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United States Department of Agriculture

Agricultural Research Service

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Research Project: Improvement of Bioherbicide Strategies for Invasive Weeds in Southern Cropping Systems

Location: Biological Control of Pests Research

2013 Annual Report

1a. Objectives (from AD-416):
The objective of the proposed research is to improve the methods of production, formulation and delivery of indigenous pathogens to safely and effectively control invasive weeds. Kudzu (Pueraria lobata var. montana) and old-world climbing fern (Lygodium microphyllum) are not well controlled by chemical herbicides and are often present in sensitive areas where herbicides may be inappropriate. Other aggressive weeds, such as marestail (Conyza canadensis) and pigweeds (Amaranthus spp.) are commonly found to be resistant to the popular herbicide, glyphosate. This research plan is designed to develop safe, affordable, and practical control strategies for these weeds using the fungal plant pathogen, Myrothecium verrucaria and combinations of M. verrucaria, other weed pathogens and compatible chemical herbicides and adjuvants. The overall objective of this project is to develop and improve biological weed control with plant pathogens (bioherbicides). Our research will focus on the following objectives: Objective 1: Develop safe and effective formulation, mass-production, and application technology techniques and systems to improve the bioherbicidal potential and utility of bioherbicides for controlling exotic, invasive or glyphosate-resistant weeds. Objective 2: Bioherbicidal and integrated control of kudzu.

1b. Approach (from AD-416):
The research plan comprehensively addresses bioherbicide development from cost-effective production methods to exclude unwanted byproducts and; to optimize formulations of bioherbicides with respect to spore and leaf-surface hydrophobicity and maintenance of free-moisture; to deliver bioherbicides with maximum spray coverage; and post-release monitoring of the agroecosystem documenting the persistence and spread of biocontrol agents. Production methods will be monitored by liquid chromatography to detect unwanted byproducts. Plant bioassays with visual and digital disease ratings will assess the effectiveness of bioherbicide formulations. Field tests will be performed in mature, naturally-occurring kudzu stands to validate bioherbicide efficacy alongside benchmark herbicides. DNA-based species-specific markers will be identified to enable post-release monitoring of bioherbicides. A strain of the bacterial pathogen Xanthomonas campestris and reduced mycotoxin formulations for biological control of glyphosate-resistant weeds such as marestail (Conyza campestris) and pigweeds (Amaranthus spp).

3. Progress Report:
A bacterial pathogen (Pseudomonas sp.) was isolated from kudzu. This bacterium successfully controlled kudzu in greenhouse experiments without a dew treatment. A strain of the bacterium Xanthomonas campestris was effective in controlling common cocklebur, monosodium methyl arsenate (MSMA)-resistant cocklebur, and glyphosate-resistant marestail in greenhouse experiments. Interaction studies of plant pathogens with herbicide-susceptible and -resistant weeds studies showed that the fungal bioherbicide, Myrothecium verrucaria (MV) controlled glyphosate-resistant and -susceptible Palmer amaranth; a strain of Xanthomonas campestris (XC) could control common cocklebur; and formulation additives can affect conidial germination formation and virulence of fungus Colletotrichum truncatum, a bioherbicide for hemp sesbania. Research continued on developing and evaluating a reduced mycotoxin Myrothecium verrucaria (MV) mycelial formulation. This formulation is efficacious against several target weeds (kudzu, redvine, trumpetcreeper, hemp sesbania and sicklepod), as well as glyphosate-resistant pigweeds and marestail in corn, when co-applied with a surfactant as a directed spray. Field plot research continued on bioherbicidal control of kudzu with MV at two locations over several years. Granular formulations of MV also suppressed kudzu and hemp sesbania significantly.

4. Accomplishments
1. Biological control of glyphosate-resistant weeds. Although herbicides are usually effective in controlling most weeds, there are weeds that are either not effectively controlled by herbicides or have developed resistance to certain classes of herbicides. Bioherbicides may offer an effective alternative control strategy, but must be properly formulated in order to maximize effectiveness. A strain of the bacterium Xanthomonas campestris, isolated from cocklebur by researchers at the USDA-ARS Biological Control of Pests Research Unit, Stoneville, MS, was shown to be effective in controlling common cocklebur and glyphosate resistant marestail in greenhouse experiments. This pathogen can be readily produced on several inexpensive solid and liquid media, and may provide effective control against these weedy species as well as other related weeds.

2. Improvement of bioherbicide efficacy. Efficacy is of paramount importance for any biocontrol agent. Researchers at the USDA-ARS Biological Control of Pests Research Unit, Stoneville, MS, have discovered that bioherbicial efficacy of several plant pathogens, such as Myrothecium verrucaria, Colletotrichum spp., and several others can be greatly enhanced using adjuvants such as vegetable oils, emulsions, surfactants, sugars, and plant extracts. These findings may also enhance the overall understanding of the infection processes of various plant pathogens.

3. Improvement of bioherbicide formulations. Undesirable secondary metabolites that are produced by many fungi may hinder their commercialization potential. Researchers at the USDA-ARS Biological Control of Pests Research Unit, Stoneville, MS, continued to develop and evaluate reduced mycotoxin Myrothecium verrucaria (MV) mycelial formulations. This formulation is efficacious against kudzu, redvine, trumpetcreeper, hemp sesbania, and glyphosate-resistant pigweeds and marestail. The further demonstration that toxin production by this fungus can be mitigated using appropriate culturing and growth media modification has enabled large scale, off-site field testing, and interest by EPA and commercial entities has resulted. This research may yield insight resulting in methodology to eliminate these mycotoxins, thus making MV and similar organisms more feasible for commercialization.

4. Biological control of kudzu. Kudzu continues to be a serious, rapidly spreading invasive weed threat. Effective control measures are needed, such as improved chemical, biological, and chemical-biological interactions for optimal weed control. Researchers at the USDA-ARS Biological Control of Pests Research Unit, Stoneville, MS, conducted field plot research on bioherbicidal control of kudzu with Myrothecium verrucaria (MV) formulations, at two locations for five years, and have demonstrated >90% control of kudzu is routinely achieved through a combination of mowing, bioherbicide and herbicide application. Granular formulations of MV also effectively suppressed kudzu growth and spread. This research indicates that an integrated control program involving cultural, bioherbicide/chemical interactions, and bioherbicide formulations may be required for successful kudzu control. Ultrastructural treated with MV and Silwet L-77 surfactant revealed a rapid (<1 h after treatment) detachment of the protoplast from the cell wall and plasmodesmata appeared broken off and retained in the wall. These results are the first to report the ultrastructural effects of MV on plants and suggest that penetration of a phytotoxic substance(s) of unknown etiology in the fungal formulation was facilitated by the surfactant analysis of effects on kudzu seedlings.

5. Biological control of weedy vines. Weedy vines are becoming increasingly problematic in southern U.S. row crop production and are either tolerant, or developing resistance to many commercial herbicides, such as glyphosate. Because these weeds are not effectively controlled by herbicides, they are becoming more widespread in many genetically-modified organism (GMO) crops, such as Roundup-ready soybeans; clearly, more effective control measures are required for these weeds. Researchers at the USDA-ARS Biological Control of Pests Research Unit, Stoneville, MS, found that Myrothecium verrucaria (MV) mycelium controlled several morningglory species, as well as redvine and trumpetcreeper, in greenhouse and field experiments. The fact that MV is capable of controlling several different weeds in addition to kudzu, improves the utility of this bioherbicide.

Review Publications
Hoagland, R.E., Teaster, N.D., Boyette, C.D. 2013. Bioherbicidal effects of Myrothecium verrucaria on Glyphosate-resistant and -susceptible Palmer amaranth biotypes. Allelopathy Journal. 31(2):367-376.

Boyette, C.D., Hoagland, R.E. 2013. Bioherbicidal potential of a strain of Xanthomonas spp. for control of common cockelbur, (Santium strumarium). Biocontrol Science and Technology. 23(2):183-196.

Boyette, C.D., Hoagland, R.E. 2013. Adjuvant and refined corn oil formulation effects on conidial germination, appressorial formation and virulence of the bioherbicide, Colletotrichum truncatum. Plant Pathology Journal. 12(2):50-60.

Hoagland, R.E., Boyette, C.D., Vaughn, K.C., Teaster, N.D., Stetina, K.C. 2012. Effects of Myrothecium verrucaria on ultrastructural integrity of kudzu (Pueraria montana var. lobata) and phytotoxin implications. American Journal of Plant Sciences. 3:1513-1519.

Last Modified: 09/19/2017
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