino acid treatment in the soil (Sands and Pilgeram, 2009), the herbicidal effect of amino acids would be maximized if the timing of application is based on the occurrence of susceptible parasitic life stages. Broomrape development depends on temperature (Eizenberg et al, 2005) and therefore, the relation between temperature and underground phenology should be recorded locally under field conditions in order to ease the targeting of vulnerable parasitic life stages by amino acid treatment.…”

Microbial Weed Control and Microbial Herbicides

“…Cohen et al (2002) transformed genes of the indole-3-acetamide (IAM) pathway to cause an auxin imbalance that increased the virulence of Fusarium oxysporum and F. arthrosporioides, pathogenic on broomrape (Orobanche aegyptiaca). Sands and Pilgeram (2009) outline the steps to enhance virulence of the biocontrol agent using amino acid overproduction. They discuss control of the parasitic weeds Orobanche and Striga, which are especially challenging to control due to the close relationship they develop with their hosts.…”