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ARS Home » Southeast Area » Stoneville, Mississippi » Biological Control of Pests Research » Research » Research Project #430266

Research Project: Bioherbicidal Control of Invasive Weeds with Indigenous Plant Pathogens

Location: Biological Control of Pests Research

2018 Annual Report

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.

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. Cyrtobagous salviniae colonies established and SPFG interaction research continued in laboratory and greenhouse. Field experiments were successfully conducted in a giant salvinia-infested pond near Shreveport, Louisiana. Viable freeze-dried formulations of SPFG and Myrothecium verrucaria (Mv) mycelium, and Colletotrichum truncatum (Ct) and Colletotrichum gloeosporioides f.sp. gloeosporioides (Cga) spores were mass-produced, freeze-dried, and stored, demonstrating weed control efficacy and shelf-life of at least 6 months for SPFG, Ct, and Cga, and >10 years for Mv. Research continued on the evaluation of a reduced mycotoxin formulation and mutant strains of Mv for control of kudzu and other invasive weeds. A process was developed to enable the bioherbicidal fungus, Ct, to control large hemp sesbania (Sesbania exaltata) (>30 cm), a problematic weed in southern U.S. rowcrops, using a hot water pre-treatment. In greenhouse tests, hemp sesbania plants sprayed with hot water (45-95°C), followed immediately by spray applications of Ct spores suspended in either 0.2% Silwet L-77 surfactant in water or unrefined corn oil with surfactant, controlled weeds 80-95% 12 days after treatment, with concomitant plant dry weight reductions, without a dew treatment. Over 80% control of hemp sesbania was achieved after 12-15 days in field experiments when a pre-treatment of hot water (65°C) was followed with a fungal application. Plants in field tests treated with the fungus without the hot water treatment were unaffected. These results suggest that hot water treatment may be useful for improving the bioherbicidal potential of plant pathogens. Studies were conducted on mycelial preparations of Mv and mutant sector of this fungus. Application of Mv and the Mv sector mycelial preparations to hemp sesbania and sicklepod plants and leaf discs (kudzu and glyphosate-resistant Palmer amaranth) showed that the sector efficacy was generally equal to, or slightly lower than Mv. Research continued on evaluating synergistic interactions between various chemical herbicides and bioherbicidal plant pathogens. Bioassays assessed possible interactions of the herbicide glufosinate and Ct for hemp sesbania control. The effects of spray applications of glufosinate and bioherbicide (applied alone and in combination) on seedling growth, Glutamine Synthetase (GS) activity and ammonia levels in hypocotyl tissues were monitored (88-h time-course). Growth (elongation and fresh weight) and extractable GS activity were inhibited in tissues by glufosinate and glufosinate plus Ct treatments as early as 16 h, but Ct treatment did not cause substantial growth reduction or GS inhibition until after ~40 h. Results provide further insight on the biochemical action of this bioherbicide and on its interactions with this herbicide.

1. Formulations to enhance bioherbicidal pathogen efficacy. Eastern black nightshade (Solanum ptycanthum) (EBN) is a problematic weed found throughout the eastern U.S. Scientists at USDA-ARS, Stoneville, Mississippi, conducted greenhouse and field experiments to compare the effects of an invert (water - in - oil) emulsion (IE) versus an surfactant - water (water + Tween 80) spore formulations of a bioherbicidal fungus, Colletotrichum coccodes (Cc) for controlling this weed. Greenhouse tests revealed that several other economically important solanaceous weeds, including the exotic invasive weed tropical soda apple (S. viarum), were infected and killed at significantly greater levels by the fungus in the IE as compared to the surfactant formulation. Field tests revealed that >90% EBN control and dry weight reductions occurred when plants were treated with the fungus/IE formulation, while the fungus/surfactant formulation failed to control the weed. These results demonstrate that this invert emulsion formulation can promote the bioherbicidal efficacy of Cc, and also suggest its utility to improve the efficacy of other bioherbicides that may have been previously rejected for further development due to low efficacy or environmental constraints.

2. Biocontrol of invasive aquatic weeds. Giant salvinia, (Salvinia molesta) is an aquatic floating fern native to South America, but has become invasive in some areas in Africa, Asia, Australia, and in subtropical and warmer temperate areas of the U.S. Scientists at USDA-ARS, Stoneville, Mississippi, and the U.S. Army Engineer Research and Development Center, Vicksburg, Mississippi, conducted experiments using two species of Myrothecium (M. rodridum and M. verrucaria) to evaluate their bioherbicidal potential for control of this weed under outdoor conditions. The results indicated that Myrothecium formulations have the potential of being effective bioherbicides of giant salvinia, as they have shown to be effective in other weed systems.

Review Publications
Boyette, C.D., Hoagland, R.E., Stetina, K.C. 2018. Bioherbicidal enhancement and host range expansion of a mycoherbicidal fungus via formulation approaches. Biocontrol Science and Technology. 28(3):307-315.
Hoagland, R.E., Boyette, C.D., Stetina, K.C. 2017. Extending the shelf-life of Myrothecium verrucaria, a bioherbicide. American Journal of Plant Sciences. 8:3272-3284.
Weaver, M.A., Shearer, J.F., Grodowitz, M.J., Boyette, C.D. 2018. Potential of Myrothecium species as bioherbicides for giant salvinia (Salvinia molesta). Journal of Aquatic Plant Management. 56:120-122.