Location: Biological Control of Pests Research
2020 Annual Report
Objectives
Objective 1. Improve the efficacy, environmental safety, and stability of two bioherbicides currently in development, SPFG and Myrothecium verrucaria (Mv), for the management of salvinia and kudzu, respectively.
Sub-objective 1.1. Develop safe, efficacious and stable formulations of the bioherbicide SPFG, and determine how interactions with insect herbivores can affect efficacy.
Sub-objective 1.2. Reduce or eliminate Mv mycotoxin levels while maintaining high product efficacy.
Approach
The research plan has two goals. The first will focus on evaluating and developing a fungal pathogen (referred to as ‘SPFG’ for proprietary purposes) for managing the exotic, invasive aquatic weed, giant salvinia (Salvinia molesta). The second deals with further developing and testing the fungus Myrothecium verrucaria (Mv) for controlling kudzu (Pueraria lobata var. montana) and other invasive weeds. Because Mv spores produce trichothecene mycotoxins, EPA registration has been hindered. A mycelial formulation devoid of, or with extremely low levels of mycotoxins should result in more likelihood for EPA registration. The development of effective bioherbicide formulations will be guided by the ecology of the pest target and an understanding of biotic and abiotic factors that influence the effectiveness of the bioherbicide. Formulations will be developed to compliment the surface chemistry of the pest host and bioherbicide for better attachment and infectivity, as well as to mitigate deleterious environmental factors that reduce or inhibit host-plant infection. The development of commercially-viable microbial pesticides for weeds could provide growers, land managers and homeowners with safe, cost-effective, non-chemical control tools for use in agriculture, chemically-sensitive environments, and natural ecosystems.
Progress Report
Research continued for evaluation of a bioherbicidal fungus (called SPFG in this report) for controlling giant salvinia in greenhouse and small field trials. Studies of effects of SPFG on Cyrtobagous salviniae colonies and interaction research continued. A manuscript detailing this research was published and accepted with corrections in the Journal of Aquatic Plant Management. Previous research showed that Myrothecium verrucaria (Mv), when combined with some commercial formulations of glyphosate, resulted in synergistic interactions on weed control, but some commercial glyphosate products were toxic to Mv. A mycelial formulation of Mv was tested alone and in combination with a commercially available glyphosate (GLY) product for controlling the invasive vines, redvine (Brunnichia ovata), and trumpet creeper (Campsis radicans) in field experiments conducted near Stoneville, Mississippi. Several application timing regimens were evaluated (Fall, Spring, Fall + Spring, and Spring + Fall). We found that a Fall + Spring application of MV + GLY controlled redvine and trumpet creeper by 95%, 12 days after the second treatment, through a synergistic interaction of the fungus and glyphosate. Neither glyphosate alone, nor MV alone, effectively controlled either weed species under any application timing regimen. No visual disease or herbicide damage occurred on glyphosate-resistant soybean plants in the treated test plots. This research resulted in a manuscript, "Interaction of a Myrothecium verrucaria Mycelial Preparation and a Glyphosate Product for Controlling Redvine (Brunnichia ovata) and Trumpet Creeper (Campsis radicans)," in the American Journal of Plant Sciences. Previously, we found two biotypes of Amaranthus palmeri (Palmer amaranth) in a population of this economically important weed that were resistant to glyphosate but differed with respect to pigmentation. One biotype was typically red-pigmented (betacyanin) while the other was green, with no visual appearance of red hue on any plant part at any growth stage. We have also reported that a strain of Myrothecium verrucaria (Mv) exhibited bioherbicidal activity against several important weeds including glyphosate-resistant Palmer amaranth. In greenhouse tests, Mv was applied to these two biotypes (red and green) at two ages (3-week and 6-week old) and effects of this fungus monitored over a 5-day time course. Initial symptoms of Mv (16 to 24 h after inoculation) were: epinastic curvature, wilting and development of lesions on leaves and stems. Generally, the younger plants tended to be more sensitive to Mv than older plants. Bioherbicidal damage increased with time leading to necrosis and plant mortality and increasing disease progress. Severe loss of fresh weight occurred in both biotypes as compared to untreated plants. Results indicated that Mv was effective on both biotypes, but effects on growth reduction and disease progression were more rapid and generally greater in the green biotype, suggesting that compounds responsible for red pigmentation may be more potent as defense against pathogen attack. This research resulted in a manuscript, "Effects of Myrothecium verrucaria on Two Glyphosate-Resistant Amaranthus palmeri Biotypes Differing in Betacyanin Content." in the American Journal of Plant Sciences.
Accomplishments
1. Biocontrol of problematic invasive aquatic weeds. Common duckweed (Lemna minor) is a rapidly spreading aquatic plant that deprives ponds of oxygen and leads to the death of fish and beneficial algae in still waters. It is important to control duckweed for the health of ponds and existing aquatic life. It can be aggressive invaders of ponds and are often found mixed in with mosquito fern (Azolla caroliniana). ARS researchers in Stoneville, Mississippi, conducted experiments using an SPFG mycelial formulation and found that 95-100% mortality of these weeds was achieved within 48 hours following inoculation. These findings are important because in addition to being important invasive weed pests, some Azolla spp. have been shown to produce a neurotoxin [ß-Methylamino-L-alanine (BMAA)], a non-proteinogenic amino acid, generally thought to produced mainly by cyanobacteria.
Review Publications
Boyette, C.D., Bryson, C.T., Hoagland, R.E., Weaver, M.A., Stetina, K.C. 2020. Interaction of a Myrothecium verrucaria mycelial preparation and a glyphosate product for controlling Redvine (Brunnichia ovata) and trumpet creeper (Campsis radicans). American Journal of Plant Sciences. 11:201-213. https://doi.org/10.4236/ajps.2020.112016.
Hoagland, R.E., Boyette, C.D., Jordan, R.H., Stetina, K.C. 2020. Effects of Myrothecium verrucaria on two glyphosate-resistant Amaranthus palmeri biotypes differing in Betacyanin content. American Journal of Plant Sciences. 11:214-225.
